Bad Logging Values

[JoshM]

Hi,

I am using an Allen Bradley Compactlogix PLC and polling data from it using KEPServerEX (OPC client). I have been logging various 4-20mA temperature sensors and found large jumps in the data. In one case the sensor usually reads around 83F, but sometimes jumps to 101F. Another sensor has been reading in the 70's and has been jumping as far down as 0F. The readings are clearly wrong, but I'm not sure how to eliminate these spikes.

Thank you,

Josh

 

[danw]

You have to isolate where the reputed bad values are coming from: sensor, transmitter, noise, PLC, or OPC

Some transmitters show fault mode by going full scale up or full scale down. If the the excursions are full scale up or down, then it is likely the transmitter is faulting for some reason.

If the excursions are not full scale but just high or low above or below what you think is real then a question is what does the PLC see those data points as? Same values as OPC? Or does PLC see values around where you think it should be?

Is the timing of the bad values related to a large electrical load, like a motor or heaters that turns on or off?

Has the data ever been good for a long period of time, or is this a new installation where the data appears sometimes good and sometimes bad?

 

[JeffKiper]

I would do some logging inside the PLC. Setup a high and low value. If the laburnum is greater than the previous value move it into the stored value. Do the same for the low value. This will tell you if the PLC is seeing the values.

 

[JoshM]

Thank you danw and JeffKiper!

-Josh

 

[JoshM]

I confirmed that the PLC is seeing these values. It is definitely the transmitter of the sensor. Possibly being caused by external noise or could be a faulty sensor.

Any suggestions for companies that make good 4-20mA temperature sensors?

-Josh

 

[adel1961]

yes as joshm said , mainly would be the tramsmitter , unless you try to wire the shield to ground if it was't.

 

[JeffKiper]

There are a bunch of manufacturers. I like foxboro because I like the sales Rep.

 

[OkiePC]

I've bought a few recently from Automation Direct which I believe are sourced from IFM efector:
http://www.automationdirect.com/adc...ors_-a-_Thermometers/Temperature_Transmitters