Length of analog cable

Got a call from a friend who's trying to move a recording device about 600 ft away from a monitoring device. The recorder uses 4-20 ma as its input. Is this possible? I've never run analog more then a few feet.

Thanks
Olias

Possible????

Yes it is, but not advisable due to excessive loading by all that wire. Tell your friend to consider using either line drivers or some other form of transmitter to overcome the distance. Direct from the transducer to the recorder with out help is ify at best.

Got any ideas for transmitters or line drivers?
I know nothing about running that distance.

Re: Possible????

randylud said:

Yes it is, but not advisable due to excessive loading by all that wire.

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Guess I have too disagree with that statement. Olias stated his device used a 4-20mA current loop which by design are immune to effects from cable lengths. You will have to look at a data sheet for the device that is generating the current loop and see how much total resistance it can support. If the device uses an external power supply, you can overcome higher resistance by stepping up the voltage. The receiving device will also have resistance (in fact it will probably be the highest resistance in the circuit, typically 250 ohm). When you consider that the resistance of the long wire will be very low, (1-2 ohms at most) the length of the cable should not be a problem. As a quick test, grab a spool of wire with 600+ feet of wire on it and connect your device to your recorder locally i.e. have your two devices a few feet apart so you can see them, but use the spool to create the effect of the long cable run. If it works with a big spool of wire, it should work with a straight run of wire of the same length.

A more important issue IMHO is the routing of your signal cable. Make sure you keep it seperate from any noise generating sources. Don't run it in any conduit or cable trays that contain high voltages. A current loop is inherently immune to noise but can still suffer.

Gregg is right - a 4-20 mA signal will read correctly up to the point you exceed the total load limit of the transmitter. This is usually 500 to 1,000 Ohms. Since a typical PLC input or indicator resistance is 250 Ohms, you can run a lot of wire. Back to basics - Ohms law says V = I x R. The resistance, in Ohms per thousand feet at 149 degrees F, of various sizes of wire is:

AWG           Ohms / 1,000 feet

22            19.0

20            11.9

18            7.51

16            4.73

14            2.97

12            1.87

If you have 1,200 feet of 18 AWG wire for example, your extra load is 9 Ohms, and the additional voltage drop with is 0.18 Volts. I routinely run 4-20 mA signals for this kind of distance without problem. I usually set my power supply output to 24.5 VDC as a precaution, but it really isn't neccessary. Most loop powered transmitters have a range of 12 to 40 VDC or thereabouts, with the allowable load determined by the voltage. A drop of 0.18 Volts won't matter much.

As long as you use shielded twisted pair for the signal and ground the shield at one end, using the precautions in routing that Greg gives, you should be just fine.

Thats what I thought, but thats why I come here. Thanks to all.
My buddy is running twisted pair now. I'll let you know if he runs into trouble. Thanks again.
Olias

really?

Guess I have too disagree with that statement.

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and you would because your opinion is what in relation to others?

a 4-20mA current loop which by design are immune to effects from cable lengths

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except when you depend on it being imune and for some reason it turns out not to be.

a 4-20 mA signal will read correctly up to the point you exceed the total load limit of the transmitter. This is usually 500 to 1,000 Ohms. Since a typical PLC input or indicator resistance is 250 Ohms, you can run a lot of wire.

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see above.

Back to basics - Ohms law says V = I x R

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if everyone of us remembered this basic law, we all would have fewer problems relating to electricity. But thanks for the reminder!

Re: really?

randylud said:

and you would because your opinion is what in relation to others?

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Seemed clear to me.

Me too!

Me too Rick!

Yes it is, but not advisable due to excessive loading by all that wire.

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I would have to disagree with that one too because we have current loop (original post was saying something about 4-20mA).
This is where current loops are at their best.
You get better noise immunity because of low impedance of the loop.
Changes of cable length don't matter because 4mA is 4mA regardless if
you measure it at the beginning or at the end of loop. If the signal was voltage, this would not be the case because of voltage drop in long cable.
We were using current loops for devices which were ca. 1km (3000ft) away. Cable was pair of #16 conductors with foil shield (cheap enough) with shield grounded on the end in control panel.

Greg and Tom are correct. There is a practical limit based on the wire resistance and connected load impedance of the receiving device, but I've personally run 4-20 ma signals well over a mile with no problems.

jUST wHAT i Needed!

tHANKS a Lot..

wHAT aBOUT IF i coNVERT a 0-10vdc FROM a tEMPERATURE SENSor TO 4-20ma uSING a sIGNAL cONVERTeR & RUN that FOR a km oR SO, wOULD IT stILL bE the SAMe?

theripley said:

wHAT aBOUT IF i coNVERT a 0-10vdc FROM a tEMPERATURE SENSor TO 4-20ma uSING a sIGNAL cONVERTeR & RUN that FOR a km oR SO, wOULD IT stILL bE the SAMe?

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At the end of the km, you would have a 4-20mA signal.

Read Tom Jenkins' post. As long as you have enough voltage to drive all the devices (including the wire) in your current loop, you will have 4-20mA signal.

use wire with cores higher as 1 mm and you will be good. use a calibrator and calibrate the loop to be sure. the resistance of the wire to ground (leaking current could be of influence a little.