Analog Transmitter failure detection

Hi Everyone,

I have a Keller pressure transmitter (Non Calibratable) having output 4-20mA. When there is zero pressure in line then its output is 3.96mA, Analog channel is configured as 4-20mA channel. So when there is a zero pressure in line, it gives zero count.

I am running a system when pressure is below the certain limit to built a pressure in line. So if there there is a case of failure of tx, the system will run continuously and may damage the line.

Is there any method to detect failure of transmitter through PLC. Or Is there any problem if i configured particular channel to 0-20mA and rest are configured as 4-20mA.

One method I use and I think it works well.
I set the input to 0 - 20ma in place of the normal 4 – 20 ma
Then set the scaled value -25 to 100 % or whatever scaled value you need
As long as the 0 value is 25% below the 4 ma value
4 ma = 0% 20 ma = 100% then 0 ma = -25%
When you read the input any value less than 0 would be a faulty reading
A problem of some sort with the sensor circuit
Open input, bad sensor or some other problem that need to be checked out.

I hope that this is some help

+ for GaryS.

Just make sure your slightly-out-of-tolerance 3.96mA is within the non-fault 0-20mA signal range.

I have seen pressure transducers fail with a less than 4mA signal, but I have also seen failures mid range and higher, so just relying on sensing a failure by looking at the 4-20mA is not guaranteed. If the high pressure is dangerous then you need a pressure relief valve and possibly a sensor telling you that the valve has operated.

Since analog card have 8 channels, all the channels are configured to 4-20mA, Configure a single channel to 0-20mA is Ok ??, Is it sure that it will not affect the function of other channels in same card??

Since the output at transmitter is 4.02mA, but due to cable length, it is 3.96 at the analog card. I want that in the case of power failure of tx ,the relative system will not run and pressurize the line.

What you actually need is a different pressure transmitter. Get a NAMUR transmitter that is configured to fail high (20mA), then you can safely shut down if the transmitter fails.

If you have a wire break, you'll get 0mA, and also safely shut down.

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First the cable length should not have any effect on the current in loop as long as the power supply and the interment can supply the necessary voltage. That’s the advantage of a 4-20mm current loop
If you are seeing a drop in the output current then you need to revaluate the loop system and make sure that all the components are within the ratings

As for setting the individual channels on a multi channel input card.
Some cards you some you cannot, you will have to check the manual for your card to answer that.

If your need is that critical then consider a redundant pressure transducer system
A 2nd transducer powered from a separated power supply returning to a separate input card
That way any one failure will be captured and can be dwelt with before it becomes a big problem.
Also consider a high and low pressure switch as well as a mechanical high pressure relief valve to protect the equipment

The Analog card should have fault detection bits built in. Under range and over range for each channel. Analog card fault. 24VDC missing.

Nyains said:

Since analog card have 8 channels, all the channels are configured to 4-20mA, Configure a single channel to 0-20mA is Ok ??, Is it sure that it will not affect the function of other channels in same card??

Click to expand...

A lot of your questions depend a lot on the capabilities of the PLC and IO that you're using (but I don't think you've mentioned). I've used some IO modules that do analog configuration with a HW plug that affects the whole module; I've used others where each channel can be individually configured, electronically.

Also depending on the card you use, some IO modules may have detection for broken channels (which is why 4-20 mA exists in the first place, to detect 4 vs 0). Note that this doesn't necessarily detect ALL possible failures, but it can detect things like broken wires. You can also do it manually, as others have mentioned, and set the monitoring from 0-20.

The suggestions about using a different type of sensor or adding a redundant sensor have value as well. You're already thinking about failure modes; sometimes the best solution is change plans.

BryanG said:

I have seen pressure transducers fail with a less than 4mA signal, but I have also seen failures mid range and higher, so just relying on sensing a failure by looking at the 4-20mA is not guaranteed. If the high pressure is dangerous then you need a pressure relief valve and possibly a sensor telling you that the valve has operated.

Click to expand...

+1

There is the possibility of a failure of the AI module itself. A failure could also occur with the control valve that adds pressure to the line.

I'd prefer to write logic to determine whether the measured pressure is responding to the requested valve operation as anticipated and trip an alarm if it does not.

Nyains said:

Hi Everyone,

I have a Keller pressure transmitter (Non Calibratable) having output 4-20mA. When there is zero pressure in line then its output is 3.96mA, Analog channel is configured as 4-20mA channel. So when there is a zero pressure in line, it gives zero count.

I am running a system when pressure is below the certain limit to built a pressure in line. So if there there is a case of failure of tx, the system will run continuously and may damage the line.

Is there any method to detect failure of transmitter through PLC. Or Is there any problem if i configured particular channel to 0-20mA and rest are configured as 4-20mA.

Click to expand...

Plenty of ways to do it depending on your type of PLC. Without you telling me what type of PLC it is all I can say is see what crazy values you get when the transmitter is not attached and check for that.

Things like the compactlogix have a bit you can just check.

Configure 0-20mA on every analog input if it's an option. There is no downside whatsoever.

If reading <3mA or >20.x mA, set an alarm and shut down whatever you need to. I do this on all analog inputs regardless of criticality.