Reverse Thermocouple Polarity Detector?

Thread Starter

Brad999

Joined Mar 24, 2020
10
I'm in the Mech Engineering field, so asking for a little guidance here.

I'm trying to see how I can detect reverse polarity on thermocouples. In this situation there will be a load going into an autoclave that may have well in excess of 100 thermocouples in play. Understandably, they should all be wired correctly... However, we're having situations where they are not, and you can't just stop the load autoclave cycle because there is a bad reading somewhere. When mounted to the tooling, the TC's are wired into a receptacle that is bolted to the tool, and it is not possible to view the wire colors or use other normal means of checking polarity.

Does anyone have knowledge of something that would work for this? A device, a circuit or whatever? Is it possible to use something like an AD594 amplifier (Analog Digital) or maybe a pair of these, and have the TC wired to both, 1 correctly wired, the other in reverse? I tried looking at the voltage generated using just a DMM and then reversing the leads, expecting the voltage to have reversed polarity. Maybe it was just my DMM, but both readings were the same magnitude and polarity, not what I expected - at least regarding the polarity. So this idea might be off the table...

I would like to have something that is portable, preferably battery powered (LED and buzzer indicators), so that the technicians can crawl in and around the load and plug into the TC receptacles to check for reversals, prior to loading, closing and running the autoclave. Not looking to measure anything, just a simple indicator. Would also be looking for this to detect opens (found a video on the AD website that seems to cover this OK, should work with the AD594) and shorts to the metal tools that the TC's are mounted to. (TC's used here are ungrounded J Type).

Thank you for anyone that may have something to offer.

Brad
 

KeithWalker

Joined Jul 10, 2017
794
My thermocouple plugs are polarized just like wall plugs can be. The wider blade is the negative (Type K).
This is the obvious solution to your problem. If the thermocouple receptacles are polarised and the thermocouples with plugs attached are bench tested before they are used, you will eliminate the problem.
There must have been something wrong with the way you measured you thermocouples if they indicated the same polarity whichever way they were connected. They generate a DC Voltage and the polarity will change if the connections are reversed. Maybe your meter was on AC Volts range.
Keith.
 

panic mode

Joined Oct 10, 2011
1,829
just reverse probles of multimeter (connect red one to COM) and 99% of people will not notice the issue and will wire things wrong and swear they tested it.
 

jpanhalt

Joined Jan 18, 2008
9,043
just reverse probles of multimeter (connect red one to COM) and 99% of people will not notice the issue and will wire things wrong and swear they tested it.
???? The red one on thermocouples (K-type) is the negative. Maybe the author will take the time to tell us what type of TC is being used and what type of connector?
 

MisterBill2

Joined Jan 23, 2018
5,741
It was not mentioned about if those thermocouples are the grounded style or not, which makes a difference in checking them.
Given that the voltage from a thermocouple is just a few millivolts, iit would be a serious challenge to read that voltage on a regular meter, even a digital multimeter. The simple method to verify the correct polarity is to have an accurate meter to check that the voltage from each TC is similar. Using a hot air blower to heat them a bit will increase the output and make reversed polarities more obvious.
The similar readings and lack of a polarity change in the check described by the TS indicate that the check process was not valid, probably due to noise from some place in the hookup.
 

Reloadron

Joined Jan 15, 2015
5,337
The trick if there is one is to make sure that when the thermocouples are wired whoever wires them gets it correct. That is a matter of a discipline problem not electrical or electronic in nature. Using a K type as an example with extension wire the Red is always negative and Yellow positive. Also in the case of K the negative is more magnetic than the positive. Comes in handy when working with bare wire.

A good indicator is when as the clave heats up temperatures begin to drop below ambient. I don't know how your auto-clave is setup but here is how I did it.

C2 Cab Inside.jpg

On the bottom I bring in about 25 TC pairs using shielded TC wire. The plugs and sockets are the larger style K connectors. These all go to Temp Transmitters giving me 4-20 mA for a programmed range. They also indicate if a thermocouple is incorrect polarity wired. Coincidentally these temps are taken on an auto clave control system. Thermocouples incorrectly wired in my book are a matter of poor work habits on the part of someone who failed to pay attention to or did not understand the task at hand. Neither of which is acceptable.

Ron
 

jpanhalt

Joined Jan 18, 2008
9,043
It was not mentioned about if those thermocouples are the grounded style or not, which makes a difference in checking them.
Given that the voltage from a thermocouple is just a few millivolts, iit would be a serious challenge to read that voltage on a regular meter, even a digital multimeter. The simple method to verify the correct polarity is to have an accurate meter to check that the voltage from each TC is similar. Using a hot air blower to heat them a bit will increase the output and make reversed polarities more obvious.
The similar readings and lack of a polarity change in the check described by the TS indicate that the check process was not valid, probably due to noise from some place in the hookup.
Since we don'r know what thermocouple is being used, it seems premature to talk about simple tests to identify each component. Alumel and iron are magnetic; chomel and constantan are not. Polarity of thermocouples using those common alloys can be easily identified with a magnet. No need for a sensitive voltmeter.
 

MisterBill2

Joined Jan 23, 2018
5,741
Since we don'r know what thermocouple is being used, it seems premature to talk about simple tests to identify each component. Alumel and iron are magnetic; chomel and constantan are not. Polarity of thermocouples using those common alloys can be easily identified with a magnet. No need for a sensitive voltmeter.
Go back and read post #1 and it will be clear why your statement is not at all related to the issue. The problem is not needing to check the polarity of unknown thermocouples, it is to detect wiring errors in large arrays on some sort of complex assembly prior to the launch of some heating process.My guess is that it is a large epoxy composite structure being cured in an autoclave. And in such a setup the extension wires are similar to the TC material.
Thus my suggestion to measure the magnitude and polarity of each with a noise resistant millivolt meter. Really, though, an equivalent digital voltmeter with a digital setpoint comparator will also work and provide an accept/reject output.
 
I used to use the finger test. Watch the temp increase when you heat the TC with your fingers.

Extension wire complicates things. A shorted thermocouple reads room temperature and there is no real temperature difference between the CJC and the measuring thermocouple.

I think you have two choices. Raise the thermocouple temperature or raise the CJC temperature.

The meter has a way to measure the temperature of the terminals. Some do it very poorly. Heating the real thermocouple makes the temperature go up. heating just the sensor on the measuring device without heating the terminals should elicit a predictable response.

A lot of times, it's a thermister, replacing that resistor inside the instrumentation with a fixed resistor or paralleling a resistor would also work.

So, heat the CJC sensor or heat the Thermocouple with a heat gun.

Here http://www.ti.com/lit/ug/tidu449b/tidu449b.pdf on page 1, there is a PT100 RTD. If you can heat that or change what it reads, you can get an instant check.
 

ebeowulf17

Joined Aug 12, 2014
3,274
Is it possible to use something like an AD594 amplifier (Analog Digital) or maybe a pair of these, and have the TC wired to both, 1 correctly wired, the other in reverse? I tried looking at the voltage generated using just a DMM and then reversing the leads, expecting the voltage to have reversed polarity. Maybe it was just my DMM, but both readings were the same magnitude and polarity, not what I expected - at least regarding the polarity. So this idea might be off the table...
Correct me if I'm wrong - it sounds like you were reading the voltage of the AD594 output in this little experiment, not the output of the raw thermocouple, as others have implied.

The AD594 has built in cold junction compensation. If both ends of the thermocouple are at a similar temperature, then the voltage level of the compensation voltage will dwarf the near-zero thermocouple reading, rendering polarity meaningless.

Backing up a step, do you understand how thermocouples work? It's a little counterintuitive, and quite different from any other temperature sensing system I'm familiar with. The thermocouple itself creates a voltage that is proportional to the difference in temperature between its two ends. This voltage has nothing to do with the absolute temperature of either end, only the difference between them. For example, a thermocouple with the probe end at 75F and the meter end at 50F will deliver the same voltage as one whose probe end is at 125F and whose meter end is at 100F. The same 25 degree difference delivers the same thermocouple voltage output, despite the two systems being at very different temperatures.

All thermocouple measuring systems must have a second temperature measuring system of some sort (thermistor, RTD, etc.) to measure the absolute temperature of the meter end of the thermocouple and then add a voltage corresponding to that temperature to the voltage from the thermocouple.

Back to your test, if both ends of the probe are around the same temperature, the thermocouple output is near zero, so the entire voltage reading from the AD594 output is the voltage corresponding to the second sensor (the built in one used for cold junction compensation.)
 

ebeowulf17

Joined Aug 12, 2014
3,274
As for a simple electronic solution for polarity checking, I can't think of one. If mechanical or visual checks aren't possible, then all I can think of is looking at change in voltage vs. change in temperature. You have to create a significant temperature difference between the two ends of the thermocouple in order to check polarity based on readings. You can't do it with the whole system idle, nor in an unknown state.

If there are any "brains" to this system, I'd consider programming in a dedicated test cycle, or maybe just building error checking into the normal operation cycle.

To begin test, system records temperature reading from all probes. Then system begins applying heat, wait some predefined amount of time, enough to be totally sure that all probes should be significantly warmer than before, well beyond any noise readings or normal temperature fluctuations. Now record all temperatures again. Compare the results - all numbers should've increased, and any numbers that dropped the same amount that they should've climbed have reversed polarity.
 

MisterBill2

Joined Jan 23, 2018
5,741
OK, now here is the technically simple way to check the thermocouples for opens and reversed polarity. Use a single channel thermocouple temperature controller device that has both heating and cooling ON/OFF outputs. Add indicator lights to show when it is requesting heating or cooling.
Start with one TC that you know is correct, and connect it to the controller. With the controller powered and stabilized, note the indicated temperature, which should be close to the ambient in the area. Set the heating limit as close as possible below the indicated temperature, and set the cooling limit as close as possible above the indicated temperature. Neither indicator should be on. Now reverse the TC connections and see that one indicator comes on, and open the connection and an indicator should come on. Next, with the local connections correct, verify that this also works with the connections reversed at the TC end. So now you have an easy to duplicate reversed polarity detector, well made and easy to reproduce. And it did not take a lot of engineering time.
 

Thread Starter

Brad999

Joined Mar 24, 2020
10
It was not mentioned about if those thermocouples are the grounded style or not, which makes a difference in checking them.
Given that the voltage from a thermocouple is just a few millivolts, iit would be a serious challenge to read that voltage on a regular meter, even a digital multimeter. The simple method to verify the correct polarity is to have an accurate meter to check that the voltage from each TC is similar. Using a hot air blower to heat them a bit will increase the output and make reversed polarities more obvious.
The similar readings and lack of a polarity change in the check described by the TS indicate that the check process was not valid, probably due to noise from some place in the hookup.
I think I did mention that they are ungrounded. The bare welded tip is covered in silicone and cured, and then bonded to the tool surface with silicone and a silicone pad to cover it. Problem with trying to heat the tool is - some of these tools weigh 10000 lbs, and are large - difficult to reach where some of these TC's are located, and lots of thermal mass. In many the egg-crating is nearly solid and would be next to impossible to reach where the TC's are. Can't do it from the top at all, as normally there is a part mounted on the tool-side surface, making the zone where the TC's are mounted impossible to affect with heat from a gun, nevermind that applying any heat at this point would start a premature reaction in the composite, and would have to throw it away.

As far as reading the voltage, it was simply to see if the voltage was opposite polarity that coincided with the reversed leads. I was hoping that a reversed voltage could be used to trigger an LED or something thru an amp cct.
 

Thread Starter

Brad999

Joined Mar 24, 2020
10
Correct me if I'm wrong - it sounds like you were reading the voltage of the AD594 output in this little experiment, not the output of the raw thermocouple, as others have implied.

The AD594 has built in cold junction compensation. If both ends of the thermocouple are at a similar temperature, then the voltage level of the compensation voltage will dwarf the near-zero thermocouple reading, rendering polarity meaningless.

Backing up a step, do you understand how thermocouples work? It's a little counterintuitive, and quite different from any other temperature sensing system I'm familiar with. The thermocouple itself creates a voltage that is proportional to the difference in temperature between its two ends. This voltage has nothing to do with the absolute temperature of either end, only the difference between them. For example, a thermocouple with the probe end at 75F and the meter end at 50F will deliver the same voltage as one whose probe end is at 125F and whose meter end is at 100F. The same 25 degree difference delivers the same thermocouple voltage output, despite the two systems being at very different temperatures.

All thermocouple measuring systems must have a second temperature measuring system of some sort (thermistor, RTD, etc.) to measure the absolute temperature of the meter end of the thermocouple and then add a voltage corresponding to that temperature to the voltage from the thermocouple.

Back to your test, if both ends of the probe are around the same temperature, the thermocouple output is near zero, so the entire voltage reading from the AD594 output is the voltage corresponding to the second sensor (the built in one used for cold junction compensation.)
Was reading raw voltage. I don't have anything yet in terms of devices or components to proceed. Was simply trying to see if reversing the leads would cause a reverse voltage polarity. The reading was the same with both, whereas was expecting the meter to read neg V for the lead reversal.
 
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