If a PT 100 probe is connected to the input of the isolation amplifier which of the following must the input be set to ?

0-60 mV
0-100mV
0-200mV
0-300mV
0-500mV
0-1 V
0-2 V
0-2.5 V
0-5 V
0-10V
0-20V
0 - 5mA
0 - 10mA
0 - 20mA
± 1V
± 2V
± 2.5 V
± 5V
± 10V
± 5mA
± 10mA
± 20mA
1..5 V
4..20 mA

Thanks.

 

None of them because it does not generate a voltage or a current. Its resistance changes with temperature, so you have to pass a small current through it and measure the potential difference across the probe. You probably need a different input card.

 

Unless I'm missing something here, an instrument characterized only as an isolation amplifier alone does not provide a resistance measurement. Usually a small constant current is sent thru the PT and the voltage drop across the element is interpreted as its temperature. I believe you are probably looking for an "isolated RTD transmitter" which is designed to provide that
function. In that case the input side of the instrument is connected directly to the probe either as a 2,3, or 4 wire hook up and you need not concern yourself with the signal levels, i.e, on the input side.

Regards
Erich Mertz
[email protected]

 

Yes, Anonymous, while anonymous, is correct. An RTD acts as a variable resistor and requires an external current source. RTD isolation amps are available, or if you can change to a thermocouple for temperature measurement, you might be able to use one of the millivolt ranges, possibly with some linearization in your software.

 

> ----you might be able to use one of the
>millivolt ranges, possibly with some
>linearization in your software.
----------
I would not recommend this solution if you want very accurate measurements without some form of cold juction compensation. Most TC modules will provide internal CJC. Software linearization is not difficult and can be made very accurate depending on the complexity of the software, but you need a way to measure the CJ temperature (usually done with an RTD or thermistor) to compensate for its effects on your TC input. Since you have the RTD, just get an RTD module.

 

Is a Pt100 mean 100 Ohm?
How much will the Ohm vary over a range of 100 degrees Centigrade?
Thanks for your help.

 

The 2 most common standards are 3.85 or 3.92 ohms. This is dependent upon the purity and annealment of the platinum.

Regards
Erich Mertz
mertz@ intac.com

 

Stephen, here is some basic information about RTDs. These are pretty basic, but they will tell you what you need to know.

"http://www.omega.com/temperature/Z/pdf/z033-035.pdf":http://www.omega.com/temperature/Z/pdf/z033-035.pdf

"http://www.omega.com/temperature/Z/pdf/z054-056.pdf":http://www.omega.com/temperature/Z/pdf/z054-056.pdf

In general a PT100 RTD has a nominal resistance of 100ohm at 0C. It has an alpha of 0.0039 ohm/C. It is not quite linear however. Since the resistance change is so small it is difficult to measure. It is best measured by a wheatsone bridge circuit.

Where does your 100C range start? 0 to 100C, or is it something else? If you want precise measurements you also need a three wire or four wire connection to the RTD to cancel out lead wire resistance. Don't count on lead wire resistance being constant, it also changes slightly with temperature, so it must be canceled out. By far the best course of action is to purchase an off the shelf signal conditioner that gives you the desired output. They are not expensive and will save you time and headache in the future.

 

Commercial RTD's have a range variation of 38.5 ohms over a temperature range of 0 to 100 degrees celcius. Where at 0 degrees RTD has a resistance of 100.0 ohms and at 100 degrees is 138.5 ohms. Resistance change is fairly linear over this temperature range. Going to -100 degrees resistance is slightly different at 60.2 ohms.
Best bet use a RTD input isolator.

Regards
Wynton

 

The 2 most common standards are 3.85 or 3.92 ohms. This is dependent upon the purity and annealment of the platinum.

Regards
Erich Mertz
mertz@ intac.com

 

Sorry about that. Haste makes waste. I read your note too quickly. Let me make a correction. What I said below is misleading and incorrect.

PT100 refers to a platinum device that measures 100 ohms at 0 deg C. The temperature vs resistance slope is specified as "ohm/ohm/deg". There are 2 common purities one at .00385 and the other at .00392 ohm/ohm/degC . For a 100 ohm device this means a slope of 100*.00385 or .385 ohms/degC and for a .00392 device this means .392 ohms per degC. A 100 deg change for a 100 ohm
device would then be 38.5 or 39.2 ohms depending on the platinum spec. The resistance at 0 deg C is 100 ohms and at 100 deg C it would be 138.5 ohms.

Again, sorry for the quick read.

Regards
Erich Mertz
[email protected]

Erich Mertz wrote:
> The 2 most common standards are 3.85 or 3.92 ohms. This is dependent upon the purity and annealment of the platinum.<

 

Don't forget to be aware of accuracy.

From the website of Thermometrics Corporation
"http://www.thermometricscorp.com/acstan.html":http://www.thermometricscorp.com/acstan.html

"The industry standard for platinum RTD's according to IEC-751 is + / - 0.12% (of resistance) at 0 degC, commonly referred to as Class B accuracy. This will provide an accuracy of + /- .3=B0C at 0 degC, which is quite good if you compare it to the + /- 2.2 degC of a standard Type J or K thermocouple. But as the temperature increases, so does the permissible deviation due to the variations possible in the TC. So, not only do we have the possible + / - .3 degC offset at 0 degC, but also the probability that the TC is not equal to 0.00385. This could account for a permissible deviation of up to + /- 4.6 degC at a maximum temperature of 850 degC . But that's still better than the K thermocouple, which could be off by as much as + /- 6.4 degC, and even more for the Type J, which is not recommended at this temperature. Because a well manufactured RTD will have high repeatability (relative to the application), Class B accuracy is generally sufficient unless there is a need for better interchangeability; or when measuring change of temperature; or if you know that you have special accuracy requirements."

Bob Pawley
250-493-6146