Thermometry Issues in Destruction Kinet

June 9, 2009

Dean Ripple

dean.ripple@nist.gov

NIST Thermometry Group

Overview of thermometer types, terminology, & gDetailed discussion of different thermometer typVerification & good practice
Effects of Temperature on Micr

Small temperature changes:

Arrhenius Equation: reaction rate constant propor

(

Ea = activation energy, R = gas constant, T = abs

Modification of protein enzymatic kinetics 

Even small changes in temperature, at relatively l

can lead to significant changes in rate of microbia

Large temperature changes:

Activation of protective pathways (e.g., heat-shoc

Physical alterations to proteins, DNA

Accurate thermometry aids in determination of da

Accurate measurements ensure validity and int

kinetics data, for both thermal and thermally as
Tolerances vs. Calibration Uncer

Tolerance band: manufacturer’s guarantee that the instrument response will conform to a standard response function to within an error equal to the tolerance. 

Calibrated thermometer: may or may not have a resnominal response function for that thermometer type. Response of individual unit is reported. Expanded uncconfidence limit, or ‘coverage factor k = 2’) typically re

calibration measurement—not including drift & user reIndividually calibrated thermometers are not interchanreadouts or software are adjusted.
Typical Measurement Uncertainty 

Note that only the first item is included in the inimanufacturing tolerance

Component Method o

Calibration uncertainty or tolerance Manufacturer, calib

tolerance

Sensor drift Literature, manufac

or in situ check

Readout uncertainty Manufacturer or indTemperature stability Logging of temperaTemperature non-uniformity Move probe or use 
Thermometer Types

Standard Platinum Resistance Thermometers (SPR

(very accurate, but susceptible to shock)

−259 °C to 962 °C 

Industrial Platinum Resistance Thermometers (IPR

–196 °C to 850 °C

Thermistors 

–50 °C to 100 °C

Thermocouples 

–196 ° C to 2100 °C

Liquid-in-Glass Thermometers 

–150 °C to 400 °C

Digital Thermometers (PRT, thermistor, or thermoc

–196 °C to 850 °C
Approximate Measurement Uncertainties of Co

0.0010.0100.1001.000

10.000

-100

0 100 

Temperature, °C

Tolerance or 95% confidence 

uncertainty, °C

PRT, ASTM Class A PRT, calibraThermistor, interchangeable Thermistor, Base metal thermocouple Organic liqu
Types of Liquid-in-Glass (LiG) Ther

Types of ASTM LiG thermometers

– Over 120 types– Total Immersion: immerse thermometer nearly u– Partial Immersion: immerse a designated, fixed 

Liquids used

– Mercury (Hg) – ASTM 

(sales regulated)

– Organic– Proprietary (non-toxic)
Non-Mercury Liquid-in-Glass Therm

• Organic liquids generally have inferior perform

but are a reasonable alternative if uncertainty requmodest 

• Beware of drainage of organic liquid down cap

cooling

• “Next-generation” organic liquids under develo

ASTM standard E2251); good accuracy, but check liquid column

• Greater sensitivity to stem temperature: a fund

for partial immersion thermometers—total immersi

• Best results for use at steady operating temperat

−100 °C to +100 °C; attainable expanded uncertaiapproximately 0.5 °C to 1 °C for properly used tota
Thermocouples

Standardized combinations of Material A vs. Material B (e.g, Type K).When combined with typical readout uncertainty, total uncertainty is fairly large, unless care is taken with calibration.Good for low-accuracy measurements over a broad temperature rangeType K is a good choice for sterilization applications; typical drift of 0.3 °C over one year at 200 °C, for fixed installation

T0

Material A

Material B

Voltmeter + 

Junction 

Compensation
Soft-Insulated Thermocouple

• Fluorocarbon insulations suitable to 200 °C (• Note that method for forming junction does n

Inner polymer insu

Outer polymer insulation

Weld or solder junction by an

Mineral-Insulated, Metal-Sheathed (MIMS) 

• MIMS thermocouples are available in small diameters (• Sheath protects thermoelements from contamination & 
Effects of Pressure on Thermoco

Bare-wire thermocouples readily withstand high-p

environments

Pressure distorts atomic lattice of thermoelements

shifts in thermoelectric response.

Thermocouple measurements may be useful in 

understanding/validating models of adiabatic heat

R. Hanneman, Symposium on the Accurate Characterization Environment, Oct. 14-18, 1968, Gaithersburg, MD

Errors are small for typical pressures ~ 400 MPa

Error for type K = 

(

−2.3x10

−

6)(

Δ

t)(

p/MPa) 

Error for type S = 

(

−1.9x10

−

5)(

Δ

t)(

p/MPa

where 

Δt = temperature span of thermocouple, p = pr

e.g., Error for type K spanning 100 °C at 400 MPa 
Platinum Resistance Thermometer

Resistance element 

• Wire wound (most accurate, or 

for wide temperature range)

• Thick or thin film (rugged)

Resistance increases as a function of temperatureNominal temperature range of use:

• –200 °C to 850 °C

Nominal resistance at 0 °C

• 100 Ω for wire wound

• often higher for film

PRTs are most common “Resistance Temperature Detector” (RTD)

Sen(wir

Steel sheath containing sensor
ASTM E1137 “Off the Shelf” Tole

0.00.51.01.52.02.5

−250 0 250 

Temperature, °C

Tolerance, °C

Class AClass B

Calibrate individual units for lower un
Which Industrial PRT (IPRT) Shou

Probes vs. Bare Element: probes recommended u

permanently mounted. Note that moisture seal at 

NOT resistant to autoclaves/steam sterilization.

Film IPRTs: good time response, small size, shock 

good as wire-wound over large (>200 °C) tempera

after 365 cycles to 160 °C 

≈ 0.2 °C.

Wire-wound IPRTs with constrained coils: low ac

resistant. Drift after 365 cycles to 160 °C 

≈ 0.1 °C

Wire-wound IPRTs with slightly constrained coils

(approaching ±0.01 °C over 400 °C span), sensitibetter than constrained coils, but highly variable a

Resistor configuration

– 2-wire for non-demanding applications (±5 °C)– 3-wire for ±1 °C measurements, or ±5 °C over 

– 4-wire for all high-accuracy measurements
Thermistors (Thermal Resis

Semiconductors of ceramic material made by sintering mioxides such as manganese, nickel, cobalt, copper, iron anTemperature Range: –50 °C to 100 °CStandard Sensor Forms:bead 300 

Ω to 100 ΜΩ

probe bead in glass rodNTC: Negative Temperature Coefficient - The vast ma

thermistors used as thermometers are in the NT

Commonly packaged in stainless-steel sheathsGood choice for applications near room temperaUse only glass-coated variety: 1-year stability <0.much higher drift at higher temperatures 
What is a Digital Thermomete

Readout

Probe

Sensor inside

ASTM specifications for thermocouples, PRTs, thermistors pertain to sensor or probe only, not complete thermometer

An electronic measurement box that converts either resistance or emf of a thermometer to temperature
Digital Thermometers

• Device displays temperature directly by using t

coefficients of the thermometer

• Uncertainty: 0.001 °C to 1 °C; Resolution: 0.000

• Careful use requires careful reading

—look at ma

specifications & know the probe type and limitation

• “Digital Thermometer” only means that you can 

numbers on a display—no guarantee of better un

• Calibration can be as a system or as readout + 

System calibration is cheaper and simpleProbe and readout calibration allows easielement fails, allows identification of sourc
Traceability of Temperature Measu

A measurement will have traceability to NIST st

following conditions are met:

• An unbroken chain of measurements back to 

must be maintained.

• Each step of the chain must have known and 

uncertainties.

• There must be a system to ensure that the the

other equipment used remain accurate betwee

Common misunderstandings:• Traceability only applies to measurements, n

• Traceability does not imply a particular level 

• There is no limit to how many transfers may 

between the highest-level standards and the 

as long as the uncertainty is properly calcula
Ensuring Good Measuremen

1. Avoid shock to the sensor and readout. With metal

thermometers, damage to the sensor will not be visual

2. For thermocouples, avoid kinks in the thermocoup

regions where the temperature is changing from one pthermocouples used above 150 °C (302 °F), best unceby using a separate thermocouple for each apparatus, the apparatus.

3. For resistance thermometers, calibrate and use with 

configuration & excitation current

4. Do not switch probes unless probes are interchangea

updated.

5. Be absolutely certain that the readout is set to the 

type (e.g., do not read a type K thermocouple with a re

6. Do not exceed the recommended temperature limit7. Check the performance of the instrument regularly

manufacturer’s recommendations or past device histor
Rules of Thumb for Installation E

• Probe location: understand the 

temperature differences between the probe location and the microbes of interest as a function of time (e.g., thermal modeling, study of wall vs. product temperature)

• Avoid large air gaps around probe: still air is a v

(500x worse than stainless steel)—can lead to large measurement of temperature transients. Keep gaps 

• Design measurement geometry so at least 15 cm 

temperature of interest. (Use fine diameter probes i

• For fluids, physical state & conditions of flow ar

Still gas (e.g., gas convection suppressed in iFlowing gas (e.g., convection oven)Condensing vapor (e.g., steam in an autocla
Thermometer Verification Meth

1. Periodically have the thermometer 

recalibrated. If recalibration indicates that thermometer drift exceeds an allowable tolerance: 

• Improve handling of the thermometer • Choose thermometer with better stability, or • Shorten interval between calibrations.

2. Check the readings of the thermometer at 

the ice (0 °C, 32 °F) and/or steam point (near 100 °C, 212 °F) . The NIST Thermometry Group can provide simple procedures for preparing an ice or steam point. 

3. Compare the reading of a thermometer to 

another, recently calibrated thermometer. 
Electromagnetic Interferenc

Not a major issue for laboratory-scale experimen

designed electronics

Design:• Noise rejection can be hard to glean from manufa

specifications; specifications for digital multimeter

• Avoid long cable runs• Use twisted pair for shielding against 60 Hz noise

• If radio-frequency interference causes problems, 

grounded shield

• Use probes with sensor isolated (“ungrounded”) fDiagnosis:• While continuously monitoring temperature, shut 

electrical noise. If EMI is a problem, temperature 

change (actual temperature change will show a c
Learning More

Good general guides:

Traceable Temperatures by J. Nicholas and D. R

2001) Handbook of Temperature Measurement (Vols 1-Bentley (Springer, 1998).

Additional reference for thermocouples:

ASTM Manual on the Use of Thermocouples in TMeasurement, MNL-12 (ASTM, West Conshohoc

Modern uncertainty analysis: 

http://physics.nist.gov/cuu/Uncertainty/index.html

Methods to set calibration intervals, see:

“Guidelines for the determination of calibration int

measuring instruments,” ILAC-G24, (International Accreditation Cooperation, 2007).