4-20ma induced current - increasing values?

[meeko]

Hey guys, got a good question that has me stumped. was at a trouble call for some odd readings from a flow meter.

-So, there is a flow meter with a little propeller in a 6" pipe, this sends a frequency signal to a GF signet 5500 flow transmitter which also displays the flow consumption.
-This transmitter sends a 4-20mA signal to an ION 7700 power meter to display the current real time flow in the water pipe. this is were it gets odd.
- I disconnect the flow meter's 4-20 and put my ammeter on and i read 3.99mA signal from gf signet to be sent to ION7700- Perfect,
- I plug the gf signet 4-20 to the ION meter and i am hoping to see 0 or very close to 0 flow, since the flow meter is disconnected or zeroed- but i read 10.7-11 gpm (max gpm is 200)

my theory - The gf signet transmitter is off the same power supply as the ion7700 meter so, could this be the problem? or The wires are in their own conduit but do run near a 5A CT secondary wire in the panel- could this be inducing small current to raise the flow rate from 0 - 10 gpm

let me know what you think

 

[danw2]

If "the flow meter is disconnected or zeroed" is that the same condition/state as when you read 3.99mA from the gf signet with your ammeter?

Because some flow meter transmitters will go to a failsafe or fault condition if they sense disconnection from its sensor; some will go to very low mA outputs well below 3.5mA (one I recall is 2mA output)\ and it isn't clear what you mean by "disconnected or zeroed". Zero flow in a pipe could be interpreted differently than disconnection of the sensor circuit, to a transmitter.

And is the ion7700 scaled for 0-200 gpm on its 4-20mA input, or is that just the output scaling for the gf Signet? The 7700 Ion analog input installation guide I found on the web said it comes 0-20mA, so 4mA would appear as 20% of the max configured range. Scales on both ends gotta match to get the right reading.

Put your ammeter in the gf Signet-to-ion7700 loop and see if it's still 4mA, give or take.

Can a current be induced? Not as likely as a ground loop caused by different ground potentials between the gfsignet, the ion7700 and the power supply. I assume the signet output is not loop powered directly by the power supply?

 

[meeko]

thanks for the quick replay,

sry should have descrbed better- the flow meter is connected but no flow- just sayed zeroed so people would no for sure its zero or no flow.

and also yes the ion is 0-20, which means if there is less than 4mA then i go into negatives 0mA = 50, 4ma = 0,

Also, yes my ammeter on the loop (gf to ion) when there is no flow is 4mA, but on the ion 7700 display I get 10.6 instead of 0. as flow rises its just 10 higher then it should be.

There is 1 single 24vdc power supply in the panel- this supplies power to the signet and the ion and to all process equipment. - could this be causing a ground loop in the meters? its a 2 hr drive to job site so im just checking with everyone here before i go for possible solutions to check.

- loop powered?- not sure what you mean, all devices connected by same power supply?

 

[danw2]

OK, you've got -50 to 200 gpm scale for the 4-20mA from the Signet. That looks good.

The Signet 9900 manual is on-line.

I have no idea how to interpret this statement:
Isolation: ... Low voltage (< 48 V AC/DC) to loop with DC power connected
No isolation when using loop power only (spec on page 61)

The wiring diagrams for loop power on page 15 has an error; on the lower right, on the "9900 terminals" drawing, the polarity of the '32Vdc power supply' terminals is reversed, assuming the Process Pro next to it is correct (Process Pro loop power looks conventional).

Since on the "Connection to a PLC/Recorder, separate supply" wiring, they use power or auxiliary or loop connections for the analog output, who knows how isolated the Signet current output is in the Signet?

The power meter had no description of its analog input other than the input resistance and a 20V common mode limitation.

I'd take a loop isolator module with me, if it were me.

 

[meeko]

ok, yah i will pick up a loop isolator before i go. Manual says use isolation on loop, and dont use isolation on loop? o well im picking one up today either way hopefully it will work. thanks for the help

 

[GTstartup]

quote "and also yes the ion is 0-20"

The reader is 0-20mA and the transmitter is 4-20mA. This means at no flow the Ion will read 20%