SLC 500 Analog Card help

[Tinker Hydro]

Calibration
What i am trying to say is: If i put a rtd in my dry well temp calibrator and it is set for 86C then I know the probe is seeing 86C. Sometimes the reading is not the same back at the PLC because of age of the probe, length of wire, etc. How can I make the input at the PLC show show 86C. Can you use a simple math add or sub function or is there another way? I guesss some people call it a offset. I am using a 4-20 ma signal for an input.

thanks

 

[Ron Beaufort]

Greetings Tinker Hydro ...

first things first ...

if you're using an RTD - and the input to the card is 4 to 20 mA - then that implies that you have some sort of transmitter located between the RTD and the PLC input module ... is that correct? ...

and if you have such a transmitter, have you checked the calibration on it? ... zero/span, etc.? ...

next item ... you really need to tell us the exact catalog number of the PLC input module that you're using ... that information definitely could affect the final answer to your question ... better yet, if you're allowed to post it, showing us the entire RSS program file will cut down on the amount of back-and-forth questions and answers like these ... check first ... some companies have strict "proprietary" and "trade secret" rules about their programs ...

also ... checking the calibration at one point only (example: 86 degs C) isn't usually going to give you enough data to do any sort of accurate calibration ... for most systems you need to check at least 5 points - and make a chart of the "expected" readings and the "actual" readings at each point ...

and welcome to the forum ...

 

[parky]

If you use SCP instruction you can scale any analog signal to suit

 

[Ron Beaufort]

using an SCP (etc.) to simply "scale" the input signal will only work correctly if the input signal is linear in nature ... unless the signal is checked in at least 5 places, there's really no indication that the signal is linear ...

what it all boils down to is the question of whether or not the individual components in the signal path are all working "correctly" ... if they ARE then we can go on from there ... but if they AREN'T then we're probably not going to have much luck making a silk purse from a sow's ear ...

the information that we have so far doesn't really allow us to do much more than guess at whether or not all of the "things" in the system are working correctly ...

 

[parky]

Yes 5 points is a common check across the span, In my experience if good quality probes & converters are used then they will be linear, It may be worthwhile checking the analog card with a stable 4-20ma source at 86 Deg C this would be somewhere about 13.107 ma

 

[Mickey]

Yes 5 points is a common check across the span, In my experience if good quality probes & converters are used then they will be linear, It may be worthwhile checking the analog card with a stable 4-20ma source at 86 Deg C this would be somewhere about 13.107 ma

That's assuming alot. The ma out would depend on how the 4-20ma device is spanned.

4ma = ???
20ma = ???