Fluke 80TK Thermocouple Accuracy and Calibration - Page 1
Forum section: electronics / general

author: David Hess
author_rank: Super Contributor
timestamp: May 17, 2023, 01:26:35 am
content: I have a project to resurrect an old fudge candy recipe and have it all figured out except the temperature measurement while cooking, which is critical. In the past I used a candy thermometer and it always ended up overcooked. Most successfully in the past I used used my Tektronix DMM916 with a thermocouple bead probe and that worked pretty well. This time I tried using thermocouple immersion probes.Testing various candy thermometers revealed that they generally have poor accuracy, which makes me question their repeatability. Immersion thermocouple probes might be accurate, however quality is questionable because of design and Chinese manufacturing; I found stripped screws in the connectors, poor mechanical design, and poor placement and coupling of the thermocouple inside the capillary tube. So I am building my own immersion probes using a thermocouple with verified good accuracy and construction. The thermocouple bead is directly exposed but still shielded from mechanical contact as shown below.I decided to calibrate the Fluke 80TKs for each individual thermocouple using boiling distilled water with the temperature corrected for ambient pressure, about 0.8F where I am. Unfortunately with the bead probe directly exposed, it has a very fast time constant and it can "see" the hotter temperature of the boiling gas as it passed the bead. I am thinking of make a baffle out of stainless steel mesh to put directly below the probe during calibration.Fluke used the TLC271 operational amplifier which is a great choice given its power limitations. Its input offset voltage is corrected as part of offset calibration, coarse R5 and then fine R4, and its input offset drift is an acceptable and good for CMOS 1 microvolt per degree C ... maybe since that is the typical specification. Noise is not apparent on the output at the 100 microvolt or 0.1 degree level. I tested the three 80TKs that I have and found TLC271 input offset voltages of 0.49, 0.07, and 0.35 millivolts.What could be improved? R4 and R16 are finicky to adjust, so maybe multiturn trimmers would be better. I thought of replacing the TLC271 with an LTC1049 or OPA187 chopper stabilized amplifier, but drift only comes from local temperature variation which affects the cold junction compensation, so is not significant. The OPA187 might be lower noise, but noise is already below the 0.1 degree resolution that I am using. The biggest improvement I have identified so far is using a baffle during calibration as described above.Despite their age, the Tektronix DM501 multimeters have held their calibration well. They each have some problems, but none of their issues affect this application. The one on the left has high leakage, probably from circuit board contamination, and the one on the right skips codes. I may rebuild them at some point, but I would prefer to rebuilt a pair of DM501As, of which I have one so far.Does anybody have any thoughts?

author: David Hess
author_rank: Super Contributor
timestamp: May 24, 2023, 11:19:49 am
content: I decides to do some upgrades. The first one is that when the water is boiling, the gas bubbles touching the bead thermocouples cause temperature variation because of their fast response which makes calibration difficult. This might not be a problem with shielded thermocouples, but I have had problems with consistency when using them.So I added a stainless steel baffle about an inch off of the bottom of the pan to break up the boiling. I used #8 mesh which ended up being too large, but it still worked by creating more nucleation points. I may make another baffle with smaller mesh, or I could set an additional piece of #8 mesh directly below the thermocouples. I silver brazed 4 sewing needles to the bottom of the mesh to act as spacers.Originally the temperature variation during calibration was more than 1 degree C making calibration difficult. With the baffle, the variation during calibration is less than 0.1 degree C, which is the resolution limit of the 4-1/2 digit multimeters that I am using. Multimeter #2 skips codes, so further improvement would require a better multimeter. I am now watching for inexpensive DM501 and DM501A multimeters.After calibration this time and cooling to room temperature, both thermocouple read out within 0.1 degrees F. Before they were within 0.3 degrees F.I ordered OPA187 chopper stabilized amplifiers and 10 turn trimmers for the Fluke 80TKs for a future upgrade.

author: mendip_discovery
author_rank: Super Contributor
timestamp: May 24, 2023, 06:13:49 pm
content: K-type is inherently inaccurate anyway. Things you can do improve it is to cover the connection contacts to avoid CJC errors.There is a well publicised method for using a ice bath for the zero end of the scale.Dont go chasing the 0.01°C stuff as you will go mad doing it.You have currently at least 4 sources of error, the CJC, and the amp on the Fluke 80TK, then the thermocouple quality, and the meter you are using.For doing what you are doing I suspect you are almost at the end of diminishing return. But don't let me stop you from playing with your project.

author: David Hess
author_rank: Super Contributor
timestamp: May 24, 2023, 09:12:27 pm
content: Quote from: mendip_discovery on May 24, 2023, 06:13:49 pmThe CJC and operational amplifier offset are calibrated out by the offset calibration, so as long as the temperature of the cold junction and operational amplifier does not change much, the accuracy is unaffected.QuoteThe major error now is from non-linearity, and the 80TK has some linearity correction. The temperature I am measuring is 238F so not far from my calibration temperature of 212F limiting the error from non-linearity and gain.QuoteI already checked the accuracy of the voltmeters, and their offset is being calibrated out as well. I am using the same probe, with the same 80TK, with the same multimeter.QuoteI would settle for 0.5 degree F accuracy, but think I am doing better than that. Other thermometers I have tested are not even within 2F which is unacceptable, and have repeatability problems which is even worse.