MIAC and 4-20 mA pressure transmitter

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go233
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MIAC and 4-20 mA pressure transmitter

Postby go233 » Thu Aug 18, 2016 6:09 pm

Hi,
I have connected the MIAC to a pressure transmitter as shown in the fig below:

I can measure with the multimeter V=1.312V at the input of ADC1 when the pressure transmitter is not under pressure (1 atm) (the MIAC ADC measure 26 in this conditions)
1)
If I connected the resistor properly I should have:
4 mA * 269 = 1.076V
20 mA * 269 = 5.380V
right?

2)
The MIAC system operating guide says divide by 20 to obtain the voltage from the ADC reading = 1.3V;
then there is an error due to the 10K input impedence of the ADC:
V real = Vm 10000 / 10000 + 269 (if I assume the output impedance of the sensor is very high) = 0.973
so, the real voltage is 1.3*0.973=1.265 ??? which is wrong because the multimeter measure 1.312V

3)
If I increase the pressure the reading of the pressure gauge is not the same of the one that I get from the MIAC.

What do you think?
Giovanni
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LeighM
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Re: MIAC and 4-20 mA pressure transmitter

Postby LeighM » Fri Aug 19, 2016 8:48 am

Hi Giovanni,

Your circuit does not show how you have connected to the MIAC,
you should be wired like this ...
current_loop.jpg
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Hope that helps,
Leigh

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Re: MIAC and 4-20 mA pressure transmitter

Postby go233 » Fri Aug 19, 2016 9:10 am

Hello,
I connect the MIAC ADC as shown in the following picture...sorry...

So, I have connected the MIAC as you suggested and when there is no pressure I have the same readings 26 from ADC and 1.312V from the multimeter.
To calculate the V measured = 26/20 = 1.3 volt
Then the impedence load of the MIAC ADC input 10000 / 10000 + 269 = 0.973
The real voltage measured is 1.3V * 0.973 = 1.265V
Is this right?
Giovanni
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Re: MIAC and 4-20 mA pressure transmitter

Postby LeighM » Fri Aug 19, 2016 10:16 am

Hi Giovanni,
If the MIAC ADC reads 26 then the voltage at the terminal is 1.3 volts, irrespective of the impedance.
Which agrees with your multimeter reading of 1.312V
The 269 Ohm resistor in parallel with the internal 10K of the MIAC gives a total resistance of 262 Ohms.
So that means that the sensor current must be 1.312/262 = 5mA
Hope that helps,
Leigh
edit: you could check the current by placing your multimeter, in series, into the current path.

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Re: MIAC and 4-20 mA pressure transmitter

Postby go233 » Fri Aug 19, 2016 12:17 pm

Yes, I agree 100%.!!!

this particular pressure transmitter sensor has to be connected in the way it is shown in the picture below.
At room pressure and temperature it transmits 5mA (what!!! I didn't know it! :shock: ).
So, when I install the control system I will need to zero the sensor relative to the room conditions (pressure and temperature).

The max load that I can add in the circuit is given by the manufacturer, in the specs:
from 0 Ohm
to (Supply Voltage-10)*50 = 700 Ohm (@ 24V)

What is my best resistor value to add in the current loop to convert 4-20mA current to Voltage ( in order for the ADC to read) ?
the MIAC ADC is linear between 0 and 12V and if I want to use all the ADC bits and achieve the best resolution I can use
Manufacturer: TE CONNECTIVITY / NEOHM, Farnell Order Code: 1083554, Manufacturer Part No YR1B590KCC
Through Hole Resistor, R Series, 590 kohm, 250 mW, ± 0.1%

With 590 Ohm
4 mA * 590Ohm = 2.36V
20mA* 590Ohm = 11.8V

Do you agree? :?:
Giovanni
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Re: MIAC and 4-20 mA pressure transmitter

Postby Steve001 » Fri Aug 19, 2016 12:54 pm

Hi Gihttp://uk.farnell.com/rhopoint/8g16d- ... 2004ovanni

we use these at work

http://uk.farnell.com/rhopoint/8g16d-250r-0-1/resistor-0-33w-0-1-250r/dp/9622004

or

http://uk.farnell.com/rhopoint/8g16d-500r-0-1/resistor-0-33w-0-1-500r/dp/9622012

then measure the voltage across the resistor this will be either

1 - 5 volts for 4 - 20 ma or 0 - 5 volts for 0 - 20 ma
2 - 10 volts for 4 - 20 ma or 0 - 10 volts for 0 - 20 ma


Steve

:Edit We also put monitoring in place so if we don't see 4ma then something has gone wrong and an alarm is raised
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Re: MIAC and 4-20 mA pressure transmitter

Postby go233 » Fri Aug 19, 2016 1:20 pm

ah! thank you steve!

I forgot to add that there is scale compression to take in account, too; the sensor can measure from 0 to 16 bar which will produce a 4 mA to 20 mA.
BUT If I add a resistor of 500 Ohm we have:
500*4mA= 2V
500*20mA= 10V
therefore a max pressure variation 16 bar will produce ONLY a max voltage variation of 8V so there is a compression factor of 0.5 .

I think the formula is:
Voltage measured = 0.5 * pressure in bar + 2
Pressure in bar = (Voltage measured - 2) / 0.5

I have attached the MIAC and Labview program.....
The MIAC is in slave mode and LabView talks to him using the MIAC USB from my PC.
Attachments
Pressure transmitter.zip
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Gas Switching System.fcfx
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