ANALOG INPUT to choose? 4 ~20mAmp or 0 ~ 10VDC

we are ordering some linear gauges for a R&D project, since I've never used ANALOG INPUTS, and have some confusions, when it comes to accuracy and reliability.

Most users prefer 0 to 10VDC, but prefer 4 to 20mAMP, claiming this gives a bigger scale, (more readout precision),
some claim mAmps are rather unstable and susceptible to noise.

all this tech talk has me more confused, can some one shed light on this?

In terms of precison of the output and reliability of the instrument there isn't any real difference between the two.

It used to be that voltage inputs to PLCs and voltage output transmitters were less expensive, but that is no longer the case.

I much prefer 4-20 mA, and that is generally the trend in the states. In Europe voltage signals are still more common, but I think that is changing.

The mA signal is less susceptible to noise, not more. A mA transmitter can be two wire (loop powered) while voltage outputs are usually four wire.

I think Tom summed it up rather nicely. The one advantage of the 4-20 mA signal that he didn't mention though was the difference in ability to detect a failed signal.

The 4-20mA signal has a 'live' zero point, the 0-10 VDC signal does not. With voltage, 0 volts is 0 volts--one can't tell if the wire/transmitter broke or the tank is just empty.

See Kirchoff's laws.

From the first law we can state that a 4-20mA signal will not degrade over long distances as it compensates for any voltage drop in the wiring and junctions. The current flowing in a circuit with no branches must be exactly the same at all points in the circuit.


From the second law we can state that the measured voltage at any point in a circuit is not the same at every point in the circuit. A 10V signal might be exactly 10.0V at the sensor, but after traveling through the wire (which has a real resistance) and connections (which all have resistance) the voltage at the instrument will be something less than what existed at the sensor. The magnitude by which it is less depends a great deal on the circuit and in many cases where good wiring practices are followed it is not significant, but it still must be less. The longer the distance from the sensor to the instrument the greater the voltage drop.

4-20mA is better in my opinion for the reasons stated above. However it is not as easy to troubleshoot as a 0-10vdc signal. With the volts you just pop your meter leads on the wires and you see the voltage.

I like to fuse my 4-20ma inputs with 32ma fuses. It makes for a great way to isolate the circuit for a variety of reasons - you want to change the sensor without powering the panel off, etc. Also you can put a meter set to check mA on the fuse terminals and then pull out the fuse. Your meter completes the circuit so you get a reading, then pop the fuse back in.

Paul B said:

4-20mA is better in my opinion for the reasons stated above. However it is not as easy to troubleshoot as a 0-10vdc signal. With the volts you just pop your meter leads on the wires and you see the voltage.

Click to expand...

Fluke has solved that problem. See link



http://www.transcat.com/Catalog/productdetail.aspx?itemnum=45876EL

Mickey said:

Fluke has solved that problem. See link

Click to expand...

That thing is awesome! I have been wanting one for some time now, but they are waaaay too expensive. Even used on eBay they are like $400 bucks! Crazy...They are nice though, I'm sure they save tons of time.

CowDung said:

The 4-20mA signal has a 'live' zero point, the 0-10 VDC signal does not. With voltage, 0 volts is 0 volts--one can't tell if the wire/transmitter broke or the tank is just empty.

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Usually you use 2-10V or 4-20mA and 0-10V or 0-20mA so the zero is alive in the first voltage case here.
With that setting you can choose if you want voltage or current with a simple 500ohm resistor between signal and common.

I have a Fluke 771. Its a huge time saver - definitely worth it if you do much work with 20mA loops.

yep saves me a ton of time

Paul B said:

I like to fuse my 4-20ma inputs with 32ma fuses. It makes for a great way to isolate the circuit for a variety of reasons - you want to change the sensor without powering the panel off, etc. Also you can put a meter set to check mA on the fuse terminals and then pull out the fuse. Your meter completes the circuit so you get a reading, then pop the fuse back in.

Click to expand...

That is an awesome golden nugget of an idea.

Thanks

Thanks Okie, I'm glad I could finally contribute something to help YOU for a change.

32ma fuses are high-resistance and were installed in a panel with 0-10vdc inputs by mistake. They were dropping quite a bit of voltage, as I'm sure you can imagine.

OkiePC said:

That is an awesome golden nugget of an idea.

Thanks

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I second that! I never thought of that before, great idea!

it is money dependend.
best is 4-20 mA but it involves electronics inside the gauge so more expensive
if needed simple use a linear potmeter slide type and a voltage input.
if speed is not important switch it with some relais so you only need one input.

4-20mA is year 2000 0-10v was the past easy solution

too much advantage of ma close loop circuit VS 0-10

but you can simulate 0-10 with a pot and ma is also easy but need to look for right resistance value