Vortex Flowmeter

[learner08]

Hi!!

Can anyone help me on this question. I am looking at a vortex flowmeter and the transmitter load line resistance is given as max 550 ohm at 24V? Does this mean the signal output from the transmitter can drive a max load of 550 ohm at 24V or 43.6mA? How is this important when I source for a digital controller? What parameter in the digital controller needs this piece of information? Thank you!!

 

[badservo]

If your loop resistance is more than 550 ohms or your supply less than 24v then you will not be able to get to 20ma. the transmitter has an internal votage drop to power the internal electronics so you will not reach 43.6ma (unless something goes very wrong).

http://en.wikipedia.org/wiki/Current_loop

http://www.datel.com/data/meters/dms-an20.pdf

 

[danw2]

A 24Vdc power supply in series in the output loop will drive up to 550 ohms of loop resistance.

A lower voltage does not necessarily limit the output current ability, but it does lower the maximum load resistance possible in the loop.

The manual for the flow meter undoubtably contains a chart similar to the one below that shows how much loop resistance any given power supply voltage will drive.

The practical implications are that any analog input on a DCS/PLC/controller/recorder/whatever develops a voltage drop when the 4-20mA current runs through the input resistor.

Many systems use 250 ohms for an input resistor. Yesterday, I discovered Automation Direct uses 125 ohms. I've seen other values used: 1 ohm, 10 ohm, 62.5 ohms and 500 ohms.

The point being that the available voltage has to support the total load resistance.

If you are supplying the flow signal to only one device it is (almost) a given that it will be OK. If you run the flow signal in series to 2 or more devices, you have to add up the load resistances of each and if the wiring is a really long run, take the copper resistance into account too.

For instance, two, 250 ohm loads and 7.6 ohms of wire add up to 508 ohms, which would be less than 550 ohms that a standard 24Vdc power supply could drive.

Dan