Dear Sir;

I have a problem about a flow totalizer that it is a local indicator. it needs to a 24VDC power supply. but we don't need any output signal (4-20mA). our vendor has proposed some loop power flow meter.

it is my question:
If I purchase a loop power flow meter, but I connect it to power supply (24VDC) directly, the purchased device will work correctly? There is not any analog input module for it.

 

No, connecting a DC power supply to a loop powered meter will not indicator or totalize or do anything and is likely to damage the meter.

A loop powered indicator does NOT regulate the current in a loop like a 2 wire transmitter does. A loop powered indicator uses the loop current and drops some loop voltage which drives its internal circuitry, but it can not regulate the loop current.

If you connect a 24Vdc supply across the (+) and (-) terminals of a loop powered indicator, there's the possibility that it could burn out the input, because there's nothing to limit the loop current.

The spec sheet for a loop powered indicator on my desk says it drops 3V as an indicator, equivalent to 150 ohms of loop resistance.
24V/150 ohms = 0.160A I^2R power = 0.160A*0.160A*150ohms = 3.8W which might be enough to do some damage.

 

If you wire a 24 VDC power supply in series with a loop powered flow meter, it will work. I would also wire a 250 ohm range resistor in series just to limit the short circuit current.

 

My reply was not clear that I was referring to loop powered digital indicators. Loop powered digital indicators can include a totalizer.

These devices do not directly measure flow, they just interpret a 4-20mA loop current signal, in this case from a flow meter, and display one or two digital values (flow rate, total). These devices can not regulate loop current and cannot be connected directly to a 24Vdc power supply.

dwpatterson is also correct when he states that a loop powered flow meter can be connected to a 24Vdc power supply. There are any number of flow technologies that have 2 wire, loop powered flow meters, many of which include a totalizer. 2 wire loop powered devices can be connected directly to a 24Vdc power supply because any 2 wire loop powered device can regulate the loop current.