Electrical noise from VFD affecting Temperature Probe 2

What signal type is the probe? Thermocouple, mV/V, 0-5V, 0-10V, 4-20mA, PWM? A more robust signal type will help eliminate noise. Otherwise, if you can identify the frequencies of the noise, a filter can be applied.

If the problem truly only occurs when probe is in water, then maybe you are using a grounded tip thermocouple and you have a ground somewhere else in the circuit... and the water is conducting?

You can get ungrounded tip thermocouples. They have a slower response time.

RTD tends to be less susceptible to EMI than T/C in general. It's not clear your problem is necessarily related to EMI.

Just my thoughts fwiw. I'm not a big instrument guy.

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(2B)+(2B)' ?

Electricpete touches on my first thought - is this a grounded tip thermocouple? In this case it may be a few microamps of stray parasitic current.

My second thought is that ferrite rings for your VFD will probably not work. Most magnetic materials saturate easily with current.

My third though is to use a ferrite on your probe. A toroid with ID sufficient to pass the probe wire through 3 or 4 times. That may deter the EMI signal from traveling on the probe cable. Possibly quick and easy.

With it in the water stop the VFD and see if the noise stops. I've heard nothing that shows they're related yet.

Keith Cress
kcress -

To answer some of the questions,

The temp probe has an Rj11 connection and I believe its a thermocouple

I have ordered a ferrite ring to try on the probe

If I turn the VFD off the noise stops

Not sure if this information helps, but the water is saltwater not fresh.

It sounds like a conduction issue to me as well. As mentioned already, a setup that insulates the probe/wires from the water would likely solve the issue.

@LinelHutz - This may be a dumb question, but the probe is designed to be in water. How would I go about insulating it more from the water?

I believe you're talking about the grey wire with the red end? I'm not sure what the red is, but it looks like tape or heat shrink which might not be that good at sealing and insulating what's inside from the outside.

I would expect to use something more like this for an industrial type of use.

Unfortunately, I'm stuck using the type of probe, otherwise I would try something else. I plugged in another probe I had and got the same results. I would be surprised if they are both defective. Also, I've used these probes elsewhere on site and not had this issue.


Here is a close up, it's seems like they dip it in some kind of rubber

For some extra info, this is how things look when the VFD is off but the probe is still in the water

What is the sampling rate. Those are remarkably linear changes and look more like it's undersampled and there is a high-resistance short circuit somewhere, though the one spike down is farther than the rest, so it's not helpfully constant.

Is there a thermostatically controlled heater?

I see it as,
1. Noise directly conducted into the probe as most likely
2. Some kind of AC voltage in the water that is inducing noise into the probe. I'd think the voltage required for this would be felt if you stuck your hand into the water.

I still don't know what type of probe that is (does anyone recognize it?).

But interesting to see the rubber coating suggests an attempt to exclude moisture. But looking at the first post photo it looks like the probe is submerged past the level of the rubber so that the grey cable (twisted shielded pair instrument cable I think) is also submerged. That would defeat the waterproofing of the rubber I'd suspect you may have potential point for water entry into the circuit either at the interface or maybe even through the grey cable. That could also potentially explain why you only see the problem when the probe is submerged (although tbh I'd think if moisture got in the problem would not go away as soon as you remove from the liquid... it might take awhile to dry).

Can you hold the probe higher in the liquid so only the rubber part is submerged? (that would help prevent moisture entry if it's happening).

Also to check for moisture entry electrically, you could lift leads at the other end and megger instrument leads to shield, leads to ground, and shield to ground. [highlight #FCE94F][/highlight]Perhaps do it with probe/cable deep in the liquid as in first photo, although you'd want to give some thought to whether such test could itself cause damage when meggering a potentially damp circuit depending on test voltage used. I'd start with low voltage like 100vdc and stop if I see bad results or work my way up in voltage if I see good results... I think instrument cable/sensors can typically be meggered to 250vdc or even 500vdc but you might want to double check that (the unknown mystery sensor adds small additional uncertainty).

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(2B)+(2B)' ?

>this is how things look when the VFD is off but the probe is still in the water (chart 3 posts up from here).
So when the VFD is off you still see downspikes as shown on the left side of the chart, 8:20 through 8:55, correct?

Observation: Sampling rate - Looking carefully between 8:40 and 8:50 you can see that it's one sample per minute.

Is the data collector (whatever the probe plugs into) battery powered or line powered?

Can you use another temp sensor/indicator? Can you borrow a battery powered DVM with a temperature input and try a 2nd device to see if it mimics this behavior?

Can you tell us what the temp sensor/indicator brand/model is?

What is the process the photo shows? Why is there open water? Is the water heated/cooled? Is it a plating bath? (room cleanliness tells me no, but I have to ask). Is the water superconductive, like coming off a softener regeneration cycle? Why is there so much PVC around and so little Sch80?

He wrote it was saltwater, so some guess work, agriculture near the Atlantic, fish tank, akvarium?
+20 C maybe a bit high in temperatur for sea living creatures but works for many too.
Not sure what can be grown in a saltwater tanks, seaweed maybe. ;-)

Maybe putting the probe and all of the cable in a separate smaller submerged plastictank in the big one with fresh water in it, would help.

BR A

@3DDave - I'm in the process of setting up a plate heat exchanger that using a different temp probe to maintain the correct temp in the system. Currently it's not up and running. The sample rate is every 2 seconds.

@Electricpete - the probe is made by avtech, they don't have the exact ones listed on the website as these are slightly customized. I have the same probe elsewhere, in other systems, fully submerged in water and don't have this issue. I can certainly try having the probe higher up in the water column.

@Danw2, Apologies I wasn't very clear, the down spikes occurred when the VFD was on, the straight line is after it is turned off. I should have annotated the graph! The data collector is line powered, but can run off an internal battery. I don't have another temp sensor I can try at the moment. Temp sensor and data collector is an Avtech. The system is for growing fish! Thus the reason for so much sch40 pvc.

Confused; Do you have a digital multimeter and if you do do you know how to use it?

If you do, set it to AC auto or 200+ volts. Plug a grounded three terminal extension cord into a nearby outlet.

Using the meter and standing within arms-reach of the water.

Put one probe in the extension cord ground hole.
Put the other probe in the narrow extension cord slot.

You should see a reading of about 120Vac on the meter.

If you do, pull the probe out of the slot on the extension cord and without moving AT ALL the probe still in the extension cord ground hole dip the tip of the other probe into to the water.

What reading do you get?

Try it with the motor running and not running.

Keith Cress
kcress -

As a comparison, this is the graph produced by the same type of probe, submerged in the same way, in another system. The only difference is, there isn't a VFD in the room with the probe.

Itsmoked - I do have a multimeter, I can just about use it. I will give it a go when I get back to work.