Flow meter troubeshooting

In most cases, incorrect readings from a flow meter are the result of broken electronic pickup unit. To diagnose, first you must determine what type of flow meter you have:

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.

Troubleshooting

It is rare that a flow meter will give erroneous readings, although it does happen. More often, the meter will break and not give any signal at all OR give a constant full scale reading.

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.

Flow total issues- In the SCADAview program, check to make sure the FLOW RATE shown on the screen is correct and in agreement with the current flow rate on the meter register itself. This may take two people- one at the computer and one at the meter at the same time to confirm the reading. Logically, an incorrect flow rate will result in an incorrect flow TOTAL. If the flow rate is incorrect, the meter may be putting out the incorrect value OR a setting has been changed.

If the flow rate is correct but the total is off, contact Micro-Comm for assistance.

More information about flow meters is available here

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