4-20mA sensor measured at two locations

[madddog]

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

 

[IRstuff]

That's probably a bad idea electrically, since that means you are cutting your current in half, not to mention compromising the noise performance and general functionality of the loop.

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

 

[cbarn24050]

Hi, I'm not quite sure what you want to do here, the 4-20ma signal is in a loop so you can put measuring resistors anywhere in the loop, ie they are all in series, so long as the loop voltage is high enough to still drive the current round the loop.

 

[madddog]

Thanks for the replys.

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

 

[cbarn24050]

Hi, i can't quite see how you can have 2 loops.

 

[IRstuff]

The issue is that you compromise the signal quality by reducing the signal level. Both of your data acquisition systems should be run in series in the loop.

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

 

[grimmer]

Maddog,

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.

 

[madddog]

Thanks Grimmer,

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?

 

[grimmer]

maddog,

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?

 

[Paulcet]

Maddog,

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.

 

[grimmer]

maddog,

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.

 

[wombatsback]

this may be total rubbish but if you know you are splitting your current in half could you not amplify each side.

 

[IRstuff]

If the common-mode problem is such a big deal, build a differential to single ended converter just in front of the A/D. All this requires is an op amp and some resistors. If you want to get fancy, you can use three op amps, with two acting as buffers.

TTFN

 

[ICSpecialist]

You are adding the potential for noise, grounding, and isolation problems. Dual power supplies are fine, but not on a single current loop, and you do have a single current loop. The theory behind current loops is that outputs are SERIAL, not parallel. Stick to established conventions, and make all of our lives simpler.

Chris Parrish
I&C Specialist
Dekalb County, GA Water & Sewer