I get it wrong..

Please tell me what's wrong with this:

1.) I've got a 2-wire pressure transmitter outputting 4-20 mA connected to a panel meter.
2.) I get the ammeter, connect the two leads in series with the line & measure the Idc from the pressure transmitter, I get 0.3mA. Unbelievable.
3.) The panel meter is connected to a motorized valve receiving 4-20mA (it works in proportion with the transmitter). I get the ammeter, connect the two leads in series with the line & measure the Idc. Surprisingly I get 0.3mA again.

Whats wrong with me?

1) It isn't clear whether the problem is
- the number your ammeter is showing you, 0.3mA, or
- the functionality of the loop: transmitter, meter, motorized valve

Does the loop function as it should and you're wondering why you get a reading outside the range of 4.0 - 20.0 mA?

Or is loop dysfunctional and you're asking why there's not even 4.0mA in either circuit?

2) Is the indicator retransmitting the 4-20mA or is the indicator's analog input in series with the motorized valve analog input?

The system is dysfunctional this time. The transmitter is connected to the panel meter (for monitoring purpose), then the panel meter outs corresponding signal to the motorized valve.

Is the indicator really a controller, sttempting to hold a pressure setpoint?

Or is the valve position just following the pressure:
Low pressure: valve closed or barely open
High pressure: valve open or mostly open

A two wire transmitter is loop powered.

Is the transmitter wired to either
- terminals on the indicator that supply loop power
- an external 24Vdc power supply

Open the meter and check if it smells "burnt".

danw said:

Is the indicator really a controller, attempting to hold a pressure setpoint?

Click to expand...

The panel meter is capable of outputting digital outs (1,0) in pre-settable set points but I didn't use it because I dont need that function.

danw said:

Or is the valve position just following the pressure:
Low pressure: valve closed or barely open
High pressure: valve open or mostly open

Click to expand...

Exactly. Its a proportional control, the valve opening will depend on the pressure reading. The panel meter is there to show values, as monitoring.

danw said:

A two wire transmitter is loop powered.

Is the transmitter wired to either
- terminals on the indicator that supply loop power
- an external 24Vdc power supply

Click to expand...

The panel meter has its internal 24 Vdc supply that can be use to power 2-wire transmitter. We use that internal supply, however, we tried also connecting it to an external power supply as it was dysfunctional. Unfortunately, we get the same result.

1) Please don't use the word 'it', because I don't know which 'it' you are referring to.

>We use that internal supply, however, we tried also connecting it to an external power supply as it was dysfunctional.

Which was dysfunctional, the internal power supply or the external power supply?


2) Is the indicator retransmitting the 4-20mA from the pressure transmitter from a separate pair of terminals as an analog output?

Or is the indicator's analog input in series with the motorized valve analog input?

You need to answer this question because I still don't understand which signal the motorized valve is using.

3) Be careful to not put power supply voltage directly across a valve positioner's analog input. That will usually destroy the valve positioner AI.

4) You need first to confirm that the stand alone pressure transmitter works with a power supply when the transmitter is not connected to the indicator or the motorized valve. Connect 24Vdc to the pressure transmitter and put your meter in series in the loop. The transmitter should regulate the current in the looop and you should see some current between 4 and 20 mA.

I'm sorry for the confusion, I was referring to dysfunctional current reading on my transmitter. First, I connected the transmitter via the internal supply, reading is 0.3mA. After, I connected the transmitter on the external power supply, reading is still 0.3mA.

danw said:

1)
4) You need first to confirm that the stand alone pressure transmitter works with a power supply when the transmitter is not connected to the indicator or the motorized valve. Connect 24Vdc to the pressure transmitter and put your meter in series in the loop. The transmitter should regulate the current in the looop and you should see some current between 4 and 20 mA.

Click to expand...

I did disconnect the transmitter from the panel meter & connected an ammeter in series, reading is 0.3mA. Is the pressure transmitter faulty?

At this point, I think a drawing of what you are talking about would be a great help.

Stu....

If the power supply is a 24Vdc power and it is functional, AND

If you wired P/S+ to xmtr + and P/S(-) to xmtr (-), AND

If you inserted a working milliammeter with its plus lead closest to the P/S + and the negative lead closest to the P/S (-),

Then, yes, the pressure transmitter is not working.

1) Did you check the P/S voltage?

2) Have you checked the meter on another mA source to confirm that it's good? (my cheapie meter reads about 0.3mA when the leads are disconnected from one another, in other words, zero)

danw said:

2) Is the indicator retransmitting the 4-20mA from the pressure transmitter from a separate pair of terminals as an analog output?

Click to expand...

The transmitter is only 2-wire, it is loop powered. On the transmitter, it has terminals & we ensure it's tightened.

danw said:

Or is the indicator's analog input in series with the motorized valve analog input?

Click to expand...

The indicator's analog OUTPUT, not INPUT, is in series with the motorized valve's analog IN.

The pressure Transmitter will need a power supply to drive the 4 to 20 mA loop, either the pressure transmitter needs to supply the power for the loop, or you need to provide a power supply to make it loop powered.

Can you give us the manufacturers part number for the Pressure transmitter and also the panel meter.

Another question that comes to mind with the fixed mA reading on your multimeter is have you checked that the mA scale of your meter has not got a blown fuse, I know my Fluke has 2 fuses and I often find the mA fuse mysteriously blown after I have loaned the meter

danw said:

1) Did you check the P/S voltage?

Click to expand...

Power supply is 24 Vdc, or around that value.

danw said:

2) Have you checked the meter on another mA source to confirm that it's good? (my cheapie meter reads about 0.3mA when the leads are disconnected from one another, in other words, zero)

Click to expand...

That I haven't tried. I suspect, the fuse may have been blown.