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 = α1 + 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
