How to get accurate results from thermocouple probes 1

[Jieve]

I have some very noob basic questions about temperature measurements using thermocouple probes. I recently purchased a fluke 568 and two fluke type K thermocouple probes (1x 80pk-1 bead probe, 1x 80pk-6A exposed junction probe). I am not yet using them professionally, but would like to practice getting as accurate results as possible, should that change. Applications would be gas/surface temp measurements.

For surface measurements, it's clear that for any thermocouple to report as accurate a surface temperature as possible it should be in as solid thermal contact as possible with the measured surface, and as far as possible insulated to reduce the effects of convection/radiation exchange with the surroundings. Fluke advertises the exposed junction probe as being applicable for surface temperature measurement. But as the shape of the thermocouple tip doesn't seem to be very conducive to good surface contact, I'm wondering if anyone with more experience than I could give me some tips on how to use it to get accurate surface readings. Maybe some type of thermal paste would be helpful? Not sure how convenient that would be for quick, non-permanently mounted measurements though. I assume the fact that the tip is somewhat angled coming out of the tube is to allow laying against a surface for maximum surface area contact.

I'm attaching a pic of the tip so you can see what I'm talking about. Any tips and info would be much appreciated!

 

[danw2]

Hmmmm. Low mass tip, but a rounded edge? Not very good for heat transfer.

I suggest that you Google "surface contact thermocouple" images and take a look at the huge variety that are available, most of which appear to offer better than Fluke's version. Thermocouples are not terribly expensive and given your concern about accuracy, then getting a viable measurement with a better surface probe is well worth looking into.

FYI, The advantage of a thermocouple measurement is that ANY thermocouple of the same type (K, J, T, or whatever) can be substituted for another. Most devices use commonly available miniature T/C jacks so any T/C probe with a complementary T/C plug with fit and work.

The disadvantage of thermocouple measurement is that it takes two temperature measurements - the thermocouple measurement and a cold junction (CJ) reference measurement. Many devices bury the CJ sensor so that its measurement does not reflect reality when a handheld T/C device is moved from a hot area to a cold area or vice versa. The error can momentarily be 10's of degrees until the CJ sensor equilibrates with the surrounding temperature (technically, the temperature of the input connection terminals). Be warned about dragging a handheld through different temp areas.

 

[Jieve]

Danw, great response, thanks for that. I think the fluke meters all use large cylindrical contacts connected with thermal epoxy as the cold junction at the thermocouple inputs, along with a thermistor for temperature reference. I'm aware of the temperature error due to the change in environmental temp, so I always try to allow the device to equilibrate with room temperature before taking readings.

I spent some time looking at different type k surface probes yesterday, and it seems the types with the little spring/ribbon thing at the tip (such as the omega 88006) would probably be best for surface measurements. There seems to be a big difference in price between companies, for example the surface probes sold by thermoworks were around $60, whereas similar probes from omega were around $200. Then there are the off brand chinese thermocouple probes for $30-40. I generally try to avoid generic equipment, but do you think there is a large quality difference between omega and thermoworks brands, for example? Accuracy claims are made by every manufacturer, but the only way to check if a thermocouple is operating within its accuracy spec is to check it against a known temp, easy ones might be ice or boiling water. But for the thermocouples that aren't designed to be submursable?

So I guess I can pretty much disregard fluke's claim in the manual of that probe being used for surface measurements? is that probe end type more designed for gases? I can't really seem to get consistent surface results with it due to the tip shape, but as mentioned, I dont have anything to accurately verify the temps of surfaces that I'm measuring.

Sorry if the above comes across as rambling, these were just things I've been thinking about. Any input is always appreciated.

 

[danw2]

That Omega 88006 looks good, it has a replaceable element and it comes with a mini-plug and an adapter; and it's color coded yellow for Type K.

I thought exactly what you thought about the Fluke tip, it looks like something designed for gas or low depth liquid measurement, not surface contact measurement. I wonder if that tip got put into the wrong, mislabeled baggie?

Omega makes nothing themselves (that I'm aware of); they private label everything in order to offer a huge product line. So, someone somewhere makes that probe for Omega.

I find the Thermoworks page more informative, with time constants for the separate probes. It isn't clear whether the elements are replaceable though.

And I don't know how they get a ±0.5°C accuracy spec over 0-100 Deg C, when the ANSI tables for standard limits of error is ±2.2°C. Special limit of error is half that, at 1.1°C; still twice the spec.

Handpicked wire? Could be, I guess. A little metal goes a long way with those sized probes.

It's tough to assess the relative quality without setting them next to each other. Even if the $60 probe does have a replaceable element, you can buy 3 for the price of the Omega. I'd probably give it a shot. But that's me.

 

[Jieve]

Interestingly, here is the description from the fluke thermocouple manual:

The 80PK-6A Type K Exposed Junction Probe is essentially a bead probe with a handle that can be used as a surface probe. The stainless steel sheath allows the bead to measure temperatures higher than bead probes with flexible leads. The handle also allows you to apply more focused surface pressure without having your hand in close proximity to the source. The 40-inch (1-meter) cable is terminated with a Type K miniature thermocouple connector with 0.792-mm (.312-in) pin spacing. The 80PK-6A can be used with any temperature-measuring instrument that is designed to accept Type K thermocouples and has miniature connector input.

It just doesn't seem to me like a good choice for surface measurements. The manual also says it shouldn't be immersed in liquids, and talks about using silicon paste for improving heat transfer. Go figure.

That's interesting about Omega. I also noticed that they provide very little information on their thermocouples. While the Thermoworks website provides a lot more info and they seem to have a wide range of products, those accuracy claims make me a little skeptical. Of course if Omega does not manufacture their own products, it's entirely possible that both omega and thermoworks products come out of the same factory, different prices points being for brand or better QC.

Two more quick questions:
1) How common is it for thermocouples to decalibrate over time? I also have 2 Fluke bead probe thermocouples (not the same as the probe above, the 80pk-1, both are special tolerance), the date on one package is 2008, the date on the other 2012. The 2008 consistently reads higher than the other by 1.5-2 degrees using the same meter. While barely within the accuracy spec of each other, this bothers me a little. I've read that oxides can form on the wires and affect the readings.

2) Fluke does not recommend that these probes be immersed in liquids. Is there a high risk of galvanic corrosion with these exposed bead thermocouples when immersed in water? I used one for checking swimming pool water temperature before I read the warning, and am wondering if I've inadvertently initiated this process.

Thanks again for the useful info.

 

[danw2]

>How common is it for thermocouples to decalibrate over time?

A T/C stored at room temperature is unlikely to change at all, especially a K with its nickel alloys.

Thermocouples degrade by chemistry changes at the junction. Once there's a chemistry change, one no longer has a pure two metal junction, one has a mix of metals plus whatever else is created in the pollution. Hence, the generated EMF/mV can not be directly related to the standard tables or the associated polynomials used to determine temperature values. The variation in EMF/mV from the standard table temperatures is drift.

The higher the exposure temperature , the more chemical pollution is induced. T/Cs used at higher temperatures can exhibit substantial drift.

>Is there a high risk of galvanic corrosion with these exposed bead thermocouples when immersed in water?

With the high nickel alloy content in K T/C, I'd be surprised if an immersion in swimming pool water could change the chemistry that radically, although I admit I have little exposed junction experience (heat treat base metal couples in protection tubes). Most of my experience is with MI sheathed T/Cs, where the chemistry changes occur at high temps.

I think you could use very fine abrasive paper and 'clean' off any light corrosion off the junction.

>The 2008 consistently reads higher than the other by 1.5-2 degrees using the same meter.

Do you know that one is closer to a traceable value than the other?

My experience shows T/Cs exhibit decent repeatability, until drift starts; even if not with tight accuracy.

If the 1.5-2 deg difference is repeatable, just consider that your calibration factor for the one and add-subtract the difference, if you know which is closer to a traceable value.

 

[Jieve]

Danw, thanks so much for that informative response.