RTD Problems

MikeVT · Feb 22, 2013

I have had fits with a machine that has RTD's on it. One has worked flawlessly, and one has been constant hassles. The latest seems to be the the 'resistor' has changed value. I have three RTD's of this type, and at room temp (+/- 70 degrees F), they read about 110 ohms. The failed one reads about 80 ohms. Any hints to look for, that may have caused this?? I replaced the bad RTD, and its working great so far, but I haven't really found a cause for this failure, so I'm not so confident the problem is fixed.

shooter · Feb 22, 2013

Just a bad one, do not look further as it is too small to repair or have a look at the problem.

Mickey · Feb 22, 2013

I would suspect a bad wiring connection. You fixed it when you replaced it.

dmargineau · Feb 22, 2013

Resistance thermometers (RTDs) just like any resistive circuit component do oppose to the passing of an electrical current; in time, the resistive component starts losing its properties and this usualy translates to a decreasing capacity to "restrict" current flow hence a lower resistance.
An environment introduced semi-shortcircuit could also reduce the overall resistance of the circuit.

Jeff23spl · Feb 22, 2013

bad wiring would increase resistance not reducing it to 80 ohms. A bad wiring to a power source may have broken it but usually i got 3/4 wires mixed up...
But RTD sometimes fail...maybe looking for a better brand or keep a spare.
You can look for mecanical issue, part knocking against the probe or overrange temperature etc...

Bit_Bucket_07 · Feb 23, 2013

Extreme vibration can cause dislocation of the resistor coil in an RTD, thereby resulting in shorted turns. Any attempt to mechanically lock down the coil in the RTD manufacturing process only results in creating a "strain gauge" effect. Exposure to very high temperatures can eventually degrade the purity of the platinum element (safe to assume PT100 in your app) over time.

BryanG · Feb 23, 2013

Just out of interest, here is a PT100 RTD table.

http://www.micropik.com/PDF/pt100.pdf

So your reading of about 110ohms at 70F/21C makes sense.

Sorry, the table uses the 'proper' scientific temperature scale of centigrade rather than one based on human body temperature and the coldest place on earth.

curlyandshemp · Feb 23, 2013

MikeVT said:
I have had fits with a machine that has RTD's on it. One has worked flawlessly, and one has been constant hassles. The latest seems to be the the 'resistor' has changed value. I have three RTD's of this type, and at room temp (+/- 70 degrees F), they read about 110 ohms. The failed one reads about 80 ohms. Any hints to look for, that may have caused this?? I replaced the bad RTD, and its working great so far, but I haven't really found a cause for this failure, so I'm not so confident the problem is fixed.

What is the make of the RTD?
I know Anderson Instruments had a bad run of RTDs awhile back, all attributed to a crimping machine causing cold solder joints.

MikeVT · Feb 25, 2013

Thanks, all, for your replies. The RTD manufacturer is Watlow. It is a PT100.

As I learned more about the details surrounding the RTD troubles, the most likely cause of the RTD changing its resistance value, was a large fault in the heater circuit. I had no more problems after replacing the RTD.

RTD Problems

MikeVT

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shooter

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Mickey

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dmargineau

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Jeff23spl

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MikeVT

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