4-20 mA water level sensor

Hope someone can help me on this

Currently there is hydrostatic level transmitter (4-20 mA, 2 wire current loop with 10-30 Vdc supply needed) already installed on location (headrace canal of small hydro power plant) and connected to existing PLC.

Our company is doing some upgrades and we'll have separate PLC for controlling some of the plant functions. So is it possible to connect this existing 2 wire sensor parallely to our newly installed PLC (analog input 4-20 mA) or we'll need another level sensor in order to get water level variable?

No- you cannot parallel a 4-20mA signal. You could run in series but I would not recommend it due to ground loop or power supply issues. The best is to get a signal splitter that will output 2 isolated signals. Phoenix, Acromag, PR electronics are a few that sell them.

Thanks for the help, so basically this is what I need: http://www.prelectronics.com/about-pr/news-and-events/news/3109-signal-splitter ?

karlek said:

Thanks for the help, so basically this is what I need: http://www.prelectronics.com/about-pr/news-and-events/news/3109-signal-splitter ?

Click to expand...

Yes- or the 3108 I believe will do "only" current signals and may be cheaper.

It depends on what type of input the PLC uses, I always use 10 volt inputs and a resistor of 500 ohms , or 250 ohms.
Voltage signals can be used multiple times in paralel.
If the input is isolated from rest (so it has a mA input and a connection mA output, and not directly going to ground you can put them in series, Like the WAGO/Beckhoff has isolated channels.
Or repeat the channel to an output.

Yes, we are using Beckhoff embedded PC. What exactly do you mean by repeating the channel to an output, what I/O cards need to be used?

shooter said:

If the input is isolated from rest (so it has a mA input and a connection mA output, and not directly going to ground you can put them in series,
Or repeat the channel to an output.

Click to expand...

IMHO Yes, I also would add my analog in, in series as long as I was certain of my power supply and loop isolation issues, as an old loop guy. This may not be the best method for everyone. There are definite considerations to be aware of, as with the next option, one must be aware of powered and unpowered loop requirements, isolation and power supply ratings etc...

If you have an extra analog output, (call it analog out 2) in YOUR new unit w/ imbedded I.O., you could feed the existing loop only to YOUR analog in, (keeping the powered/ unpowered concepts in mind) then, in YOUR logic, (in addition to whatever else you were going to do with the signal) move that value to your analog out 2 and feed the existing PLC's analog input with that. (Keeping the same powered, unpowered concepts in mind) i.e. "repeat"

Just my humble 2 cents,

Regards

https://www.youtube.com/watch?v=0v9ZCD3oL1M

long but comprehensive

Whether you parallel a voltage signal (from the dropping resistor across the analog input) to a 2nd AI or run the current signal in series to a 2nd AI, the risk is common mode voltage between the sender and either of the receivers or between the two receivers. 'Single ended' AI's are very likely to have common mode problems.

If the common mode voltage level is low, it might result in just an offset of the interpreted readings.

If the common mode is high, but not destructive, it could drive both signals off scale

If the common mode is high and destructive it could burn out either analog output.

Try it and see, but if you get a shift in the signal when you connect the wires, there's common mode.

Signal splitters typically isolate the input from the output and isolate one output from the other output, so there is no 'common' mode between the sending device and receivers, or between the receivers.

Sherlock4me said:

If you have an extra analog output, (call it analog out 2) in YOUR new unit w/ imbedded I.O., you could feed the existing loop only to YOUR analog in, (keeping the powered/ unpowered concepts in mind) then, in YOUR logic, (in addition to whatever else you were going to do with the signal) move that value to your analog out 2 and feed the existing PLC's analog input with that. (Keeping the same powered, unpowered concepts in mind) i.e. "repeat"

Just my humble 2 cents,

Regards

Click to expand...

Yes, of course, after you acquire the signal with analog input you just feed it to to analog output (by means of software/program), I had some other thing in mind, nevermind

Thanks everyone for the help, now I have a bunch of directions and tips to work with.

Do you even need to do any of this? If the PLC's are networked you could just share the level reading across the network.

If the card has isolated inputs (for each channel separate in and out ) so no common ground.
both versions are available. so if 1 is in and 2 is out, you can use them in series with another PLC as the input is isolated to 4000 volts
they have sometimes 2 channel (they are isolated, but the 4 channel has both possibilities.