4-20mA sensor measured at two locations
4-20mA sensor measured at two locations
(OP)
Normally the return wire of a 4-20mA sensor terminates with a 100 ohm resistor at the input to a data acquisition system.
My question is can you have two return wires each terminated with a 200 ohm resistor monitored by two data acquisition systems. The parallel resistance would equal 100 ohms.
This would allow measurement at two locations.
Thanks
My question is can you have two return wires each terminated with a 200 ohm resistor monitored by two data acquisition systems. The parallel resistance would equal 100 ohms.
This would allow measurement at two locations.
Thanks





RE: 4-20mA sensor measured at two locations
I'd assume that you are supposed to put the stuff in a daisy chain to maintain the current in the loop at the correct value.
TTN
RE: 4-20mA sensor measured at two locations
RE: 4-20mA sensor measured at two locations
Unfortunately I have one sensor powered by two independent 24v power supplies that must be monitored by two data acquisition systems, one or the other is the backup. The systems are located about 100ft apart. Each power supply has a diode on the + output inturn connected to the + input of the sensor.
The sensor's 4-20mA return output will then be split into two loops each terminating with a 200 ohm resistor, ie., each loop will conduct 2-10mA in series with a 200 ohm resistor to the appropriate supply ground. With 200 ohms the voltage measured will be 0.4-2.0 volts.
From what I can tell the sensor never knows it is powered by two loops.
Are there any problems in doing this?
Thanks
RE: 4-20mA sensor measured at two locations
RE: 4-20mA sensor measured at two locations
This should be trivial if your data acquisition systems have differential measurement capability. Simply place the resistors in series and measure the voltage drop across the resistors. If these are multichannel data acq systems, you can also use two single-ended inputs on each resistor and then take the difference in post processing.
TTFN
RE: 4-20mA sensor measured at two locations
This response is probably way too late, but here goes anyway.
The only applications I've seen with 4-20mA loops terminate
on a 250 ohm resistor at an input. You can actually measure this resistance at an input with an ohmmeter.
Typically, you can have up to three 'devices'(read loads) in a loop, for a total loop impedance of 750 ohms (some can only work with 600 ohm max. which means two devices)
You must connect these devices in series in order to utilize
the loop signal - just make sure that you can have only one
source of power and one sensor! In most situations, your power supply for the loop is external to the actual input so you need only modify the wiring to suit the application.
RE: 4-20mA sensor measured at two locations
My problem with your solution is that my input amps have to be referenced to ground and can not float up. By using two separate 200 ohm resistors in parallel runs from the sensor I actually split the 4-20mA in half. Each resistor then "sees" 2-10mA and generates 0.4-2.0 volts.
The sensor's power supply and ground would be common to each 200 ohm reistor.
See any propblem with this situation?
RE: 4-20mA sensor measured at two locations
I don't think I've been much help here - I just tossed in
my two cents worth.
I'm not familiar with the type of input you're using.
I have seen cases where only part of the range of a 4-20mA input is actually utilized, but what you're suggesting is actually going to be out of range for a standard 4-20mA
(1-5V) input, especially at the zero signal point.
If I understand you correctly, you're going to have two separate sources of power running through this sensor. Even with a common reference point (ground) this setup could be a little flaky - it just doesn't sound right.
If your inputs are voltage, rather than current reference, you could create one loop with one sensor, one source of power, and one 250 ohm resistor. Ground one side of the resistor, one side of the power supply, and take your signal from between the resistor and the sensor. This kind of defeats the purpose of having a 4-20mA loop, though.
Let me know how you end up doing it, OK?
RE: 4-20mA sensor measured at two locations
I think I see your point of having redundancy: If one power supply fails, the system will continue. (No problems here because of your diodes.) If one recorder/controler fails, the system will continue.
However, consider what happens if one of your resistors opens or shorts. Both recorder/controlers fail. For example, suppose the sensor outputs 10mA because the PV is at half-scale. If all is well, each resistor will see 5mA (1 volt). If one resistor, or loop opens, the sensor will still output 10mA and the remaining resistor will see all 10mA (2 volts).
If you have just a single (series) loop, only an open circuit or short from sensor output to ground will cause both recorder/controlers to fail.
RE: 4-20mA sensor measured at two locations
I'm kicking myself for not thinking of this sooner.....
You didn't mention anything about a budget, but if it's not too tight, consider this option: How about a single loop, using the one sensor, either powered from the measuring device or from a separate power supply, and feed the signal through a loop isolator at each of your data acquisition systems.
That would be the simplest, easiest solution - but it'd cost a few bucks.
I still think two power supplies feeding the same sensor will lead to nightmares - and worse.
RE: 4-20mA sensor measured at two locations
RE: 4-20mA sensor measured at two locations
TTFN
RE: 4-20mA sensor measured at two locations
Chris Parrish
I&C Specialist
Dekalb County, GA Water & Sewer