4-20ma splitter

[redlinej]

Hi guys,
I have four fuel storage tank feeding two different power plants ,each tanks have two p/i tranducer for each plant level reading on the hmi ,my problem is that most sensor on the tanks not work and need replacing,but i was thinking of only buying four p-i tranducer and use 4-20ma splitter.
Anyone have any experience with these splitter would like some advantage and disadvantage.thanks

 

[xnuke]

What would you gain by using splitters? Right now, you have redundant measurements on each tank, so if one transducer fails, you have a backup measurement. If you go down to one transducer per tank and it fails, you will have no idea what level is in the tank. Not only that, if all of your four transducers are working and you install splitters, even though you will still be showing eight measurements on the HMI, you won't really have eight measurements because you'll have four pairs of measurements with identical readings in each pair due to the splitters. This gains you nothing. Replace your broken transducers. I know it's more expensive than installing splitters, but think about the cost to your operation if you have a failed transducer on a tank and the tank leaks and empties all of its fuel. If that happens, you've not only the cost of lost fuel, but you have the cost of cleanup as well.

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[Compositepro]

Instrument redundancy is a separate issue. You can put as many receivers as you want into one 4-20 loop, as long as you have enough supply voltage and the loop resistance is not too high.

 

[xnuke]

Compositepro's statement is almost correct. You can connect multiple receivers in series if either every receiver or all of the receivers but the last in the loop are able to function correctly when floating above the zero potential of the power supply. Most display indicators, signal converters, and signal splitters can be floated and will function correctly. However, many analog controller inputs are nonisolated and require that they be installed as the last device in the loop because they return the signal to the negative of the power supply via the card's connection to DC common. Thus, putting two controller inputs in series will often give one input that reads correctly (the first in the loop) and one that reads incorrectly (the last in the loop). In that case, a splitter would be required for both inputs to read correctly.

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[Compositepro]

Xnuke, I think I see your point but I've never seen an industrial instrument with a non-isolated 4-20 input. And I don't know why they would exist because of the problems you mention in addition to the fact that that would create a ground loop that would introduce all kinds of noise into the signal.

 

[xnuke]

Almost every PLC, DCS, and DDC manufacturer has multichannel analog input modules with non-isolated inputs (when I say non-isolated, I mean the channels share a common terminal, not that they aren't using optoisolation from the input circuitry to the interior electronics). They exist because they offer twice the input channel count on the card than you can get with isolated channels. That reduces cost a lot if you have a ton of analog inputs required since you only have to buy half of the number of analog input cards. They don't typically create ground loops when connected properly, but you cannot connect two input channels in series as I mentioned above. Very few 4-20 mA signals I've seen in process control require isolated (i.e., differential-mode) analog inputs, except those from separately-powered (4-wire) transducers. For those, signal isolators are usually provided separately and the repeated signal is then used on a non-isolated input.

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.