PT100 on V350
Posted 22 November 2011 - 01:04 PM
Is there something that I am missing out??
Any helps or pointers would be appreciated, Thanks!
Posted 23 November 2011 - 12:51 AM
Posted 23 November 2011 - 01:24 PM
To get the most accurate measurement we highly recommend to use 3-4 wire RTD sensor.
Using 2 wire sensor may lead to very unstable (sensitive to noises) and un-precise (affected by wire impedance) reading.
If anyway you have two wire RTD sensor, you need to connect one of the wires to T+ and T- (both have PT label as well) and the second wire connect to CM.
You need to define selected input as PT100 in hardware configuration in accordance to used PT100 type (alpha=0.0385 or, alpha=0.0392) assign MI and select C(celsius) or F.
Then you will see in assigned MI raw number, which is 10xt.
Example: you see raw number 247. Temperature is 24.7 C.
When MI = 32767 Sensor is not connected to input, or value exceeds permissible range,
or sensor is different than PT100 (for example, PT1000).
When MI = -32767 Sensor is short-circuited
Posted 23 November 2011 - 05:51 PM
I have used 2 wire 1000 Ohm RTDs for about 12 years and have never had a problem with noise (that I am aware of). I have not used an RTD with a Unitronics, but is there any test data that anyone knows of that would show me the relative affect of having a 3 wire RTD over a 2 wire RTD in similar noisy environments. Or is it simply a matter of accuracy in that a 3 wire is more accurate than a 2 wire in all conditions. I know that the 2 wire units we use have always had better accuracy and much lower noise that any thermocouple I have ever used which is why we went that way in the first place.
It is completely accidental that I have allways used 2 wire units, it is just what was selected from a companey that has a very inexpensive converter from 1000 Ohm RTD to 4-20ma that we needed at the time and we have never altered what we are doing with that product.
Posted 23 November 2011 - 09:42 PM
A 1000 ohm RTD is typically two wire because the ohm or so of lead resistance has a small (< 0.1%) effect on the temperature reading. A three wire 100 ohm RTD is much more common and allows accurate resolution of 0.1 degree but obviously lead resistance becomes an issue. The third wire is used in an adaptation of a 4-wire ohmmeter to actually measure the lead resistance and subtract it from the RTD resistance.
Posted 24 November 2011 - 12:42 AM
If you only have a 2-wire probe and the interface expects 3 or 4 wires, you can trick the interface by bridging the compensation terminal to one of the RTD terminals locally at the interface. This of course defeats the purpose of compensation, but at least allows the interface to operate. If the resulting accuracy is sufficient for the application, then you have achieved your purpose. Take a close look at the Unitronics wiring diagrams when doing this, as wiring errors are a common reason for non-functioning RTDs.
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