RTDs and Incorrect Temperatures

Guvnor - are the lead resistances correct?<Splutter>

yes,

And does it correspond with the standard PT100 RTD temperature/resistance table? <Cough, wheeze>

If it doesn't, then it's fu broken and you need to change it. <Cough>

Hi Pete,

It doesn't sound logical. If:

• the temperature indicator is calibrated properly
• the RTD checks out as okay and
• the wires are all the same resistance

it should work okay. If it doesn't, one or more of the above is incorrect.

regards,

Vulcan

The readout and the probe should be aligned at a known value. Usually ice point. This will align the probe to the readout and if all else is linear and cal'd correctly should correct for any error in the system.

Some indicators can have an offset which can cause some brain strain.

What is the resistance value for your RTD and what is the indication of your meter at the same conditions.

Are you able to put a known value resistor to the indicator input and read the value?

I did see a similar instance once. I removed the RTD wire at the TB in the JB between RTD and Panel and cleaned the TB, cleaned the lead of the wires and put it back and the 10 dc difference was gone.

You could try though

Regards

Naveen

Hi all,

Firstly - Temperature measurement with RTD or Thermocouple is definately not linear.

Now to the problem.

Measurement circuits that accept three-wire inputs minimize the effects of lead wire resistance as long as the outer legs are equal. However factors such as terminal corrosion and loose connections can still create sinificant differences between the lead resistances seen by the measurement circuit, because only one ohm of difference between the legs results in an error of 4.7 deg F (2,6 deg C).

So this should give you your starting point - lead wire lengths, loose connections, corroded terminals. I suspect it would be one or more of the above mentioned if your element is 100% as well as the transmitter.

Regards

Craig

When calibrating , do you have the RD hooked up to the final display or are you just reading resistance with a multimeter in the shop and comparing to the RTD resistance/temperature chart.

RTDs work by having a constant current source pass through the RTD.

Measure the loop current on this defective circuit and compare to a known good circuit.

Maybe you cc source is a bit low

Some indicators use a 24 V DC supply. If this voltage is incorrect then you may face such problems

Is the rtd in a well or in direct contact with the process?If in a well, do you use a thermal compound between the rtd and the well bore?If in a well, is the external portion of the well clean and free of debri?Are connections tight and clean?(both ends).There are 2 curves for RTD's. Both are 100 ohm at 32 degrees, but the response is different. Are you sure you have the right curve?

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hi harsh check with rtd simulator from rtd point to controller ie ; remove 3 wire from the rtd and in place of rtd connect simulator or decade box and feed the resistance as per the resistance chart if the error persist check the lead contacts in junction box or any unbalance lead resistance . www.telcoenvirotrols.com/ACI_100_ohm_resistance_chart.pdf hope u will resolve it

hi dear ,

set the tempture controller off set parameter ,then see what happened.

how much to set the offset 8deg or 10 deg

HI DEAR ,

you send me temperature controller type , model no , company name , then i tell you how you set off set parameter.