[thingstodo]

I purchased 14 surplus meters, tentatively 0 - 50 ua (micro-amp) for 0 - 100% of the display, from an ebay vendor several years ago for another project that did not happen. I tested them last night with a decade resistance box, a digital ammeter, and a 9V battery. 6 of the 7 that were shipped to me without shorting bars have a typical resistance but the meter does not move at all - so they are dead. All 7 that were shipped with a shorting bar installed work fine. I guess I should have checked on them earlier and installed shorting bars. I know for next time - a lesson learned.

The PLC analog outputs are 0 - 10VDC. I thought that they were 4 - 20 milliamp - I got the part number wrong. I can change the whole card from 0 to 10V range or -10V to 10V range. I have a couple of the meters that range from -500 - 500 ua, so I will likely have one card set for 0 - 10V and one set for -10 - 10V. 10V / 50 ua = 200K. Compared to this value, the resistance of the analog meter (around 3K ohms) is rounding error, so I'm going to ignore it.

The method I'm using
- adjust each analog output to maximum
- use the decade box to calibrate maximum deflection, starting at 300K just to be paranoid
- record the required resistance value
- select resistors from the junk box
- install on the meter, verify full scale at 10.0V

A search of my junk box(es) yields a few resistors. These resistors are put in series with the meter. I'm using at least two resistors to make up each value. One on the meter + and one on the meter -. The connections to the meter are covered with electrical tape to prevent unintentional contact. The network cable I'm using (category 6 solid core 24 awg unshielded twisted 4 pair) is crimped and covered with electrical tape (this is temporary, so I'm not using heatshrink). The results are listed for each meter.

DC amperes gauge - use 19K. Gauge uses 500 ua from 0 to full scale, so -10 will give -500, 10V will give 500. 16K7 + 3K3, 16K7 + 3K2

Square gauges - 50 ua to full scale. 181K. No resistors located yet

Small square meter - 50 ua to full scale, 192K. No resistors located yet

The search through the junk box for suitably sized resistors continues ...