4-20MA devices??

ok no one was able to answer my previous post maybe the question needs to be simpler,, how can i read a 4-20 ma device linear on my plc? my hmi seems to drift ,, i dont know how to set ofsetts and gains or maybe im not setting it up right my mitsubishi plc. i read d0 at 1000 at 72 degrees or so and in my hmi software i can set my ofsetts and gain calc, the default is offset-32767 to 32767 and gain -32767 to 32767 i changed the offset to 0 to 10000 gain to 1 - 150 and the prob reads right on that temp but drifts hugly as the temp goes up or down ,, any help in the right direction would be greatfull,,

The first thing you need to do is figure out what temperature corresponds to a 4 mA signal and what temperature corresponds to 20 mA. Then you need to figure out what value you see in the PLC corresponding to a 4 mA signal and what PLC value corresponds to a 20 mA signal. Armed with that data, you should be able to set up the scaling in your HMI software.

thanks ill try that tomorrow at work ,, ill have to get a bath set up to get thos figures

Instead of trying to get the information experimentally, I would try to get the data sheet for the device that's converting the temperature sensor signal to 4 - 20 mA.

As you are reading Temperature, Do you have the Cold junction compensation setup on your converter?

I had this problem on one site that I was called to - The Temperature floated everywhere as the ambient changed - The installers had thrown away the CJC as they did not know what they were for.

You need to know about your input device. It may well be non-linear. RTD's are usually quite linear but thermocouples are far from linear. You can convert thermocouple inputs quite well with a table of about 20 breakpoints with their millivolt to temperature conversion at each point. You can easily interpolate between points effectively drawing a straight line between each pair of adjacent points.

Otherwise your PLC may well have thermocouple conversions available.

Also usually you would read a thermocouple directly into a high quality low level analogue input channel rather than as 4 - 20 mA.

the temp prob is an anderson instruments and here is the specs for the probe and analog modual im tring to work with ,, how do i know what d0 is on my plc at 4- 20 ma without puting it in a bath


http://www.andinst.com/PDFs/2001.pdf

You configure the temperature transmitter for the temperature range you want to correspond to the 4 - 20 mA output.

how do i know what d0 is on my plc at 4- 20 ma without puting it in a

Click to expand...

The manual for the Mitsubishi analog module will tell you what values in D0 correspond to the 4 mA and 20 mA signal levels. I'm not familiar enough with Mitsubshi hardware to tell you whether that range is 0 to 10000, 4000 to 20000, 0 to 4095, or something else.

Set defaults to 0-32767. If span is the full range of -50 to 350, then -50 will be 6553 (20% of 32767).
4mA = -50 degrees = 6553. 20mA = 350 degrees = 32767.

thanks for the help guys i seem to have gotton it to work , the mitsu manuals are horrible at explaning things and i could not find a converstion for 4-20ma in the book all it said was something +1000 but the nubers defenitly go higher than that,, so i did it the hard was and pulled a high and low temp situations for the probe and took thos numbers from d0 low and high and it did work in my software it was somwthing like 2265 to 3745 and the range was 0 to 180 degrees.