RTD Foul Up

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big john

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Portland, ME
Somebody goofed up and permanently installed 100 ohm platinum RTDs in an application that needed old 10 ohm copper RTDs.

Short of changing out the measurement equipment, is there anything that can be done to make the 100 ohm compatible?

I've had no luck finding a signal conditioner.

Thought about paralleling a resistor to knock it down into the 10 ohm range, but I imagine I'd quickly lose accuracy.

Ideas?
 
How old are 10 ohm RTD's I have only seen 100 ohm.

Can't even recall seeing 10 ohm option on process equipment, mostly just 100 ohm RTD, thermocouple, or 0-10V/4-20mA as analog input options.
 
171111-1223 EST

big john:

You need an impedance converter.

If the measuring circuit used AC excitation to the RTD, then a suitable transformer might be a solution. In a transformer impedance transforms as the square of the turns ratio.

If the measuring instrument uses DC excitation, then more would need to be known about the instrument, and it might be possible to design a DC impedance transformer.

Is it impossible to replace the RTDs?

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What device is being used to measure the resistance? Does it attach to a front end that uses a programming interface where you could create a custom resistance to temp table for the 100 ohm RTD? It's that or just replace them as the time and cost to install 100 converters if they even exist may not be practical.
 
I don't think there is a converter available. I just recently ran across some old 10 Ohm copper RTDs. The AB RTD input module worked fine with them. Good thing too. They are buried deep inside a large generator. No way to change them without spending a lot of money.

if you can't replace the RTDs with the right ones, you will need to replace the other end to use 100 Ohm RTDs.
 
I don't know what gear is actually looking at these, whether it's an old PLC or standalone temperature monitor. I haven't been out there in person yet.
petersonra said:
...They are buried deep inside a large generator. No way to change them without spending a lot of money....
Click to expand...
Exactly the problem here, so replacing the RTDs is basically impossible.
 
big john said:
I don't know what gear is actually looking at these, whether it's an old PLC or standalone temperature monitor. I haven't been out there in person yet.
Exactly the problem here, so replacing the RTDs is basically impossible.
Click to expand...

Temperature monitors are relatively cheap you may well have spent more talking about it on the internet then it would cost you just to replace them.
 
171112-2239 EST

Try to deduce what the measuring circuit is. My guess is a bridge, not most likely, a constant current source, or an approximate constant current source (a moderately high source voltage with a current limiting resistance).

Use an adjustable resistance box, 1 or 0.1 ohm increments to 200 ohms to determined the type of measuring device.

Voltage across the test resistor will be linear relative to resistance up to a possible saturation point for a current source.

If a bridge circuit with 10 ohm resistors, then open circuit voltage will be about 2 times that with 10 ohm load.

An approximate constant current source will have substantially less than 1/2 voltage with a 10 ohm load compared to open circuit voltage.

Also you want to know the voltage across the 10 ohm resistor when it is in place of the RTD.

With this information one can determine possible translation circuits, but is it worth it? If the measuring circuit is programmable, then reprogramming the set point and source current may provide a solution.

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Also be aware of failure modes if you try to do some kind of translation.
I inherited a design where a parallel linearizing resistor was imbedded in the RTD probe. Unfortunately, that rendered detection of an RTD that failed open unreliable. We even had some fires and ultimately an expensive field retrofit of tens of thousands of installations.
 
big john said:
Somebody goofed up and permanently installed 100 ohm platinum RTDs in an application that needed old 10 ohm copper RTDs.

Short of changing out the measurement equipment, is there anything that can be done to make the 100 ohm compatible?

I've had no luck finding a signal conditioner.

Thought about paralleling a resistor to knock it down into the 10 ohm range, but I imagine I'd quickly lose accuracy.

Ideas?
Click to expand...

I'd talk to the folks at Omega Engineering. I had some questions long ago and they were very helpful.
 
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