# 4 - 20 mA???

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Thread Starter

#### Rakesh

Can anyone tell me why 4-20mA is standard in industries?
Also why 3-15PSI???
This is the regular question I am facing at various places.
Thanks in advance.

rgds,
Rakesh

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#### T. Connolly

There are several advantages to the 4-20mA current loop. This list is not all inclusive:
1) current loops are less prone to signal loss vs voltage signals over large distances.
2) By stetting the lower threshold at 4mA, two advantages can be realized; the instrument can still be powered directly from the loop if its total current consumption is less than 4mA making two wire loops easy to set up, and the 4mA ouput can be used to tell if the instrument is still functioning even when the process variable is zero.
3) The 4-20mA can easily converted to a 1-5 volt signal by placing a resistor across the end of the loop.
For pneumaitic controls which are much less common now but were very common at one time the same idea prevails, the 3psi serves to verify instrument function. I've never used an old style nozzele/flapper pneumatic pid controller, but I have seen them. I suspece that you would need to have some small pressure signal always present in such a device, otherwise the flapper position would be indeterminate and control action therefore unpredictable.

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#### Al Boake

Hello

4 mA is a "live" zero. If signal is << 4 mA, programming can detect this as an open loop condition. In plain english, the circuit has failed open.

Al Boake P.E.
Metso Minerals (USA) Inc.

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#### Jim Uthgenannt

In addition to the other comments...

1) Analog current signals are more noise-immune than analog voltage signals.

2) 4-20mA at 24VDC poses a low risk (of handling) to humans.

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#### R.Sundar

For 4 to 20 mA DC explanation, there is a very good Appliation Note from Measurement Technology Limited (MTL) called 2 wire transmitters. It is
number used to be AN 9002 or 9004. In addition to explaining benefits fo 2 wire concept, this AN also explians how to size power supplies for IS
applications.

For pnematic signals, 3-15 psi is chosen (and not 1-10 or 1-12 for example) because 3-15 psi is the linear range of pressure variation with respect to
flapper movement. For details, please refer to "Industrial Controls" by E.B. Jones.

Sundar

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#### Dean Reimer

Hi Rakesh,

4-20mA is a standard analog format because it is widely available, and, unlike voltage, can support long transmission distances. i.e. 4-20mA at the source comes out as 4-20mA at the destination. With a voltage signal resistance in the wire causes a drop at the destination.

As for the pressure, I have no idea.

Dean Reimer
Westroc Inc.

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#### Toni Kaesbeck

Dean (and all other 4..20 gurus):
What is long transmission distances on 4..20mA?
1 meter, 50 feet, 1 km? 3 miles?
Anybody, who can share experiences and anyone who has experience with a distance, it didn't work anymore?

best regards,
Toni

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#### DBaird

The drive distance would be only limited by voltage drop from long cable runs and could be calculated using ohms law and if excessive larger
conductor size normally eliminates the problem.

David Baird
Senior Controls Engineer
GEM CITY ENGINEERING
DAYTON,OH 45404
(937)-223-4600 x1456 FAX (937) 226-1908
[email protected]

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#### Dean Reimer

Holy cow! One simple answer and I'm a guru! If only everything was so easy.

Actually, I suggest that someone else answer this, because everyone else who has contributed to this thread is much more knowledgable than I am. To be honest, I've done very little work with analog I/O of any form. I know that in our
plant our longest 4-20mA cable is probably in the area of 200ft, and we have had no problems.

Dean

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#### Al Pawlowski

When they were introduced, they were found to work well and so became popular.

As far as I am aware, the main advantages over other current, or flow, signaling methods are the raised zero (4mA/3psi) and convenient amplitude
range. The raised zero makes it easier to read and allows power to be supplied over the signal media (wire/tubing).

I think Foxboro introduced the 4/20mA standard and maybe the 3/15psi too.

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#### Brett Haas

Verify the maximum capacitance away from the measuring ADC, e.g., cabling and sensing device, and see what effect the 1/RC time constant has on the circuit. E.g., a fast changing signal may not reach the ADC with the correct voltage (IxR), thus misreadings via time (phase) lag. Ohms law is a good thing.

Brett Haas

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#### Bruce Durdle

It depends on the circuit voltage, the voltage needed by the transmitter, and the resistance of the conductors.

4-20 mA will work as long as there is sufficient residuaol voltage at the transmitter to operate it when the source voltage and resistance drops are taken into account. Typical distances are of the order of 2 km, but IS barriers or other series components such as indicators can reduce this significantly.

Bruce

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#### Eminent

Length of transmission need not be a problem for 4 to 20mA signal under theorotical conditions. The power should take care of driving through the
load.

However, in practice, the design of the current control circuits in the transmitter or the way it is being measured at the measuring end could pose
some limitations. Therefore each transmitter comes with some resistance-impeadance-capacitance factors. Use these factors and use proper cables in order to get maximum transmission length. I have gone around 1000Metres w/o problems.

Anand

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#### Al Pawlowski

I once put in a tank level monitoring loop that was ~6000ft and it had very little trouble in the 3 years that I was involved with it. I took care of another that was ~4000ft and ran fine for 20 years before being replaced by a radio telemetry link.

Theoretically, the only limitation you have is total loop resistance. Practically, long loops begin to become tenuous because of the probability
that they will be damaged from digging, car wreck, water shorted connections, lightning strikes, gopher chewing, etc. I remember the ~4000ft loop I mentioned above being cut by back hoes about 4 times during its life. Of course, 4 in 20 years is not bad considering the cost of laying was low because it followed a pipeline and there was pretty much no other service the cable needed.

Al Pawlowski, PE
[email protected]
dba ALMONT Engineering
Baton Rouge, LA USA

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#### Kevin Totherow

The simple answer is that different instrument companies had different "proprietary" standards for transmitters and for pressure. The 4-20mA and 3-15PSI won the market share and thereby the project specifications and became the standards. Later, ISA and others wrote standards that specified what was already a defacto standard. 4-20mA uses a smaller loop power supply than 10-50mA transmitters - that was the main advantage over the 10-50mA - which is still in use in many plants. I think Foxboro used the 10-50mA.

3-15 PSI won out over 6-30psi and others.

Bottom line: You're looking to an engineering answer to a marketing question. They all worked fine. Slight advantages and disadvantages, but mostly it was a battle between proprietary systems of instrument companies. It really wasn't that important of an issue until the DCS had specific I/O capabilities. Eventually, someone won and a standard emerged without a multi-national organization.

KevinT

> Can anyone tell me why 4-20mA is standard in industries?
> Also why 3-15PSI???
> This is the regular question I am facing at various places.
> Thanks in advance.
>
> rgds,
> Rakesh
>

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#### Osmo Kokko

Also notice how all signal standards have a 5:1 ratio and a live zero (ie 4 mA, 3 psi) to detect supply problems easier.