false high flowrate

hie! I have been trying to add a scanner which requires a flowrate signal, to an already existing belt weigher to PLC loop. the moment l wire the scanner, the flow rate indicates high on the HMI even if there is nothing flowing on the belt. What could be causing this and how best can l wire the scanner without disturbing the HMI reading?

We need more specifics: What PLC and HMI are already installed? What scanner and flowmeter are being added?

You may need to troubleshoot the system first by verifying the flowrate signal is correct into another readout (i.e. is the frequency/ analog voltage or current what is expected?) If this is correct, is the scaling correct in the PLC/ HMI?

Also make sure you are not connecting a 4-20 Milli-amp transmitter to a voltage input without a resistor across the terminals.

RUE said:

hie! I have been trying to add a scanner which requires a flowrate signal, to an already existing belt weigher to PLC loop. the moment l wire the scanner, the flow rate indicates high on the HMI even if there is nothing flowing on the belt. What could be causing this and how best can l wire the scanner without disturbing the HMI reading?

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Ideally, you should use a signal generator to determine how the HMI responds to a known good signal at the input point to which you are connecting the instrument. This will insure that the input is functional, and will also verify the range of the HMI display, if you don't already know that.

Then you should verify with a multi-meter that your instrument is generating a signal of the appropriate range for the input to which you are connecting it.

If you have a valid connection between the analog input and the instrument, you could still have a problem with a ground loop if your instrument is non-isolated, which is pretty rare these days.

As has already been mentioned, many PLC 4-20mA analog current inputs are actually 1-5 VDC voltage inputs, and you have to install a 250 ohm resistor in parallel with the input terminals when you connect your instrument.

Thank you so much. I had not connected the terminals properly and the current was not flowing through the circuit. I am amazed at how answers came from you guys, I really would love to be of some aid by giving and answer to someone and not just ask questions. thanx for you time.