Calibrating the temp measurement of a DMM? - Page 1
Forum section: electronics / general

author: n3mmr
author_rank: Regular Contributor
timestamp: August 23, 2020, 11:10:42 am
content: Is there a reasonably simple and not too costly way to calibrate the temp measurement of a DMM?I'd like it to be known to be accurate to <±0,5 °C for measurement of air temperatures around 30 - 50 °C and around 200°C in small enclosed spaces such as an oven.

author: n3mmr
author_rank: Regular Contributor
timestamp: August 23, 2020, 01:00:03 pm
content: Just to clarify: i understand fully well how to calibrate at 0°C and 100°C. I need to understand how to know the actual accuracy at around 50°C and at around 200°C and then in dry air, not in concentrated steam or liquid water.

author: ap
author_rank: Frequent Contributor
timestamp: August 23, 2020, 01:16:49 pm
content: Gallium triple point cells are used for the temp around 30C. Lots of information available on the web. You can pretty easily build one yourself if you only need 0.5K uncertainty.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 23, 2020, 01:19:20 pm
content: The temperature calibration usually uses a few fix points (like the 0 C and 100 C) and than interpolation according to the sensor curve. So the accuracy in between depends a lot on the sensors. The calibration would be for the sensor, not so much the DMM.The usual ones are type K thermocouples, NTCs and RTDs (e.g PT1000). Thermocouples are tricky to get +-0.5 K accuracy. There are quite accurate NTCs - but there are also poor ones and the curves may be slightly different between types / brands.PT1000/PT100 or similar can be quite accurate also in between the fix points. So if 0 C and 100 C are OK it should also be quite good in-between and a little beyond 100 C.PT100 may need a 4 wire connection.A calibration in between the fixed points and also for lower demand usually uses a platinum thermometer as a reference.There is an additional fixed point at some 39 C from gallium. This can be handy for medical temperatures. AFAIK the next higher common fix point is the melting temperature of tin.

author: TUMEMBER
author_rank: Regular Contributor
timestamp: August 23, 2020, 03:20:20 pm
content: Quote from: Kleinstein on August 23, 2020, 01:19:20 pmWrong, gallium it's29,76indium 156,60https://en.wikipedia.org/wiki/Triple_pointvery stable, very expensive can be replaced by a mixture of ice and distilled water.https://www.measurement.govt.nz/resources/#collapse-control-1-5

author: tszaboo
author_rank: Super Contributor
timestamp: August 23, 2020, 03:52:13 pm
content: Which type of measurement?PT100? Calibrate resistance range.Thermocouple? Calibrate 100mV rangeNTC? Throw out NTC and use something else.Built in thermometer? Forget it.

author: 2N3055
author_rank: Super Contributor
timestamp: August 23, 2020, 04:47:31 pm
content: What sensor, and what multimeter?0.5 C absolute accuracy with some stability with thermocouples is doable but not trivial, for instance..

author: Gyro
author_rank: Super Contributor
timestamp: August 23, 2020, 05:53:15 pm
content: Quote from: TUMEMBER on August 23, 2020, 03:20:20 pm29.7646'C - A useful Fluke reference (pun intended)...https://us.flukecal.com/products/temperature-calibration/its-90-temperature-standards/its-90-fixed-point-cells/its-90-fixed-?quicktabs_product_details=2I wonder how much they cost.

author: ejeffrey
author_rank: Super Contributor
timestamp: August 23, 2020, 06:17:14 pm
content: Quote from: n3mmr on August 23, 2020, 01:00:03 pmThe reason that calibrations are often done with water or steam is that the heat transfer is good enough to make sure the thermometer is at the same temperature as the fluid which is mixed well enough to be uniform temperature. that's all you can really ask out of calibration -- that the measurement result accurately reflects the temperature of the sensor itself. In air, especially still air or passively convecting air the temperature variation will be more than the calibration error and poor thermal contact to the sensor will result in heat conduction along the leads changing the sensors temperature. These are not calibration errors they are measurement errors, and they are likely to dominate your accuracy.

author: HighVoltage
author_rank: Super Contributor
timestamp: August 23, 2020, 07:48:20 pm
content: I have a calibrated Fluke Secondary Reference PRT.- The calibrated accuracy is ± 0.011 °C at 0 °C- The short-term repeatability is better than ± 0.010 °C (± 0.004 °C is typical)- The long term drift is ± 0.007 °C at the triple point of water when exposed up to its maximum temperature (420 °C) for 100 hoursI am using a jar filled with oil and use the Fluke PRT together with the Keithley DMM7510 or an Agilent 34420A as a reference.And then compare other probes against this one.

author: nfmax
author_rank: Super Contributor
timestamp: August 23, 2020, 08:56:48 pm
content: There are three tasks here:Calibrate the combination of the temperature sensing element + DMM at a set of temperatures over the operating range, using either a stirred liquid bath or a dry-block calibrator, and estimate the calibration uncertainty at these temperatures,Construct (or look up) an appropriate interpolation function between the calibration points, and estimate the additional uncertainty at intermediate temperatures arising from the interpolation,Measure, calculate, or estimate the effects of using the thermometer in air as opposed to the calibration medium(s) - basically the effects of self-heating and lead conduction.For ±0.5˚C uncertainty the use of water, gallium or tin fixed points is unnecessary. As @HighVoltage suggests, calibration using a reference thermometer of known uncertainty is the way to go. You can use a combination of a water bath and a dry block calibrator, or an oil bath: in both cases the aim is to get intimate thermal contact of both the UUT and the reference thermometer to the heated medium. Only a few temperatures need to be measured, covering the operating range.The interpolation function represents the 'ideal' sensor response, based on known physics. For example, a PRT over this temperature range can be modelled using the Callendar polynomial, whereas a thermistor could be modelled using the Steinhart & Hart function. Given the interpolation function, its sensitivity to each of its parameters, and the uncertainty at each calibrated temperature, a curve of uncertainty versus temperature can be produced.Finally, the effect of lead conduction and/or self-heating can be directly measured, and either compensated for or made negligible (e.g. by choice of measuring current, immersion depth, and disposition of leads). For some senors, the dissipation constant in different media may be provided by the manufacturer.You don't say what type of sensor you are planning to use. Be warned that achieving ±0.5˚C uncertainty with a thermocouple is difficult. My first choice of sensor would be a PRT, measured using a 4-wire configuration. Note that an IEC-751 class A PRT has an initial tolerance of ±0.5˚ at 200˚C without calibration: if the DMM could measure resistance 'perfectly' no additional calibration would be needed. A glass-encapsulated bead thermistor would also be suitable, although finding one able to reach 200˚C would be tricky.The best reference I know of is:J. V. Nicholas and D. R. White,Traceable temperatures: an introduction to temperature measurement and calibration, 2nd ed. Chichester ; New York: Wiley, 2001. ISBN 978-0-471-49291-7It's not cheap: however I got lucky and found a remaindered copy on eBay for only £36.Rod White is also one of the authors of the BIPM thermistors guide:D. R. White, K. Hill, D. del Campo, and C. Garcia Izquierdo,Guide on Secondary Thermometry - Thermistor Thermometry, BIPM, Sèvres, Temperature Measurement Guide, Aug. 2014. Available at:https://www.bipm.org/utils/common/pdf/ITS-90/Guide-SecTh-Thermistor-Thermometry.pdf.This has worked examples of calculating the uncertainty arising from interpolation.