Flow meters

Flow meters typically come in two varieties- pulse output or 4-20 milliamp DC current output. Either the pulse signal or 4-20 mA signal is connected to the proper input on the Micro-Comm PLC.

A pulse output type flow meter means that a single DC pulse from the meter represents a number of gallons. Based on the manufacturer, a single pulse could represent one gallon or as many as ten per pulse. Usually this information is printed on the meter head or elsewhere in the documentation. Testing of a pulse type meter connected to a Micro-Comm RTU will require the use of a voltmeter that has the capability to measure frequency in hertz. The frequency is cycles per second, so to calculate GPM, multiply the frequency reading on the voltmeter by 60. Then, multiply by the pulse per gallon number specified by the flow meter manufacturer.

The 4-20 mA type flow meter will provide a flow rate where 4 milliamps is zero flow and 20 milliamps is the full scale output. Any current in between algebraically can be calculated to represent gallons per minute. To calculate flow, use a voltmeter set to read DC milliamps. Take the number you read and subtract 4. Divide that number by 16 and you will have a decimal/percentage. Multiply that percentage by the full scale output of the meter to determine the current flow rate.

Example:

Your measurement shows 11.23 milliamps. Full scale output of the flow meter is 1000 gallons per minute.

11.23 mA subtract 4 = 7.23 mA

7.23 divided by 16 = .452

.452 X 1000 = 452 gallons per minute.

Troubleshooting

It is rare that a flow meter will give erroneous readings, although it does happen. More often, a pulse type meter will break and not give any signal at all. A 4-20 mA output meter may give a low value like zero, or well below the 4mA minimum. In some cases, the meter may put out current in excess of 20mA. In any case, if you read current well above 20mA or below 4mA, there is most likely a problem with the meter output unit.

M1500, M1550, M1650, S3000, S4000, S4500 and similar PLC’s have a light associated with the pulse input that will flash when the RTU is receiving a pulsing signal from the flow meter. Consult your RTU drawing to determine exactly where the flow meter in question is wired in so you can verify the correct light is flashing.

The only valid way to test a 4-20mA signal is by use of a voltmeter described above.

More information about flow meter diagnosis is available here

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