Piggyback metering

MetroEngineer
Member • Oct 11, 2011

Piggyback metering

Hi,

I am wondering if it is possible to "piggyback" off existing meters/indicators in a diesel generator?

The generator has traditional analog 'VU' meters for voltage, current, and frequency. It also has standard analog guage fuel meter, oil pressure, water temperature, rpm, and lamp/led indicators for overspeed, low oil pressure, low fuel level, battery charge, high water temperature and earth leakage.

Ideally I would like to monitor all these readings, I have the MOXA AD convertors with analog (0-10V or 4-20mA) and digital (Logic 0 = 0-3V and Logic 1 = 10 to 30V).

If I 'piggyback' off these device taking for example the voltage range that is applied to the fuel meter or the voltage applied to the lamp indicators will it affect the circuit? I realise that for the ammeter I would have to put it in series, if the current range is higher than the MOXA input I can just use a current transformer right?

I have the wiring diagram and the single line diagram but not the full schematic for this Japanese made generator. One colleague I spoke to said to not mess with the existing circuit and just add in the required metering (i.e. a new modbus output fuel meter etc), but it seems a waste when the relative signals are already there?

Is this possible?

This is what the panel look like:
#-Link-Snipped-#
Behind Panel:
#-Link-Snipped-#

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KenJackson

Member • Oct 12, 2011

If you are talking about the #-Link-Snipped-#, it has an input impedance of 500 Kohms. I don't know what it would be for a VU meter, but I'm sure the A/D input impedance will be huge by comparison, so it seems unlikely it will have any effect on the existing display.

It's likely that the current meter has an external shunt, so it's really measuring the voltage developed across a small resistance and displaying it as current. You could also use the same high-impedance device for it.

The downside could be accuracy. Is this for one or two units? Or for hundreds? If it's for one or two, you can calibrate each one manually, but that might not be practical for a larger operation.

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MetroEngineer

Member • Oct 12, 2011

I'm actually using the #-Link-Snipped-# series particularly the E1242 and E1210, the product specifications for the analogue inputs are 10M input impedeance, and the digital inputs are NPN, PNP, and Dry contact with 3K VDC or 2K Vrms isolation. Every other forum I have posted on they are saying like my colleague, not to touch the existing circuit, but as I said particularly for the analogue voltage input with 10M input impedance isn't just like putting a multimeter across the terminals, and what about using a voltage divider if the range is greater than 0-10V?

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KenJackson

Member • Oct 12, 2011
MetroEngineer
..., but as I said particularly for the analogue voltage input with 10M input impedance isn't just like putting a multimeter across the terminals, and what about using a voltage divider if the range is greater than 0-10V?
Yes, that sounds very reasonable to me. Did they say why?

Though I guess there are some concerns worth considering:
- If there is any conditioning for the signal to the VU meters, it may not be as accurate or precise as you might need,
- If the existing equipment is under warranty, modifications may void it,
- The modifications could get in the way of a factory technician who services the equipment, or
- If it's old equipment, it could be fragile with hard-to-find replacement parts.
But engineering is very much about judgment. Sometimes you have to pick the least bad choice.
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