1794-ie12 scale below 4mA at 4...20mA setting

[Pavel]

Good afternoon.
1794-ie12 module available, channel set to 4...20 mA.
According to the table from the manual, we get from 0 at 4mA to 30840 at 20mA.
And what will be the number when it drops below 4mA? Logically, for every 1mA, we have 1927 units, so when it drops to 3mA it should be -1927, when it drops to 2mA -3854, etc., and approximately -7780 at 0mA. If the sensor breaks, this should show approximately this value -7780.
Is it really so?
There is currently no way to test this on real hardware.

 

[JTCat]

0 - 3.999 ma and 20-22ma is usually reserved for fault conditions, aka , 'fault current'.

You will not get negative engineering units unless your scaling parameters allows for it.
Your raw data will also not be negative below 4ma. It will just be zero.

 

[JTCat]

You also bring up a good point that I encourage everybody to do when tasked with programming.

Build in routines that determine whether or not the data is both correct and reliable, and what the program should do if a field device is determined to be not working.

Don't be the guy who programs a pump to fill a tank to 100% only looking at the level sensor's reported value. If the sensor is dead the pump won't shut off. Have failure planning built into your process.

 

[ndzied1]

I think you need to set the module for 0-20mA to read below 4mA.

 

[Pavel]

0 - 3.999 ma and 20-22ma is usually reserved for fault conditions, aka , 'fault current'.

You will not get negative engineering units unless your scaling parameters allows for it.
Your raw data will also not be negative below 4ma. It will just be zero.

Good afternoon.
As a result, it was experimentally found that engineering units decrease below zero, at 0mA it turns out approximately -7708.

Of course, this test was not a real sensor, but a current signal generator. Most sensors will give an error when they drop well below 4mA.
But there was a question about the capabilities of the 1794-IE12 module, and the answer is that it is possible to measure current below 4mA when the channel is set to 4...20mA.

 

[Robb B]

Something to be aware of, a lot of sensors implement NAMUR protocol, https://instrumentationtools.com/4-20ma-transmitter-namur-signal-levels/ and you can use these values to implement crude diagnostics in PLC or HMI.