Thermocouple noise

theprocessguy

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Nov 2013
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I read here a lot but this is my first post. I have an ungrounded type K T/C that is inside a stainless sheath. It is about 1M long and inserted into an electric heating element to monitor internal heater temp. The T/C is literally 1cm from the heating element conductors. The heater is driven by a VFD which as you knows is a very noisy animal. As the voltage is ramped to increase heat, I get a corresponding immediate rise in temperature. Too fast to be a real temp increase. An offset to the tune of 30-50C. As soon as I stop the drive the temperature immediately drops back to normal. The temperature reading itself is also noisy while the drive is running. The T/C is wired directly to an AB Flex I/O module. I have tried grounding the sheath. I have tried grounding one of the T/C pins. I have connected the drain wire, module end only, to an isolated "clean" earth. Someone mentioned removing the grounding spring on the bottom of the Flex terminal base to eliminate the earthing there but I have not done that. I have read about placing a small capacitor across the T/C connection to filter noise but I don't think this would solve my offset issue.
The only thing I have not tried is to use a 4-20 transmitter head. I have these unplugged for now as they seem to also affect my other process T/Cs where I must have a good reading.
Does anyone have an experience with this or any suggestions.

Out of ideas
 
Move the thermocouple.

Seriously, you are inducing a voltage onto your TC, and unless you can somehow adequately shield it from the heater element, you will continue to read this induced voltage. A transmitter installed locally in the field will probably only exacerbate the problem, since TC transmitters tend to be vulnerable to radio interference from walkie-talkies. Took me a while to figure out what was going on the first time I encountered that problem.

You might be better off with an RTD, since it measures resistance, rather than voltage, but I suspect you'd still see the noise if the temperature sensor has to be in close proximity to the heater element. Installing an inductor on the VFD output might help also.

I don't understand why you require variable frequency for a heater. A simple variable voltage drive should give you adequate control for a heater element.
 
A VFD is the wrong type of drive for a heater. You need an SCR controller.
If that's not possible, try adjusting frequency to maximum and minimum. That will verify noise from the VFD, and minimize it.
Wind some T/C wire around a toroid. Use one with high permeability.
Here's one that's 2.4" OD. A little big, but a good start. Wind a single layer. At least half the ring if you can.
 
Perhaps you could add line reactors on the input and output of the VFD. I dont know if it will help or not, just a suggestion.
 
You need to have a truly isolated input. Not sure that the Flex I/O can do that. There needs to be channel-to-channel isolation. I have successfully measured temperature using a thermocouple that had over 100 volts riding on top of it(induced from heater).
 
Great feedback guys. VFD is a customer spec that I am stuck with. I have a line reactor on the input and a transformer on the output to step my voltage down. Not sure whether the transformer acts the same as a line reactor. Heaters are 90V/70A. My Powerflex 700 drive is in adjustable voltage mode and is fixed at 60Hz so it simply changes the amplitude of my output. It needs to run at 60Hz b/c I have a 60Hz transformer. I have tried all of the VFD carrier frequencies and it does behave better on one than the others. Not sure I can use RTD as my heater gets to 1100C. Spectrum Controls does have an isolated input module I could try. I like the toroid idea too.
I have not tried to plug the T/C into a handheld to see if it sees the same offset. I will try it.

Thanks
 
isolated input or indeed a isolated transmitter (same object)
cheaper is isolate with a metal grounded shield (normally the cable, and have a twist in the wires,
 
theprocessguy said:
I read here a lot but this is my first post. I have an ungrounded type K T/C that is inside a stainless sheath. It is about 1M long and inserted into an electric heating element to monitor internal heater temp. The T/C is literally 1cm from the heating element conductors. The heater is driven by a VFD which as you knows is a very noisy animal. As the voltage is ramped to increase heat, I get a corresponding immediate rise in temperature. Too fast to be a real temp increase. An offset to the tune of 30-50C. As soon as I stop the drive the temperature immediately drops back to normal. The temperature reading itself is also noisy while the drive is running. The T/C is wired directly to an AB Flex I/O module. I have tried grounding the sheath. I have tried grounding one of the T/C pins. I have connected the drain wire, module end only, to an isolated "clean" earth. Someone mentioned removing the grounding spring on the bottom of the Flex terminal base to eliminate the earthing there but I have not done that. I have read about placing a small capacitor across the T/C connection to filter noise but I don't think this would solve my offset issue.
The only thing I have not tried is to use a 4-20 transmitter head. I have these unplugged for now as they seem to also affect my other process T/Cs where I must have a good reading.
Does anyone have an experience with this or any suggestions.

Out of ideas
Click to expand...

I don't know how quickly your temperature is rising, but if the T/C is 1 cm from the elements, you *could* be seeing a quick rise in temperature. Do you really want to measure the element temperature or are you trying to measure the air temperature in the area? If it's the air, them move the T/C. Otherwise, while you could be getting noise on the T/C, I'd be thinking the temp is rising pretty quickly too.

I won't bother entering into the electrical noise discussion, there are enough opinions already.
 
I'll bet that the extension wire is parallel wires, not twisted. You need to get or make twisted pair as close to the transition out of the RTD sheath as possible.

I agree with Shooter - twist the extension wires and use an isolated transmitter.

An RTD will not solve the problem because the RTD signal that the analog input reads is a voltage signal, the voltage (IR) drop of a constant current source through the RTD's resistance.

FYI, zero cross SCR controllers (that are conventionally used to fire resistive heater elements) produce orders of magnitude less noise than the SCR phase angle controllers. I'm not sure what mode VFD runs in but it wouldn't surprise me if it chops the waveform.
 
The objective is to prevent overheating the heater element. This immersion heater is inserted into a protective tube therefore limiting my T/C position to where it is now.

I do have a short run of 1-3M from the T/C to a junction box that uses high temp (fiberglass) unshielded untwisted exetension wire. I can try replacing this section with shielded and/or a twisted type of wire.

First I want to try plugging a handheld directly into the T/C to see if I still get the instant rise/fall of this offset.
 
You can buy shielded thermocouple cable.
The head mounted transmitter will probably cause problems with other analogue readings unless it is an isolated type - non isolated loop powered head mounted transmitters are to be avoided at all costs.
If you get an isolated loop powered transmitter make sure it is of the 4-20ma type - pretty immune to induced voltage as it works on current.
Make sure you use screened cable too - only ground at one end. If you have your 24VDC or 24VAC power supply with the neutral/negative grounded that can cause problems too. Use a switch mode power supply for 24VDC or double wound 24VAC transformer for 24VAC and do not ground the secondary - let it float.
 
You probably have a high common mode voltage being induced onto your thermocouple. I know that AB TC modules supposedly have common mode voltage rejection but IME its poor at best. I had similar difficulties with common mode voltages and no AB product could filter it. I suggest you put in a transmitter - one that has excellent common mode voltage rejection. Try Dataforth. I had to try several different brands before finding one that could handle what we were throwing at it.

It would also be well worth your time to help the customer understand the difference between a drive and an SCR power controller and why the latter is a better choice for control.
 
I still think twisting any of the extension cable is critical, even if you do it by hand (secure the transition end of the cable in a vise, so that the transition is not stressed by the twisting. Most T/C cable is not twisted, it's parallel.

If you already have one of those transmitters, you probably have the PC configuration program (and presumably a cable). Something to consider because sometimes the cost of the software/cable is free, 1x the cost of the transmitter or 5x the cost of the transmitter.

None of the approvals relate to the issue at hand.

But if you have one avaiable, try it and see.
 

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