RTD low reading 1

[advnceed123]

hello everyone. I have one question regarding the RTD Platinum PT100 output.

we have 3 RTD install on heat exchanger after replacing shell and tube reading reduces from 117 to 111

As per our process indication and energy consumption the Temp Transmitters it actually should read higher than current reading.

What will be the possible cause for temperature reduce while process temperature should be around 118 C.

 

[danw2]

Which of the situations listed in the reply to your control.com thread makes the most sense?
Link

 

[hacksaw]

There are two standards.
One has a 0 DegC resistance of 100 Ohms, the DIN standard calls for 98.129 Ohms

If DIN RTD's replaces the first, then 118 Deg C would read cooler say 115 Deg C

 

[danw2]

The error difference for a 392 alpha vs a 385 alpha RTD at 117°C is only about 2°C, so the 6 degree C error (117 vs 111) comes from somewhere else.

The 392 alpha RTD curve is the standard for SAMA (American Scientific Apparatus Makers Association) RC21-4-1966 and the US Department of Defense (MIL-T-24388). I've heard people refer to the 392 alpha as the Burns Engineering alpha, because Burns was a major proponent and vendor for such. The Asians frequently refer to the 392 alpha as the American standard.

I have to say that I haven't run into 392 alpha RTD's since the 1990's, everything I've run into in the past 20 years has been DIN 385 RTDs in the US, but that might be a limitation of my industrial social neighborhood;

The 60751 DIN standard is the 385 alpha.

The instrument interpreting the voltage created by the current source driven through the RTD resistance has to match the RTD alpha curve.

If the instrument is configured for the 'other' alpha' curve, then yes, there will be an error:
- a 392 alpha instrument reads low with 385 RTD.
- a 385 alpha instrument reads high with 392 RTD.

Specifically, for a 392 alpha instrument reading low with a 385 alpha RTD:
At 117.0°C, a 392 alpha instrument expects 145.75 Ohms, sees only 144.937 Ohms from a 385 alpha RTD;
the 392 instrument reports a temperature of 114.9°C, a low error of 2.1°C

2.1°C is 1/3 of the 6 degree error reported.