Thermometry

SIM Summer School
Thermometry Workshop
Karen Garrity
National Institute of Standards and Technology
Gaithersburg, MD USA
[email protected]
www.nist.gov/cstl/process/thermometry
•
ITS-90
•
Overview of thermometer types
•
Discussion of SPRTs
•
Discussion of Liquid-in-Glass
•
Discussion of thermocouples
•
Discussion of thermistors and platinum resistance thermometers
•
Digital Thermometers and other types
•
Humidity Measurement
1
NIST Thermometry Group Calibration Staff
Dean Ripple – Group Leader
– [email protected], 301 975 4801
Karen Garrity – Thermocouples
– [email protected], 301 975 4818
Michal Chojnacky – SPRTs for T > 83 K and Bridges
– [email protected], 301 975 4821
Gregory Strouse – ITS-90 for T > 83 K, SPRTs, Fixed-points, and PTs
– [email protected], 301 975 4803
Wes Tew – ITS-90 for T < 84 K, SPRTs
– [email protected], 301 975 4811
Dawn Cross – Industrial Thermometers
– [email protected], 301 975 4822
Wyatt Miller – Industrial Thermometers, Humidity
– [email protected], 301 975 3107
Greg Scace – Humidity
– [email protected], 301 975 2626
www.nist.gov/cstl/process/thermometry
2
What is the ITS-90?
The ITS-90 is an approximation of the thermodynamic temperature scale
with specific recipes for “realizing” temperature, including:
17 defining fixed-point cells (triple point of water, freezing point of silver, etc.)
with assigned temperatures
RECIPES !
Defined thermometer types
Cake:
Bake at (190 ±10) °C
Silicon Wafer: Oxidize at
Defined interpolation equations between fixed points
for
(2100±300)
seconds
(974±1)°C for 18 seconds
Defining
interpolating
instruments:
Standard Platinum Resistance Thermometer (SPRT) from 13.6 K to 962 °C
Spectral Radiation Thermometer from 962 °C to all higher temperatures
Why use the International Temperature
Scale of 1990 (ITS-90)?
• The ITS-90 is easier to realize than thermodynamic temperature.
• The ITS-90 is internationally accepted, leading to global
consistency
• Establishment of temperature uncertainty more straightforward
on the ITS-90 than with “Process measurement scales”.
3
• Redundant subranges provide calibration flexibility
ITS-90 Equations
Two reference functions
– One for above 0 °C and one for below 0.01 °C
Ten deviation functions to cover temperature calibration subranges
– Required fixed point measurements depends on temperature
subrange
– All subranges in overlapping temperatures considered equal
Two approximate inverse functions to calculate temperature
– Error < 0.13 mK
A simple, but important equation to remember:
R(T )
W(T ) =
R(273.16 K)
90
90
4
ITS-90 Temperature Subranges for SPRTs
13.8033 K to 273.16 K
54.3584 K to 273.16 K
83.8058 K to 273.16 K
234.3156 K to 302.9146 K
273.15 K to 302.9146 K
273.15 K to 429.7485 K
273.15 K to 505.078 K
273.15 K to 692.677 K
273.15 K to 933.473 K
273.15 K to 1234.93 K
H2, 17 K, 20 K, Ne,
O2, Ar, Hg, TPW
O2, Ar, Hg, TPW
Ar, Hg, TPW
Hg, TPW, Ga
Ga, TPW
In, TPW
Sn, TPW
Zn, Sn, TPW
Al, Zn, Sn, TPW
Ag, Al, Zn, Sn, TPW
a1, b1, c1, c2,
c3, c4, c5
a3, b3, c3
a4, b4
a5, b5
a11
a10
a9, b9
a8, b8
a7, b7, c7
a6, b6, c6, d
SPRTs must meet ITS-90 specifications:
W(302.9146 K) ≥ 1.118 07 or W(234.3156 K) ≤ 0.844 235
For above 933.473 K
W(1234.93 K) ≥ 4.284 4
5
ITS-90 Fixed Points for SPRTs
Material
T90/K
e-H2 (TP)
13.8033
e-H2 (VP)
17.0
e-H2 (VP)
20.3
Ne (TP)
24.5561
O2 (TP)
54.3584
Ar (TP)
83.8058
Hg (TP)
H2O (TP)
Ga (MP)
In (FP)
Sn (FP)
Zn (FP)
Al (FP)
Ag (FP)
234.3156
273.16
302.9146
429.7485
505.078
692.677
933.473
1234.93
t90/°C
–259.3467
–256.15
–252.85
–248.5939
–218.7916
–189.3442
–38.8344
0.01
29.7646
156.5985
231.928
419.527
660.323
961.78
Wr(T90)
dWr(T90)/dT90/ mK
0.00119007
2.407E-07
0.00844974
1.227E-06
0.09171804
3.903E-06
0.21585975
4.342E-06
0.84414211
1.00000000
1.11813889
1.60980185
1.89279768
2.56891730
3.37600860
4.28642053
4.037E-06
3.989E-06
3.952E-06
3.801E-06
3.713E-06
3.495E-06
3.205E-06
2.841E-06
6
TP: triple point
MP: melting point
FP: freezing point
VP: vapor pressure point
Thermometric Fixed Points
Temperature fixed points of pure substances
Freezing (melting) points, S-L, P-T
Triple points, S-L-V, P/T
Boiling points, L-V, P-T
Sublimation points, S-V, P-T
Transitions (superconductive), S-S, H-T
Requirements for fixed-point temperature realization
– Constant pressure
– Sample purity ≥ 99.9999%
– Thermal gradients of furnace ≤ 10 mK
– Thermometer readings must represent the phase equilibrium state
– Corrections for pressure and immersion depth
SPRT Measurements
– W(T90) = R(T90)/R(273.16 K)
7
Anatomy of a Fixed-Point Cell
Ga MP or TP
In, Sn, or Zn FP
Teflon valve
Port to gas
handling system
Glass outer
envelope
Glass
thermometer well
Compression
Fitting for
(HT)SPRT
Rubber Stopper
Cap plug with
hole for
evacuation of
head space
Graphite
Heat Shunts
Teflon crucible
Structural
integrity
rings
Nylon
support ring
Ceramic fiber
insulation
Glass outer
envelope
8
Checklist of Things to Consider in Realizing
Thermometric Fixed-Point Cells
Fixed-Point Cells and Furnaces
– impurities in fixed-point cell
– immersion profile
– length of freezing or melting curve
– acceptable change in realization temperature (e.g. slope)
– temperature corrections (HH, P)
– furnace gradient
– furnace temperature
– furnace control
Use check thermometers for measurement assurance
– Total system check on measurement process
9
Type A and Type B Standard Uncertainties
for fixed-point cells to consider
Type A
Freeze to freeze repeatability, W(T90) – from check SPRT
Bridge-measurement errors
– effects of changes in reference resistor(s)
– non-linearity of bridge
– ratio error
Type B
Chemical Impurities
TPW Isotopic concentration
Hydrostatic-head (location error of SPRT sensor)
Propagated TPW
SPRT self-heating
Heat Flux (Immersion)
Effects of moisture on leads and insulation degradation
Gas pressure
Choice of fixed-point value derived from a short plateau
10
Check SPRTs and Control Charts
Each fixed point is assigned its own check SPRT
SPRT is measured only at its assigned fixed point and the TPW
– fixed-point reproducibility over time (Type A uncertainty)
– SPRT stability over time
Verification of:
– fixed-point cell plateau realization
– measurement system
– measurement procedure
DOES NOT assign a value of temperature to the fixed-point cell
Fixed-point calibration
– check SPRT, customer SPRTs (as many as 5), check SPRT
Control charts are a graphic display of check SPRT data
11
Control Chart for IST Zn FP Cell
Check SPRT 0051, ASL F18, 1 mA
0.6
[Wx(Zn)-Wavg(Zn)] / mK
n=406, s.d.=0.18 mK, r=0.81 mK
0.4
0.2
0.0
-0.2
-0.4
-0.6
0
100
200
300
number of readings
400
500
12
Triple Point of Water (TPW): 273.16 K
Defines the unit of the kelvin
The temperature of the triple point of water (TPW) is
the temperature of pure water, ice, and water vapor
in thermal equilibrium.
– Isotopic composition knowledge is critical for
lowest uncertainty
– Cell envelope material impacts impurities
R (273.16 K) propagated error in W (T 90), mK
0.8
0.1 mK error
at 273.16 K
Ag
0.6
Al
0.4
Zn
Sn
0.2
Hg
Ar
Ga
In
TPW
0.0
0
300
600
Temperature, K
900
1200
W(T90) = R(T90) / R(273.16 K)
Fundamental measurement for
the ITS-90 calibration of
SPRTs
TPW propagates over the entire
13
temperature range
The Isotopic Definition of Water Redefined
“NEW” water isotopic definition
– Current compositional equivalent of V-SMOW (Vienna-Standard Mean Ocean Water)
• D / 1H = 0.000 155 76 (mol / mol)
• 18O / 16O = 0.002 005 2
• 17O / 16O = 0.000 379 9
Isotopic concentration correction algorithm
Tmeas = TV -SMOW + ADδD + A Oδ O + A Oδ O
17
17
18
18
– xδD, δ17O, and δ18O values are isotopic deviations from VSMOW
– A(D) = 628 µK, A(17O) = 57 µK, and A(18O) = 641 µK
Using the isotopic correction algorithm decreases the TPW realization temperature
differences
– VSMOW equivalent TPW cells have corrections of ±5 µK w/small uncertainties (2 µK)
– New TPW cells should be tested for isotopic composition
Old cells – no isotopic analysis
– Correction based on Deuterium concentration and estimating algorithm
• Type B uncertainty (normal distribution) of 20 µK
14
Preparation Techniques for the TPW
Most mantle preparation techniques do not affect the final realization
temperture
– Crushed Dry Ice
– Immersion Cooler
– liq. N2 cooled rods
– liq. N2
Crushed Dry Ice Method
– Pre-chill TPW cell
– Wipe reentrant well dry
– Fill well with crushed "dry ice" and maintain full for 15 to 20 minutes
– Place cell in ice or maintenance bath until "dry ice" sublimates
– Fill reentrant well with ice water and allow 2 to 3 days for relieving of
strains in mantle
Testing for TPW conditions
– Solid-Liquid-Gas Interface
– Insert rod into reentrant well to form inner melt
– Ice mantle must be free to rotate when TPW cell is tilted
15
Preparation Techniques for the TPW
Step 1
Step 2
Technique for estimating the amount of gas in a TPW cell
– 1 cm3 of air is equivalent to a temperature depression of about 0.01 mK
– sharpness of the water hammer depends on amount of air in the cell and
16
the amount of that air absorbed into the water
Considerations in Selecting a Thermometer
Accuracy: Uncertainties range from < 0.001 °C (0.002 °F) to >1 °C (2 °F)
Cost of Thermometer: Range from $6 to $6000
Cost of Calibration: from $50 to $12,000
Temperature Range
Stability and Durability during use
– chemical contamination
– resistance to high temperatures, moisture, vibrations, and shock
Compatibility with measurement equipment
– resistance thermometers, thermocouples easy to integrate to
electronics
– liquid-in-glass, digital thermometers much easier for quick visual
inspection
Compatibility with object being measured
– sheath construction
17
Tolerances vs. Calibration Uncertainties
10
TC, type K,
special
Tolerance or uncertainty / °C
LiG, partial immersion
1
PRT,
grade A
0.1
LiG, total immersion
Thermistor, accuracy class 1
0.01
0.001
-200
0
200
400
600
Temperature / °C
Colored lines: ASTM tolerances (ASTM E1, E1137, E230, and E879).
18
Dashed lines: NIST calibration uncertainties (k=2)
Electronic Thermometer Types
Standard Platinum Resistance Thermometers (SPRTs)
(very accurate, but susceptible to shock)
–259.4 °C to 962 °C (–435 °F to 1764 °F)
Industrial Platinum Resistance Thermometers (IPRTs, PRTs, or RTDs)
–196 °C to 850 °C (–321 °F to 1562 °F)
Thermistors
–50 °C to 100 °C (–58 °F to 212 °F)
Thermocouples
–196 ° C to 2100 °C (–321 °F to 3810 °F)
Digital Thermometers (PRT, thermistor, or thermocouple in disguise)
–196 °C to 850 °C
19
Standard Platinum Resistance Thermometer
(SPRT)
Types
Construction
Selecting
Calibration
– Ranges
– Methods
– Sources of Error
– Uncertainty
– Measurement assurance
Measurement Equipment
Recalibration Consideration
Uncertainties in Use
Technical Contacts: Michal Chojnacky, 1-301-975-4821, [email protected]
Gregory Strouse, 1-301-975-4803, [email protected]
Wes Tew, 1-301-975-4810, [email protected]
20
Standard Platinum Resistance Thermometry
The SPRT is the ITS-90 defining instrument for interpolation of
temperature between the fixed points using the ITS-90
formulas
The SPRT may be calibrated at the ITS-90 fixed points over the
temperature range from 13.8 K to 962 °C
A unique set of deviation function coefficients are calculated
for each calibration of the SPRT
Three types of SPRTs
– capsule
– long-stem
– high-temperature
13.8 K to 232 °C
83 K to 660 °C
0 °C to 962 °C
21
Anatomy of an SPRT
Birdcage
Single-layer
bifilar
Coiled helix
Pictures courtesy of Goodrich and Hart Scientific
22
SPRTs: Advantages and Disadvantages
ADVANTAGES
– Defining standard instrument of the ITS-90
– Stable if treated properly
– Covers wide temperature range
– R vs T90 well characterized
DISADVANTAGES
– Fragile
– Elaborate and expensive equipment for measurement
– Expensive to purchase
– Expensive to calibrate
– Requires properly trained personnel for use
Essential for highest accuracy, but trained hands
are essential for use
23
Suitable Calibration Ranges for SPRT Models
ominal
R(TPW), Ω
Sheath
Material
Sensor
Support
Material
Lowest
ITS-90
Fixed-Point
Highest
ITS-90
Fixed-Point
Borosilicate
Mica
Ar TP
Zn FP
Temperature
Range of Use,
°C
–200 to 500
Mica
25.5
Fused silica
Fused silica
Ceramic
2.5
Ar TP
Zn FP
Ar TP
Al FP
–200 to 661
Stainless
Steel
Ceramic
Ar TP
Zn FP
–200 to 500
Inconel®
Ceramic
Ar TP
Al FP
–200 to 661
Fused silica
Fused silica
TPW
Ag FP
0 to 962
0.25
Fused silica
Fused silica
TPW
Ag FP
24
SPRT Calibration Process
“Simple World View”
Step 1:
Login SPRT
5 Main Steps in an SPRT Calibration
Step 2:
Stabilize SPRT
Step 3:
Calibrate SPRT
Step 4:
Analysis of Results
Step 5:
Logout SPRT
25
SPRT Calibration Procedures at NIST
Step 3:
Calibrate SPRT
Batch Measurement Pattern:
Check SPRT
SPRTs under test
Check SPRT
check SPRTs measured at all fixed points
appropriate corrections applied: pressure,
hydrostatic heat, ESH
SPRT must meet official ITS-90 criteria
R(TPW) before & after R(Al) and R(Zn) ≤ 0.3 mK
Range of R(TPW) measurements ≤ 0.75 mK
Redundant fixed-point measurements validate
calibration
– W(T90) measured − W(T90) assigned
≤ calibration uncertainty/non-uniqueness
Measure
R(Al), R(TPW)
Measure
R(Zn), R(TPW)
Measure
R(Sn), R(TPW)
Measure
R(In), R(TPW)
Measure
R(Ga), R(TPW)
Measure
R(Hg), R(TPW)
Continue to
Step 4: Analysis of Results
Measure
26
R(Ar), R(TPW)
Measurement Assurance in Use and
SPRT Recalibration Cycles
Periodic checks at the TPW is necessary
Drift at other temperatures correlates well with drift at the TPW
Anecdotal evidence shows that W(T90) is somewhat insensitive to
a change in the R(TPW)
R(TPW) Use Recommendation
– New R(TPW) should be used to calculate W(T90)
Time interval is NOT valid for determining recalibration cycle for
SPRTs
Conservative Recalibration Cycle Recommendation
– When the change in R(TPW) propagated TPW change (∆T)
exceeds uncertainty requirements at the temperature of interest
27
SPRT Care and Handling
Protection from mechanical shock
– Upward changes in R(TPW) are a sign of mechanical abuse
– “Abuse” can be as seemingly innocuous as:
• setting an SPRT down on a hard surface,
• SPRT lead connectors bumping into sheath, or
• rattling tip when inserting SPRT into furnace.
– Practice, plus a belief that mechanical shock is not inevitable, is the key
Protection from overheating
– Each SPRT model has a specific temperature of maximum use
– Excess temperature can cause permanent changes in Pt or, for micainsulated SPRTs, release of water into sheath
Recalibration
– Restabilizes the SPRT at a new R(TPW) starting point
– Generates new coefficients with new uncertainties
– Measure R(TPW) as soon as the SPRT arrives home and compare with
Calibration Report !!!
28
Liquid-in-Glass Thermometers
Types
Calibration
– Ranges
– Methods
– Uncertainty
Ice Melting Point
Measurement Equipment
Software
Uncertainty in the Use of the Liquid-inGlass Thermometer
Measurement Assurance
Technical Contacts
Dawn Cross, 1-301-975-4822, [email protected]
Wyatt Miller, 1-301-975-3107, [email protected]
29
Types of Liquid-in-Glass (LiG) Thermometers
Types of ASTM LiG thermometers
– Over 120 types
– Total Immersion
– Partial Immersion
Liquids used
– Mercury (Hg) - ASTM
– Organic
– Mercury Thallium
– Proprietary (non-toxic)
Low temperature limit of −196 °C (freezing of thermometer liquids ).
High temperature limit of 400 °C (softening of the thermometer glass).
30
THERMOMETER NOMENCLATURE
A LiG thermometer consists of four main parts:
BULB - The liquid reservoir
–The volume of the bulb depends on the coefficients of expansion of
the thermometric liquid and the bulb glass.
–For mercury in normal glass, the bulb volume is approximately 6,222
times the volume of a 1°C length of unchanged capillary or 11,200
times the volume of a 1°F length of unchanged capillary.
STEM - The glass capillary tube
–White or yellow stem backing, Magnified scale
–Red line along the stem.
LIQUID - The thermometric fluid
–Mercury (Hg) - ASTM, Organic, Mercury Thallium, Proprietary
GAS FILLING - The gas above the liquid column
–The gas is dry, inert, and under pressure. It is usually nitrogen or
argon.
31
TOTAL IMMERSION
The bulb and the liquid column are
immersed in the bath with the
top of the liquid column is
approximately one centimeter
above the top of the bath fluid.
PARTIAL IMMERSION
The bulb and a specified portion of
the stem are immersed in the
bath fluid.
32
Tolerances & Calibration Uncertainties
Typical ASTM tolerances: 1 or 2 scale divisions.
Typical calibration uncertainties:
0.2 to 0.5 of a scale division
Additional uncertainties in use:
•
Emergent stem temperature different from ASTM specification
(partials only)
•
Drift due to changes in bulb
•
Drift due to capillary distress at high temperatures (> 150 °C)
33
Recalibration of LiG Thermometers
• NIST SP1088 recommends recalibration by adjusting old
calibration values by the ice-point shift for Hg thermometers
below 150 °C.
• LiG thermometers used above 150 °C should additionally be
periodically recalibrated at the highest temperature of use.
34
Temporary Changes in Bulb Volume
After use at high temperatures, there is a temporary enlargement of the bulb
This type of temporary bulb expansion will cause a depression in the reading at
the ice-point.
It takes 72 hours at room temperature for a thermometer to recover from
exposure to high temperatures.
Easiest solution is to use thermometer with increasing temperature.
Alternative solution is to apply a scale correction (contact Dawn Cross)
Permanent Changes in Bulb Volume
High temperature use sometimes causes a permanent increase in the icepoint reading due to annealing effects
35
Main Steps in LiG
Thermometer Calibrations
Inspect for defects with 10X
microscope
First measurement: Ice MP
Measurements in ascending
temperature
Last measurement: Ice MP
Stem corrections applied using
Fadens for Partial LiGs (≥ 150 °C)
36
Ice Melting Point, 0 °C
Temperature equilibration of water, ice and air-saturated water at
101.3 kPa
Ice MP components
– Dewar flask
– siphon tube to remove excess water from Dewar bottom (necessary
only for liquid-in-glass thermometers)
– distilled water and distilled water ice
Ice MP preparation
–
–
–
–
–
–
fill Dewar 1/3 full with water and pack with shaved ice (2 or 3 mm size)
OR, fill Dewar with shaved ice saturated with distilled water
voids between ice particles are filled with water
allow 30 minutes for temperature equilibration
siphon water and repack with ice as necessary
wear gloves to prevent contamination
U (k=2) = 0.002 °C for distilled water
U (k=2) = 0.02 °C for tap water that is not too hard
37
How to Build an Ice Melting Point (0 °C)
Distilled water - solid
38
How to Build an Ice Melting Point (0 °C)
Shaved distilled water ice
39
How to Build an Ice Melting Point (0 °C)
Dewar flask with
siphon hose
40
How to Build an Ice Melting Point (0 °C)
Fill Dewar Flask with
shaved distilled water ice
and distilled water (liquid)
Note: gloves to maintain
water purity
41
How to Build an Ice Melting Point (0 °C)
Mound ice above Dewar flask
Distilled water (liquid) fills
voids between shaved water
ice
Note: There should never be
liquid water above the ice or
floating ice.
42
Measuring an LiG in an Ice Melting Point (0 °C)
Clean thermometer with shaved distilled water ice prior to
pre-chilling
Note: gloves to maintain water purity
43
Measuring an LiG in an Ice Melting Point (0 °C)
Pre-chill thermometer in shaved
distilled water ice
44
Measuring an LiG in an Ice Melting Point (0 °C)
Use a clean rod (metal or glass) to
make thermometer hole, prior to
insertion of thermometer.
45
Measuring an LiG in an Ice Melting Point (0 °C)
Thermometer in an Ice Melting Point
Pre-chill thermometer in ice
Clean thermometer with ice
Use rod (not the thermometer!) to
make hole
0 °C mark just above ice.
10 minute equilibration time
– LiG is pre-chilled in ice
– 3 min equilibration time in
the ice melting point
Siphon liquid as needed
Repack ice prior to each thermometer
46
insertion
EXAMPLE OF A DEFECTIVE THERMOMETER
10x Microscope used to identify glass chip in the capillary just to the
left of the graduated scale. Even though the chip is off scale it can
cause problems.
The amount of gas around the chip, which will expand more than the
mercury, could vary and produce erratic readings.
NIST would refuse to calibrate this thermometer.
Separated liquid columns can be fixed—not grounds for rejection.
47
CALIBRATION SYSTEM CHECK
The calibration baths always contain a NIST liquid-in-glass
thermometer in addition to the SPRT to check that all parts of the
calibration system are operating properly.
The NIST liquid-in-glass check standard are always used
The reading order for this group of thermometers is:
–
–
–
–
–
–
–
SPRT,
The NIST check standard,
The customer's thermometers from left to right,
SPRT,
The customer's thermometers from right to left,
The NIST check standard, and the
SPRT.
The SPRT is placed in front of the digital video camera when it is
being read so that it is in the same part of the bath as the other
thermometers when they are being read.
48
Check
Standard
49
LiG Thermometer Calibration
Uncertainty Components
LiG thermometer
– Repeatability, A
– Short term stability at ice MP, A
– Scale accuracy, B
– Emergent stem temperature correction, B, (Partial only ≥150
°C)
LiG thermometer measurement system
– Resolution, B
Reference Temperature Measurement System
– Standard platinum resistance thermometer, B
– Resistance ratio bridge, B
Comparison Baths
– Instability, B
– Spatial uniformity, B
Ice MP, B
50
Thermocouples
What is a Thermocouple?
Reference Functions
Types
– Letter designated
– Non-letter designated
Construction
Selecting
Calibration
– Ranges
– Methods
– Uncertainty
– Measurement Assurance
– Measurement Equipment
Software
Sources of Error
Recalibration Considerations
IST Technical Contacts
Karen Garrity: 301 975 4818
Dean Ripple: 301 975 4801
51
Thermocouples...any two dissimilar conductors,
joined at one end
T0
Material A
Voltmeter
Material B
T1
Although total signal
depends on temperature of
two ends (1 & 4),
thermocouples generate
signal primarily in regions
of strong thermal gradients.
(Region 2-3)
The junction itself does not
generate a voltage!!
52
Advantages of Thermocouples
Cheap
Wide temperature range
Small (down to 0.25 mm diameter)
Easy to integrate into automated data systems
Disadvantages of Thermocouples
Small signals, limited temperature resolution, typically 0.1 °C (0.2 °F)
Thermocouple wires must extend from the measurement point to the
readout. Signal generated wherever wires pass through a thermal
gradient.
At higher temperatures, thermocouples may undergo chemical and
physical changes, leading to loss of calibration.
Recalibration of certain types of thermocouples or in certain applications is
very difficult.
53
Emf-Temperature Relationships
for the 8 Letter-Designated Thermocouple Types
100
Emf / mV
80
E
60
Base-metal
types
J
K
N
40
R
20
S
B
T
0
−20
−400
0
400
800
1200
Temperature / °C
1600
2000
Notation: E = emf = Electromotive Force = Thermoelectric Voltage
54
S = dE/dT = Seebeck Coefficient = Sensitivity
ASTM E230 Tolerances for Type K and Type N Thermocouples
10
Standard tolerance
Tolerance, °C
5
Special tolerance
0
-5
-10
-200
0
200
400
600
800
1000 1200 1400
Temperature, °C
Note that even special tolerances does not meet goal of 0.5 °C (0.9 °F)
uncertainty goal for retort applications: need calibration of wire ! 55
Annealing of Pt-Rh Alloy Thermocouples
• Places thermocouple in
uniform thermoelectric state
• Alleviates any mechanical
cold working
• Only done with noble metal
thermocouples
• TCs annealed by direct
passage of AC current, at
1450 °C, then 750 °C
56
Thermocouple Preparation for Comparison Calibration
• Junction first welded on each TC, using gas-oxygen torch
• Up to 5 test TCs + reference TC welded into a common junction
57
Comparison Calibrations in Furnaces
Reference TC
High-purity alumina tube,
inserted in tube furnace
Test TC
• TCs measured simultaneously,
to cancel temperature drift
• Measuring junctions at center of
furnace to minimize gradients
• Reference TC calibrated at
fixed points
Common welded junction
NIST Exp. Uncertainties in °C
Base TCs
0 °C
0.1
400 °C
0.4
900 °C
1.0
Noble TCs
0.1
0.2
0.3
58
Emf Measurements
Copper
wires
Dedicated Readout: equivalent
to voltmeter + reference junction
compensation + software.
Reference Junction
Compensation (RJC)
• Tables assume 0 °C (32 °F)
ref. junction temperature
DVM
Ice
Point
TC
TC connector & extension wire
• RJC mimics “missing”
section of thermocouple
when reference junction
temperature is not at 0 °C
• Emf addition by hardware
or software
Readout
TC
Equivalent circuit, with internal RJC
59
Ballpark Drift Rates for Thermocouples in Clean
Environments
Noble Metal Thermocouples
• Au/Pt: no drift observed with proper maintenance over 1000 h
• Type R, S: 0.1 °C every 100 h or 20 uses, in regular thermal cycling to
1100 °C & handling in calibration service
• Type B, R, S: <1 °C after 120 h at 1380 °C
<5 °C after 10000 h at 1330 °C in air
<65 °C after 10,000 h at 1330 °C in Ar or vacuum
• Changes in immersion depth can make apparent drift many times worse
• Chemical contamination, use in inappropriate atmospheres, or overheating
can cause extremely large changes
• Up to 30 % change in emf seen without any physical failure of sensor!
60
Thermocouple Calibration Uncertainty Components
Temperature reference
• reference calibration uncertainty
• reference stability
• readout uncertainty
• reference junction temperature
Test thermocouple
• test thermocouple stability and homogeneity
• readout uncertainty, including effects of extraneous emf
Thermal equilibrium of test and reference
• Comparison bath/furnace stability
• Comparison bath/furnace uniformity
61
Care and Feeding of Thermocouples
Noble Metal Thermocouples:
• Use at the same immersion at which the calibration was performed.
• Protect from contamination by the furnace environment, using single
lengths of alumina insulator when possible
• Protect from mechanical strain and kinks
Base Metal Thermocouples:
• Monitor drifts in base metal thermocouples by in situ tests
• Protect from contamination using alumina or silica tubes, or use
mineral-insulated-metal-sheathed thermocouple wires.
• For each temperature environment to be measured, a new
thermocouple should be made, and it should always be used at the
same immersion.
• Obey the ASTM upper temperature limits for bare wire
thermocouples.
62
Recalibration of Used Thermocouples is Problematic!
If the furnace is
isothermal, there will
be NO difference
between the original
TC calibration and the
recalibration.
This is a known
problem for t ≥ 200 °C
or for strained TCs.
Unknown effects for
exposure to 150 °C
(300 °F)?
63
Alternatives to Recalibration of Used Thermocouples
Option 1. Recalibrate thermocouples in situ.
Example: a reference thermocouple is inserted into a furnace with a
control thermocouple. The control thermocouple may be recalibrated
without removal.
Option 2. Periodic replacement, based on in situ data or literature.
64
Thermistors (Thermal Resistor)
Semiconductors of ceramic material made by sintering mixtures of metallic
oxides such as manganese, nickel, cobalt, copper, iron and uranium.
Temperature Range:
–50 °C to 100 °C (–58 °F to 212 °F)
Standard Forms:
bead
300 Ω to 100 ΜΩ
probe bead in glass rod
disc
0.5 cm to 1.3 cm thick, 5 kΩ to 10 kΩ
washer 2 cm diameter
rod
moderate power capacity, 1 kΩ to 150 kΩ
NTC:
Negative Temperature Coefficient - The vast majority of commercial
thermistors used as thermometers are in the NTC category.
65
Advantages and Disadvantages of Thermistors
40
ADVANTAGES
R (t ) / R (25 °C), kΩ
Easy to miniaturize
30
Rugged
Fast response time
20
Easy to use
Digital thermometer readout
10
Inexpensive
High sensitivity
0
−60
Stability: 4000 h at 100 °C
bead-in-glass
0.003 °C to 0.02 °C
disc
0.01 °C to 0.02 °C
IPRT
Thermistor
−30
0
30
Temperature, °C
60
90
DISADVANTAGES
Small temperature range
Non-linear device
Needs frequent checks on calibration when exposed to t > 100 °C
Interchangeability is limited unless the thermistors are matched
Self-heating may be large
66
What is an IPRT?
2, 3, or 4-wire resistance element – nominally
100 Ω @ 0 °C
– Wire wound
– Thick film
– Thin film
Resistance changes as a function of
temperature
Positive temperature coefficient
Nominal temperature range of use:
– –200 °C to 850 °C (–328 °F to 1562 °F)
Nominal resistance at 0 °C
– 100 Ω wire wound, others for film
Pictures courtesy of SDI and Minco
67
ASTM and IEC Reference Functions for IPRTs
where
δ 
A = α1 +

 100 
α = A + 100 B
B=−
αδ
1002
1002 B
δ=−
A + 100B
C=−
αβ
1004
1004 C
β=−
A + 100B
ASTM E1137 and IEC 751 have adopted values for the
coefficients A, B and C for ITS-90
– A90 = 3.9083 x 10−3
– B90 = −5.775 x 10−7
– C90 = −4.183 x 10−12
ASTM E1137: –200 °C to 650 °C
IEC 60751: –200 °C to 850 °C
68
ASTM E1137 “Off the Shelf” Tolerance and Uncertainty
2.5
Tolerance, °C
2.0
Class A
Class B
1.5
1.0
0.5
0.0
−250
0
250
500
Temperature, °C
Calibrate individual units, or buy special manufacturer’s
tolerances for lower uncertainty
69
“Simple” Questions to Consider in Buying an IPRT
Temperature Range
– probe, head, and wire compatibility
Specifications of probe
– Diameter
– Length
– Type of sensor and support
– Number of wires and insulation type
– Type of end seal
Uncertainty
– In use at your facility
– Stability – (e.g. 10 thermal cycles)
Environmental Conditions
– Pressure, vibration, moisture
Pt purity – α
– 385, 390, 392
Time Response
Calibration
– “Off the Shelf” – Tolerance and Interchangeability
– Actual calibration – Lower Uncertainty
Pictures courtesy of
NANMAC
70
Differences in IPRT Sensor Types
Film IPRTs: good time response, small size, shock resistant. Typical
stability spec. of 0.25 °C/year
Wire-wound IPRTs with highly constrained coils: accuracy similar to
film IPRTs, similar shock resistance
Wired wound IPRTs with slightly constrained coils: best accuracy
(approaching ±0.01 °C over 400 °C span), sensitive to shock.
Performance is highly variable with model.
Drift of PRT with vibration.
Less Vibration
Drift
Thin-film PRTs
Wire-wound “fully supported” with powder
Wire-wound “partially supported”
Less Thermal
Drift
71
Sources of Errors When Using IPRTs
Stability
– Repeatability at 0 °C or 0.01 °C
Immersion
– Check immersion characteristics of IPRT on insertion in thermal
environment
Insulation Resistance
– May degrade at t > 500 °C and high-moisture environments
Self Heating
– Calibration and measurement current must be the same (nominally 1 mA)
– Thermal contact with temperature of interest is important
– Air probe or fluid probe will influence calibration method
Mechanical Shock and Vibration
– Vibration or dropping the IPRT will cause the IPRT to drift or fail
Hysteresis
– Measurement of temperature with increasing temperature may be
different than measurement of temperature with decreasing temperature
72
Drift Mechanisms in PRTs
Water ingress into sensor through moisture seal
Moisture causing electrical short between lead wires in head, just
outside of sheath seal. (Beware of effects of steam cleaning.)
Mechanical shock
Strain due to thermal cycling
Chemical contamination
User should minimize shock and exposure to moisture
Special care/validation/testing needed if thermometer head
is exposed to steam
73
Table 1. Uncertainty components for temperature
measurement with a PRT or thermistor
Component
Calibration uncertainty
or tolerance
Sensor drift
Method of evaluation
Manufacturer, calibration laboratory, or
ASTM tolerance
Manufacturer’s specifications, or user
history
Hysteresis of alternative ASTM Test Methods for Testing Industrial
sensor (PRTs only)
Resistance Thermometers (E 644),
implemented by manufacturer or user
Self-heating
Manufacturer or independent evaluation
Readout uncertainty
Manufacturer or independent evaluation
Readout drift
Manufacturer or independent evaluation
74
Recalibration Interval for an IPRT
Widely varies by design
Widely varying performance based on use
– Thermal history
– Mechanical shock
Behavior is not as predictable as an SPRT
Drift at 0 °C or 0.01 °C does not always correlate well at other
temperatures
Recommendation:
– Measurement at 0 °C or 0.01 °C as a minimum
– Measurement at highest temperature of use is better
– If probe is out of tolerance, replace or re-calibrate probe with a
full set of points (3 points evenly spaced above 0 °C suffices for
retort applications).
75
Digital Thermometers: Sensor is a PRT, Thermocouple,
or Thermistor in Disguise
What is a digital thermometer?
– An electronic measurement box that converts either resistance
or emf of a thermometer to temperature
Pictures courtesy of Agilent, ASL, Brookstone, Hart Scientific, and Omega Engineering
76
Digital Thermometers
Device displays temperature directly by using the calibration
coefficients of the thermometer
Uncertainty: 0.001 °C to 1 °C
Resolution: 0.0001 °C to 1 °C
Device may allow two thermometers to be connected directly to unit
for differential thermometry
Some have software that allow “real time” calibration
SMART thermometers have electronics in the thermometer head
for an RS232 (or similar) port connection
Calibration as a system (probe + readout), or as components
77
Comparative Thermometer Types:
Calibration Methods, Uncertainties, and Costs
Thermometer
Type
Nominal Cost
$
Temperature
Range, °C
Calibration
Method
Calibration
U (k=2), °C
Calibration
Cost, $
SPRT
3000
–196 to 660
Fixed-Point
<0.001
7000
SPRT
3000
–196 to 500
Comparison
<0.01
1750
IPRT
100 to 1000
–196 to 500
Comparison
<0.01
1750
Thermistor
50 to 1000
–10 to 100
Comparison
<0.005
1200
Thermocouple,
Noble Metal
600
0 to 1100
Comparison
0.3
1000
Thermocouple,
Base Metal
6 to 50
0 to 1100
Comparison
1
1000
LiG, Hg
60
–20 to 400
Comparison
<0.5
1000
78
Comparison Temperature Baths
Comparison
Bath
Temperature
Range, °C
Maximum number
of thermometers
liquid nitrogen
–196
cryostat
–97 to 0
5
water
0.5 to 95
24
stirred water
oil
95 to 300
24
stirred liquid
(Dow Corning 210H)
salt
300 to 550
12
Hastalloy tubes in stirred
molten salt mixture
10
Description
copper block
in stirred liquid nitrogen
stirred ethanol
79
Comparison Baths
80
Comparison Baths Uncertainties
Four Comparison Baths from −20 °C to 400 °C
Bath
Type
Cryostat
Water
Oil
Salt
Range
°C
–20 to 0
0.5 to 95
95 to 275
275 to 400
Instability
±°C
0.005
0.002
0.003
0.004
Spatial
Non-uniformity, ±°C
0.006
0.001
0.002
0.005
Stability
– measured with an SPRT every 25 °C
– length of measurements: 15 minutes
– Treated as a Type B (rectangular) calculated from
(half of the range)/SQRT(3)
Spatial Non-Uniformity
– measured with three SPRTs every 25 °C
– triangular horizontal spacing: 6 cm each side
– vertical spacing: ±10 cm
– Treated as a Type B (rectangular) calculated from
(half of the range)/SQRT(3)
81
Comparison Bath Stability
Instability, ± °C
0.006
Cryostat, ±0.005 °C
Water, ±0.002 °C
Oil, ±0.003 °C
Salt, ±0.004 °C
0.004
0.002
0.000
-100
0
100
200
300
400
temperature, °C
Bath stability was determined by measuring an SPRT immersed 30 cm
over a 24 h period using a moving window average
15 minute average - duration of a normal calibration measurement
The largest instability was at the lowest and highest temperatures.
82
Comparison Bath Spatial Non-uniformity
1.2
Spatial non-uniformity, mK
cryostat
water
0.8
0.4
20 cm
30 cm
40 cm
▼
0
−50
0
50
100
Temperature, °C
Two SPRTs by ratio measurement
– Reference SPRT resistance is first measured
– Then Reference SPRT becomes the reference resistor and the check
SPRT is the unknown
– Reference SPRT position is fixed at 30 cm
– Check SPRT is measured at a three depths at four equally-spaced positions.
Maximum difference gives Spatial Non-uniformity Uncertainty
83
Humidity
Basic units and terminology
Humidity Generation Basics
Laboratory Check Standards
84
Humidity Quantities and Terminology
Saturation vapor pressure (ew ):
The equilibrium pressure of water vapor over iso-thermal water or
ice. Value increases exponentially with absolute temperature.
Value calculated using Wexler* converted to ITS-90.
Dew / Frost-Point Temperature (TDP):
The temperature at which liquid and vapor phase water (for a given
water vapor concentration and total gas pressure) are in equilibrium.
At TDP, the water vapor partial pressure ,e, equals ew.
* Wexler A., Journal of Research of the National Bureau of Standards, 1976, 80A, 775−785
Wexler A., Journal of Research of the National Bureau of Standards, 1977, 81A, 5-19
85
More Quantities and Terminology
Relative Humidity (%RH):
For a given gas temperature, the ratio of the partial water vapor
pressure, (e) to the saturation vapor pressure, (ew).
Expressed in percent - (100 × e/ew).
Mole Fraction (X):
Ratio of the moles of water vapor to moles of the water vapor and
carrier gas mixture.
Usually expressed as parts per million or billion (ppm or ppb).
Synonymous with Water Vapor Concentration.
86
Relationships Between Humidity Quantities
For Temperature, T, and gas pressure, P, at a given point
of interest (the hygrometer undergoing calibration):
Mole Fraction / vapor pressure relationship:
x = nw/(nw+ng) ≈ e/P
Mole fraction at the dew/frost-point temperature is approximated by:
x ≈ ew(TDP)/P
Relative Humidity is approximated:
%RH = e/ew(T) ≈ xP/ew(T)
87
1 Last Quantity - Enhancement Factor
Enhancement Factor, f(T,P), reflects non-ideal gas effects, and
departures from non-ideal solution behavior.
Magnitude of enhancement factor ≈ 1.004 at T = 20 °C
and P = 100 kPa.
f(T,P) greater when molecules are highly mobile and / or
closer together. Value can become > 1.01.
Calculated from correlation developed by:
R.W. Hyland, A Correlation for the Second Interaction Virial Coefficients and
Enhancement Factors for Moist Air, Jour. Res. NBS 79A, 551−560 (1975).
88
Saturator-based Humidity Generator Operating Principle
ew(Ts )
x=
f (Ts , Ps )
Ps
Water / ice and gas at constant Ts
liquid water / ice and water vapor are in
equilibrium.
Conservation of mass assumed once gas
leaves saturator.
x
mole fraction of H2O vapor
ew
sat. water vapor pressure
f (Ts, Ps) enhancement factor
gas
Water or ice
Ps
Saturator pressure
Ts
Saturator temperature
89
Primary Standard Dew-point Generator
TDP
The temperature at which liquid and vapor phase water (for a given water
vapor concentration and total gas pressure) are in equilibrium.
At TDP, the water vapor partial pressure ,e, equals ew.
Gas saturated with
H2O. Vapor
pressure = ew(Ts)
Dew-point hygrometer
∆e → 0
e = ew(Ts) - ∆e ≈ ew(Ts), and
Tdp≈Ts
Water or ice
As gas expands thru tubing connecting generator to hygrometer, the H2O Vapor
pressure reduces by ∆e , such that e = ew(Ts) - ∆e.
For ∆e → 0, the case when flow thru downstream plumbing is laminar and head
loss is small, e = ew(Ts) - ∆e ≈ ew(Ts), and Tdp of calibration gas ≈ Ts.
Uncertainty dominated by u(Ts).
90
The Two-Pressure Generator
ew (Tdp ) f (Tdp , Pc )
ew (Ts )
ec
x=
f (Ts , Ps ) =
f (Tc , Pc ) =
Ps
Pc
Pc
Constant Ts
Ps varied to produce
different
gas values of x
Tc constant for RH
Pc near ambient
Test Chamber
Water or ice
Expansion Valve
• Ps varied to produce different values for x. Gas expanded to near ambient pressure in test
chamber. Uncertainty dominated by Type B uncertainties u(ew) and u(f(Ts,Ps))
• Tdp found by iterating:
•
RH % = 100
ew (Tdp ) = es (Ts )
x Pc
ew (Tc ) f (Tc , Pc )
Pc f (Ts , Ps )
Ps f (Tdp , Pc )
91
The Chilled-Mirror Hygrometer
Gas Outlet
Peltier Heat-pump Stack
Photo
Detector
Heat Sink
Light Source
Sampling Cell
Gas Inlet
Metallic single-surface
mirror with imbedded
PRT
92
Chilled-Mirror Hygrometer Function
Light Source
Photo detector
Mirror temperature is cooled by Peltier
stack until condensate (dew / frost) forms
on mirror surface.
Peltier stack controls mirror temperature,
using reflected light from the mirror as
the feedback signal for the Peltier stack.
The amount of condensate remains
constant and in equilibrium with H2O in
the calibration gas.
The temperature of the mirror, Tm, is
measured by a platinum resistance
thermometer imbedded immediately
below the mirror.
Dew-point temperature, Tdp = Tm, since the
water vapor partial pressure in the
calibration gas ,e, equals ew At Tm.
93
Summary
Standard humidity generators at NIST are based on invariant fundamental
properties of water. They are extremely precise, and under the right conditions
may be considered as primary generation standards.
In order to improve usability, the two-pressure technique is usually employed.
Such generators lose their primary nature, but maintain high-precision.
Chilled-mirror check standards are similarly based on the properties of water.
Given proper implementation, they may be considered primary measurement
standards for dew-point.
94
THANK YOU
QUESTIONS95