4-20mA help - using a local display for a flow encoder between sensor and PLC

[emiller233]

we have a customer that has a DCS that is monitoring a flow sensor over a 4-20mA signal. we do not have any info on their DCS.


They would like to add a local display(for the flow) out near the sensor. and the displays that would be used is a Omron# K3HB-XAD 24VAC/VDC with a option card inside of it that can transmit a 4-20mA as well (Omron# K33-L1A)


https://www.ia.omron.com/products/family/1535/specification.html






The flow sensor is a FLOTECH F6206-AIV-DS


https://www.google.com/url?sa=t&rct...r-Manual.pdf&usg=AOvVaw0n8qYNOYO8_A9v6tZDN6ae




my question is, can i just add the Omron display in series between the flow sensor and the DCS, using the current + and - on the display?
or do i need to use the retransmit capabilities?

I've tried attaching the manuals, but it keeps saying there are errors. the links above should get to the data sheets for these (if they're not attached)

 

[petersonra]

most likely you can do it either way. however since you don't know anything about what is at the DCS the retransmission capability is probably the way to go.

 

[emiller233]

most likely you can do it either way. however since you don't know anything about what is at the DCS the retransmission capability is probably the way to go.

is there any ways to verify that? I tried calling Omron tech support, spoke to 3 people and no one could verify this device...? they couldnt even verify that it could supply power to the sensor (active vs passive device)

we wont be able to test these devices here, as this is a last minute addition to a system that is already torn down and in the truck for shipping. we will wire them into the box quick, but i don't want it to not function when they get powered up at the customers location. we wont be on site to fix it if needed...

 

[Ingenieur]

as far as I can tell you need a power source for the display (120 vac, etc)
it will supply power to the xmtr and display the xmtr output
then you can use the repeat 4-20 from the display to input to the plc

 

[don_resqcapt19]

I would just connect the display in series with the 4-20 mA signal and supply power to the display. Most transmitters can drive 2 or 3 250 ohm loads.

 

[emiller233]

Sorry, I do have a power source for powering the display (24VDC).
I would rather just power up the display and put it in series, as i cannot verify the display will power the transmitter. called Omron a few times with no luck...

I've never ran multiple 4-20mA devices in series. Just to verify, when daisy chaining devices i just connect them like:
+ from the XMTR, to the + of the DCS input card
- from the XMTR, to the + of this display analog signal
- of the display analog signal, to the - of the DCS input card

 

[emiller233]

I spoke with someone from Omron that was knowledgable with these, and he advised against running this display in series with the DCS and the XTMR. Only bc he couldn't find anything on his end that stated if it would (or how much it would, %) deteriorate the signal if wired in that fashion. He agreed that it 'should' be fine to wire in that fashion.
So we will go with this retransmit option that Ingenieur suggested. we will try do some internal testing on these to verify this if we have time after they get here, before it ships right back out...

 

[don_resqcapt19]

I spoke with someone from Omron that was knowledgable with these, and he advised against running this display in series with the DCS and the XTMR. Only bc he couldn't find anything on his end that stated if it would (or how much it would, %) deteriorate the signal if wired in that fashion. He agreed that it 'should' be fine to wire in that fashion.
So we will go with this retransmit option that Ingenieur suggested. we will try do some internal testing on these to verify this if we have time after they get here, before it ships right back out...

That answer comes from the transmitter. The information tells you how many ohms the transmitter can drive the signal into. The display information should tell you the ohms of the display input and the DCS should also. As long as the total resistance is less than what the transmitter can drive into, there is no loss in signal. it is a series circuit and the current is the same at all points. Where the total resistance is greater than what the transmitter can drive into the current will drop and the results will not be correct.

Most of the ones I have worked with would drive the signal into about 800 ohms. Typical devices are 750 (should read 250) ohms, so 3 devices in series and a bit left for the resistance of the conductors.

I did work on one where there were 4 or 5 devices in series. It worked fine until it tried to drive over 12 mA. Above 12 mA the signal did not match the process variable. It was not discovered for a long time as the normal process range was between 8 and 12 mA. It was a temperature loop and was discovered when they did a actual calibration check of the transmitter and took the temperature up to the point that the transmitter should have been putting out 20 mA.

 

[Smart $]

...
I've tried attaching the manuals, but it keeps saying there are errors. the links above should get to the data sheets for these (if they're not attached)

Enhanced data sheet...
http://www.ia.omron.com/data_pdf/ca...73.474571487.1507079602-1039516761.1507079602

 

[Smart $]

...
Most of the ones I have worked with would drive the signal into about 800 ohms. Typical devices are 750 ohms (?), so 3 devices in series and a bit left for the resistance of the conductors.
....

(?) Are sure you didn't mean 250 ohms?

I usually go up to 22mA on 24VDC system... 24V ÷ .022A ≈ 1kOhm . Higher voltage supply at 20mA works out to 1.2kOhm. As you mentioned, that includes wire resistance and the transmitter device itself.

 

[don_resqcapt19]

(?) Are sure you didn't mean 250 ohms?

I usually go up to 22mA on 24VDC system... 24V ÷ .022A ≈ 1kOhm . Higher voltage supply at 20mA works out to 1.2kOhm. As you mentioned, that includes wire resistance and the transmitter device itself.

Yes, it should say 250 ohms. Thanks

 

[emiller233]

Very helpful info, thanks!

I was looking at the data sheet for the display, and didn't see where this actually lists the resistance on the analog input signal for it?

Out of curiosity i looked up a few random 4-20mA devices and they all had a 250ohm resistance. is 250ohm a 'standard' for 4-20mA devices? Is this an accurate assumption?

Enhanced data sheet...
http://www.ia.omron.com/data_pdf/ca...73.474571487.1507079602-1039516761.1507079602

 

[Smart $]

Very helpful info, thanks!

I was looking at the data sheet for the display, and didn't see where this actually lists the resistance on the analog input signal for it?

Out of curiosity i looked up a few random 4-20mA devices and they all had a 250ohm resistance. is 250ohm a 'standard' for 4-20mA devices? Is this an accurate assumption?

250ohm isn't a standard that I know of. I'd say more like a recommended maximum.

For this device look at page 4, Input Range table, under Input Impedance 10ohm.

This device does not look like it can be loop powered. Look at Specifications on page 2. Power consumption appears to exceed 24V x 4mA.

 

[don_resqcapt19]

Very helpful info, thanks!

I was looking at the data sheet for the display, and didn't see where this actually lists the resistance on the analog input signal for it?

Out of curiosity i looked up a few random 4-20mA devices and they all had a 250ohm resistance. is 250ohm a 'standard' for 4-20mA devices? Is this an accurate assumption?

It is very common. Most devices actually work off of voltage and not current. 4-20 mA across a 250 ohm resistance results in a 1-5 volt signal.