Changes of temperature coefficients in time for Fluke 742A-10k  - Page 1
Forum section: electronics / general

author: Alex Nikitin
author_rank: Super Contributor
timestamp: June 28, 2024, 10:24:00 pm
content: I have a well aged (=old) Fluke 742A-10k Resistance Standard, with the original measured values of resistance at 23C at 9.999996K, and temperature coefficients Alpha = 0.120 E-6 and Beta = -0.049 E-6. I did measure the value at ~23C as 10.00006K with a freshly calibrated HP3458A a few years ago. However up until recently I couldn't measure the temperature coefficients as these are quite small. I also been told in the past by people from Keysight calibration lab that the stated Alpha and Beta on the 742A should be still valid after all these years (30+ perhaps). Now I have the HIOKI DM-7275 7.5 digit voltmeter and the HP3245A as a reasonably stable current source, so I have tried to measure the tempco with my reptile box "temperature chamber". The result was somewhat surprising. As you may see from the graph, measured Alpha is about +0.027E-6 and measured Beta about -0.016E-6, about 4 and 3 times better than stated on the unit.CheersAlexP.S. - the complete measurement cycle took about 8 hours, the current was set at 1mA so the measured voltage was about 10V. Thermal offsets were less than 1uV.

author: KT88
author_rank: Frequent Contributor
timestamp: June 29, 2024, 11:38:24 am
content: This deviation coincides with 3 sigma (68,3%)…just a hunch.

author: Kleinstein
author_rank: Super Contributor
timestamp: June 29, 2024, 01:32:32 pm
content: The temperture dependence is not a perfect parabola. The calculated alpha and beta values thus depend on the temperature range used for the test. The 15 -30 C range used here looks reasonable for later use, but the original values may be from a different range.

author: Alex Nikitin
author_rank: Super Contributor
timestamp: June 29, 2024, 01:40:23 pm
content: Quote from: Kleinstein on June 29, 2024, 01:32:32 pmI've received an email from one of the forum members, apparently the 742A has a very long thermal time constant and for 1 degree C change it needs 5-6 hours time to settle properly (so for the required 5C change to determine Alpha and Beta coefficients according to Fluke manual it needs at least 25-30 hours at each of 18C, 23C and 28C temperature points), and the "reduced" tempco I've observed is not real but the effect of the long time constants, as an 8 hours cycle is far too fast for this unit. I will at some point repeat the measurements on a longer time scale using 3 stable temperature points only.Quote from: Kleinstein on June 29, 2024, 01:32:32 pmHere is the quote from the manual for 742A:QuoteCheersAlex

author: Andreas
author_rank: Super Contributor
timestamp: June 30, 2024, 09:01:19 pm
content: Quote from: Alex Nikitin on June 29, 2024, 01:40:23 pmHello,strange: usually I get a hysteresis slope for a complete temperature cycle in this case.with best regardsAndreas

author: Alex Nikitin
author_rank: Super Contributor
timestamp: July 05, 2024, 03:35:41 pm
content: An update: I've tried to do a long time (days) measurements at three temperature points. The main problem is that the HP3245A stability as a current source is not good enough, the temperature coefficient is too high, with up to +/-1ppm variations over a 24h period due to the room temperature change by about 2-3 degrees C and an occasional 0.7-0.8ppm "jump" . The good news is that the temperature stability of my reptile box is very good, with variations of the 742-10k case temperature +/-0.05 C. I can confirm that the time needed for the 742 to settle after a 5C change in the case temperature is close to 24h, however my equipment is not good enough to trace a fraction of a ppm change over a 24h period reliably and accurately. I need to build a better current source for 1mA! On the graph below is the resistance variations plotted according to the original Alpha and Beta, as you may see the p-p value is less than 2ppm for 18C to 28C range, so I need a current source with the stability of at least +/-0.1ppm over 24 hours! Or another 742A-10K.CheersAlex

author: Kleinstein
author_rank: Super Contributor
timestamp: July 06, 2024, 06:34:41 am
content: A stable current source would be a difficult way, as one would include the votlage reference at the voltmeter and usually also a voltage reference inside the current source. The best reference to compare to is another stable 10 K resistor and than use some kind of bridge or similar circuit to compare the resistors.

author: guenthert
author_rank: Frequent Contributor
timestamp: July 06, 2024, 09:57:33 am
content: Quote from: Alex Nikitin on July 05, 2024, 03:35:41 pmNah, I don't think 'nother 742A-10k will be necessary, just one resistor with sufficientshorttime stability (I'd think, most precision WW or metal foil resistors would do) and a 2nd oil/air bath or temperature chamber. See .https://www.researchgate.net/publication/337693822_DETERMINATION_OF_THE_STANDARD_RESISTOR_TEMPERATURE_COEFFICIENTS_AND_THEIR_UNCERTAINTIES/fulltext/5de5c724299bf10bc33a7422/DETERMINATION-OF-THE-STANDARD-RESISTOR-TEMPERATURE-COEFFICIENTS-AND-THEIR-UNCERTAINTIES.pdf