There is nothing to stop a plug from being wired wrong.

Exactly.

 

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.

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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

I don't have direct access to the control panel on the autoclave. And I agree 100% about the workmanship issues. One problem is - new supplier of the TC wire - and pretty easy to confuse the 2 leads. One is magnetic, but I doubt that they check for that. Management issue. The system will indicate a fault, problem is, by then, the load is already running and won't be stopped.

 

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.

The system will detect these problems, however due to that system is closed, and in the run mode, they will not stop it. The loads are approx.. 80,000 lbs including rack and tooling, and so once started the only thing that stops a load is a power outage. The autoclaves are pressurized vessels, and would be a big and quite interruptive process to unload and make the correction. One saving factor is that there are typically multiple TC's on any given part. And even though the part may be end up being perfectly fine, if there is a faulty TC identified, the part is quarantined and goes through an lengthy and extensive quality check to verify it good. So adds a lot of time, cost and handling to the process.

 

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.

I'm going to try again in a few days when I'm back to the shop - on a rotating schedule due to the virus. The meter I used is pretty new but not the most expensive (Agilent), was getting +35mV either way at ambient temp (20C or about 68F). I may try one of the electrican's meters (Fluke) and see if the results are the same. I was expecting a polarity reversal, so it may be that I had wrong setting or something. Assuming what you say is correct about the DC voltage reversal, I am hoping to use that to come up with a simple detector.

 

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.

Your assessment of the situation is quite correct MisterBill. I'm going to check out what is available for a millivolt meter, as mentioned.

 

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.

I think this may well end up being the best way to go. In the past, we (actually a former colleague) had set up a bank of 3 Watlow SD31 controllers, with alarms set for various conditions. It worked, however was clumsy and required 110V power, and not very portable. For that reason there was reluctance by the technicians to use it, and it disappeared from use. It looks like we'll have to revisit this and see if something more user friendly can be put together. Ultimately hoping for a smaller and simpler device, but willing to go back to this. I thank you for your input.

 

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.

I missed that part the first time. Sorry about that. I am not infallible, nor invincible, any more. Lost it all back in 1969. Bad year, that one!
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.

I described what you are asking for in the second part of my post #18. And the good news is that you can buy it and not need to design or build it.

 

I described what you are asking for in the second part of my post #18. And the good news is that you can buy it and not need to design or build it.

Thank you MisterBill2 and all the others that answered. First time posting on here, and was please with the response.

 

Thank you MisterBill2 and all the others that answered. First time posting on here, and was please with the response.

Your initial post included enough information to allow answers that were not just guesses.

 

I occasionally use thermocouples in my work, often with a hand held single channel unit using a polarised plug to connect the thermocouple probe. I have noticed that if the plug is forced in in the wrong polarity, then the meter temperature reading decreases due to an increase in thermocouple temperature. You may be able to use this effect to easily identify miss-wired connections before starting the autoclave in earnest.

 

I occasionally use thermocouples in my work, often with a hand held single channel unit using a polarised plug to connect the thermocouple probe. I have noticed that if the plug is forced in in the wrong polarity, then the meter temperature reading decreases due to an increase in thermocouple temperature. You may be able to use this effect to easily identify miss-wired connections before starting the autoclave in earnest.

I am suspecting that there is a lot more involved with these thermocouples than we are aware of, and that the installation is not being done by electronic instrumentation folks at all. And in some shops and some areas the lighting is poor and getting the wires reversed into the plug is more likely. And not all TC extension cables are that nice bright red and white teflon material. The braided fiberglass has much less contrast.

 

I am suspecting that there is a lot more involved with these thermocouples than we are aware of

I'll buy that. How many places for something to go wrong like reversed polarity? Then as MisterBill2 points out the extension wire is not always pretty.






Next I have no idea how the extension cable(s) are attached to the thermocouples? If attached at the actual thermocouple using a TC connector that is the first place things can get reversed or even shorted.



Eventually the extension wire needs to terminate somewhere like the images I posted earlier in the thread so more plugs and sockets for things to be incorrectly wired. We would really need to check every point along the way from start at the thermocouple to finish wherever it all terminates. In my picture in a previous post at a temperature transmitter (hockey puck).

Ron

 

I'll buy that. How many places for something to go wrong like reversed polarity? Then as MisterBill2 points out the extension wire is not always pretty.
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Next I have no idea how the extension cable(s) are attached to the thermocouples? If attached at the actual thermocouple using a TC connector that is the first place things can get reversed or even shorted.

View attachment 202389

Eventually the extension wire needs to terminate somewhere like the images I posted earlier in the thread so more plugs and sockets for things to be incorrectly wired. We would really need to check every point along the way from start at the thermocouple to finish wherever it all terminates. In my picture in a previous post at a temperature transmitter (hockey puck).

Ron

Ron, you stated the points I was trying to make very well. What is interesting is that I am not aware of steel structures that need to be autoclaved, although some do need to be heated and stress-relieved. But those huge blades for wind turbines are autoclaved to fully cure the binding material. And I imagine that the reliability requirements are such that proper curing temperature must be verified. That may also be the case with large aircraft parts. And that would certainly be a process that could not be stopped and then re-started. So having all of the TCs working right would be very important. And I think that my idea of using a simple temp controller would work. At least it would be cheap enough to be worth trying.

 

This https://www.omega.com/en-us/resources/thermocouple-types reference says:

QUOTE
Also, some materials are strongly to slightly magnetic:

Type J Positive (strongly Magnetic), Type K Positive (slightly magnetic).

To determine polarity, connect the thermocouple to a voltmeter capable of measuring millivolts or microvolts and looking for increasing output when the tip is heated slightly.
End Quote

 

Ron, you stated the points I was trying to make very well. What is interesting is that I am not aware of steel structures that need to be autoclaved, although some do need to be heated and stress-relieved. But those huge blades for wind turbines are autoclaved to fully cure the binding material. And I imagine that the reliability requirements are such that proper curing temperature must be verified. That may also be the case with large aircraft parts. And that would certainly be a process that could not be stopped and then re-started. So having all of the TCs working right would be very important. And I think that my idea of using a simple temp controller would work. At least it would be cheap enough to be worth trying.

The reference to Auto-Clave always piques my attention or trips my trigger. I worked with quite a few large claves. Here is an example of me checking the spring tension on some custom made heating elements.


I actually had brown hair so guess it was awhile back. The pictured clave vessle has a 24" ID and 3" thick 316 stainless steel walls so a 30" OD. The heater bands are 240 Volt 6 KW per half so 12 KW per band. Top to bottom this clave has 72 bands so calling for full heat about 865 KW. We ran Grade A deionized water through the claves at a high temperature. We placed a 3,000 PSI nitrogen blanket in the surge tanks. Once a hot clave test began you really did not want to terminate it before the test was complete. Some yellow nylon jacket Type K extension wire can be seen exiting a conduit on the left. The clave will eventually have large insulated covers blanket what you see here. Thermocouples used for measuring a few points of skin temp were wired using high temperature Glass on Glass extension wire. TC junction boxes were placed on the cover walls.

We fed the power in using again glass on glass insulated AWG 8 wire and linked the elements as seen here.


The little ceramic caps were a nice touch. Two 240 VAC halves in series. Part of making sure every step of the way is done right is once the thing is all together you really don't want to take it apart for several years.



All in all the clave pictured is about 40 feet top to bottom.

Less any detail the product we built and tested were CRDM (Control Rod Drive Mechanisms) as used on US war ships, The carriers and subs which today are all nuclear powered. Nothing I posted here is of a confidential or classified nature. The work we interesting and I enjoyed it. We could run as many as six of these things once the new power sub station went in a few years before I retired. No shortage of temperatures, pressures, flow rates and other things to measure and record.

Ron

 

OK, Ron.I had not thought about reactor control parts being run in an autoclave, that is another example of an area where it is rather important to get everything right the first time. And a great example of why identifying wires at assembly is not the solution for the assembly error problem. And I am sure that the data acquisition system was a rather complex package indeed.

 

OK, Ron.I had not thought about reactor control parts being run in an autoclave, that is another example of an area where it is rather important to get everything right the first time. And a great example of why identifying wires at assembly is not the solution for the assembly error problem. And I am sure that the data acquisition system was a rather complex package indeed.

A very sinking feeling to look up from my desk in my nice little office and see one of the control rod technicians as they say, Ron, we have a problem with clave #? when I know that clave has a hot test running.

Ron