4-20mA question

I have a Ozone Monitor with a 4-20mA output. The scaling for the O3 Monitor is 0 to 2.0 PPM, so 0 PPM = 4mA and 2.0 PPM = 20mA.......right? The problem I am having is when connect to an AD DL06 with 4 ch analog IN card it reads full scale (digital display reads 0 PPM)....4095 EU's. I spoke with their tech service guy he said to put a 1K Resistor between the two wires and check with a Multimeter. I am getting 130mA. I have also hooked the 4-20 out to a Micron Panel meter, it starts flashing and the numbers jump around. So, I called thier tech support again, this time I passed on to an engineer. He said that it is a common problem for prople to get incorrct reading from their O3 Monitor because it is a Current Sourcing Device and the meters or PLc have to be set up for this type of signal compaired to most 4-20 signals being passive. Does this make anysense to anyone...... I am lost need some help.
thanks,
Micheal

Current sourcing just means it has its own power supply and GENERATES a 4-20ma output signal on its own. Does this seem to be the case? You should be able to read 4-20ma with a MA meter with no connection to the PLC. Can you do that?

Stationmaster

yes it would seem tohave its own power source. When I wired it to the PLc Analog card I only used the two wires from the O3 Monitor. Yes I can get a reading with a MA meter without being connected to either the PLc or Panel Meter. I did at the tech said and placed a 1K resistor across the two wire. I placed the MA meter after the resistor and before the 4-20 out wire and it was reading around 130mA. I have used a MA meter between a Pressuretransducer and the power source before to get a mA reading. I hooked the MA meter up the same way, should it be different?\

Thanks,
Micheal

If a field device like a transmitter or in your case, analyzer is a Current Sourcing (or 'ACTIVE') device it is providing the 24Vdc to power the loop.

It also generates the 4-20ma signal.

If it didnt provide the 24v it would be a Current SINKING or 'PASSIVE' device.

In a loop there can only be ONE SOURCE. There can be multiple SINKS, such as a transmitter, a chart recorder and a PLC all in the same loop.
(there can be loop impedance issues if you use too many devices in the same loop, but in this case it won't apply.)

It sounds like you may have 2 SOURCES, the Analyzer and the PLC.
You can check by measuring for 24vdc at the terminals of each device with the loop wires disconnected. If they both have 24vdc then they are both ACTIVE or SOURCING devices.

If this is true, there are two solutions:

1) Make one of the ACTIVE devices PASSIVE (change it from a SOURCE to a SINK). Sometimes there are jumpers that do this, sometimes different terminals to wire to. As my old supervisor used to say , RTFM!

2) Use a loop isolating module. This will couple two active loops together and pass the signal.


Hope this helps.

SLC_Integrator said:

As my old supervisor used to say, RTFM!

Click to expand...

OK, I have to asked,,,,,,
Oh... But wait..... I get it......

Another consideration is that your PLC input card needs to be setup for differential, or isolated mode. This means you do not connect the (-) terminal to the common ground on the card or rack. The 4/20ma signal should just be looped through the input card.

I don't get the bit about the 1K resistor, if it is sourcing 4/20ma then it should read right on your current meter regardless of the load.

pw3 said:

I don't get the bit about the 1K resistor, if it is sourcing 4/20ma then it should read right on your current meter regardless of the load.

Click to expand...

I agree. The resistor seems to be a bit of a distraction. Unless the tech support guy wants it in SERIES with the MA meter (in the loop) to limit current into a particular range. That's NOT what is being described, but it may make sense.

There is also the concept of using a BURDEN resistor to convert a voltage signal to MA, but it takes more than 1k (I can't remember, 50k?, 5k?) Somebody?

Stationmaster

We use 250-ohm resistors in a 4/20ma loop to provide a 1-5v signal to our paperless recorders. We typically do this so the recorder can be unplugged/removed for service without breaking the loop.

What do you expect the meter/analyzer to read?

If the meter is ranged for 0-2.0 ppm and it is open to the atmosphere at 20% Oxygen, the sensor will saturate.

20% O2 is 200,000 ppm, some 5 orders of magnitude greater than your analyzer scale.

Dan​

​

danw,
O3
http://en.wikipedia.org/wiki/Ozone

Thanks, Mickey for the correction. I guess I read the OP too fast.

Well then, let's look at wiring.

The analog card wiring is in the options manual D0–OPTIONS–M at

http://web1.automationdirect.com/adc/Shopping/Catalog/PLC_Hardware/DirectLogic_06/Documentation/D0-OPTIONS-M

pg 62 (pdf):
"The F0–04AD–1 does not supply power to field devices. You will need to power transmitters separately from the PLC."

So either the transmitter must power the 4-20 internally or you need to supply external power to the transmitter. The PLC does not provide transmitter power.

Does the O3 analyer supply internal power to its 4-20mA output, or does its output require external power?

Which of the wiring options (see below) are you using?



"The F0-04AD-1 provides 125 ohm resistance for each channel." (pg 63 pdf).

That 125 ohm resistor develops a voltage drop, which means that you should be able to put a DC voltmeter across the plus & minus input terminals on the DL06 input card and read 0.5 to 2.50 volts dc proportional to the 4-20mA output:
0.004A * 125 ohms = 0.50V
0.020A * 125 ohms = 2.50V

When you put a DC voltmeter across the input what voltage do you read and what should the output of the transmitter be?

Dan

Dumb question - does the O3 analyzer have a local indication, and what does it show?

1) Let's assume the ozone meter is a four wire device. In this case the top wiring diagram danw provided applies. You need to check the transmitteri to make sure.

2) I don't know houw you managed to get 130 mA out of the ozone transmtiter. Verify the instrument set-up is correct for 4-20 mA output. Some of these analytic instruments have menus or dip swtiches to set up lots of output ranges.

3) Pressure transmitters and such are typically two-wire loop powered devices, and are wired like the bottom two instruments in the danw diagram.

I've use both types mixed on a single input card successfully. Draw out your loop diagram and make sure you have continuity and correct polarity. Current is out of the + terminal on a four wire transmitter, and goes into the + terminal on the PLC.

The brand/model of the ozone monitor may have been nice to know. I looked up a few and many offered the option of 4-20ma or 0-10vdc. At first I was thinking it may have been set for voltage.

I have no idea where the 130ma comes from, unless my calculations are wrong that would mean there was a 130v drop across that 1k resistor. You sure the meter read 0.130 or could it have been .013?

If it really is outputting 130v then I assume it may be 120/240vac powered and something is allowing the input voltage to get on the output line.

Technically a decent multimeter should not need anything to test a 4-20ma transmitter, just connect it in series with the 4-20ma loop.

Another thought, if there are other analog devices connected to this card and they use a separate power source that may have an effect. The picture shown for the AD card shows the 4 types as being connected to a single source separate from the devices.

Could you tell us what monitor, maybe link to the manual, then someone may see how it needs to be connected.

Thanks for all the help thus far. The Ozone monitor is an Ecosensors DOM-1. The manual can be found here.....http://www.ecosensors.com/dom1.pdf
I have not been able to work on it today. As far as hooking it up to the PLC it was a AD DL06 with an F0-04AD analog card. When I wired it to the PLc I had nothing else connect to the PLc and I only used the two wires from the DOM-1. With nothing in the Logic except the required setup for analog the analog value was full scale, 4095 EU's. I hope tp be able to work on it some this afternoon.

Thanks,
Micheal