Remote Metering by “piggybacking” existing meters

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:

Behind Panel:

Hi Raymond, your colleague is right, unless you know the genset really well, you will make nasty expensive smoke in your logger. You will need not-cheap isolated 4-20mA or 0-10V transducers. Earth potentials not being equal destroys data loggers regularly, it's quite good business for the logger makers, they get to sell it to you twice! The second time, you use the right stuff and avoid the destruction. Many automotive meters use gas pressure to read temp or unusual pulse width drive methods, not lending themselves to easy connections to loggers. Ask the old guys, they usually know this stuff, and approach it like your colleague suggests. Good luck, Will

I'm actually using the Moxa ioLogik E1200 Series - Daisy-chain Ethernet I/O for Distributed Data Acquisition Systems (http://www.moxa.com/Event/Net/2009/ioLogik_E1200/) 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. The output of these devices is Modbus TCP/IP. Surely the 'lamp' indicators can be monitored as these are just on/off? As I said particularly for the analogue voltage input with 10M input impedance isn't it just like putting a multimeter across the terminals, and what about using a voltage divider if the range is greater than 0-10V? Obviously Ill have to calibrate!

If your data logger has common earth terminals you can run into trouble. A multimeter has two wires, like you said, hard to get into trouble. But with many inputs, if the ground on the generator isn't consistant, or there are things like current meters where both terminals may float at the output voltage it's a different situation. If your data logger has totally galvanically isolated inputs, it'll work fine, so long as the isolation voltage is within limits.

I don't care if you don't want do the work of understanding what you are connecting to and burn the generator up. I'm just letting you know that I've seen some weird circuitss with generators in the past and I would trace the generator and metering wiring and draw my own diagram before doing any more.

It's about risk and consequences. If the risk is blowing a 50 cent transistor, try it. If the risk is wrecking a $10k generator, I'd do the work and be very certain. In this case, the risk is high, the consequences are high. You work it out. I developed a healthy respect for power circuits after a few years. When they get damaged, it's always expensive.

Frankly, do what you like, I don't care. I've wasted enough time. Good luck. Colin

Sorry I was not trying to be dismissive of your advice I am just trying to understand the possibilities of trying this, to determine if it is a common practice, which it is obviously not! I am actually a student intern and this problem seems like something I should already know, in my mind it still seems possible at the very list to read the on/off voltages of the indicator lamps as I said. I have posed these questions to ensure I do not make the mistake of "burning the generator up", I do not intend to just go and hook my devices, IF I attempted anything I would definitely trace the generator circuit to make sure I understood it's electrical function exactly and furthermore I would attempt to obtain a schematic from the manaufacturer as well as speak to a representative of the company and ask them if it is possible despite any warranty issues. I will of course heed your warnings however as I do understand the tradeoffs, and as I am managing this project under supervision from my managing director who is a very experienced engineer I will bring it up with him and he will probably explain it exactly as you and my colleague have, thanks for your time.