Custom Shunt Calibration

[AquaticsLive]

I am working on making a new custom shunt to convert a Peterbilt amperage gauge from using a calibrated length of wire to a shunt.  I have had some good success by just trial and error using a bench power supply to give me a set amperage and using a file to get it correct.  The hardest part now is moving up to larger amp draws I really need to get a meter that can do many more decimal points than what I have to get it correct.  I am off by 13 amps at 225 amps.  I can't use the load though to make the change since its not stable enough and just a slight amount filed off changes it allot. 

So question for techy the folks out here, do any of you have a suggestion for a voltage meter that has multiple decimal places in the mV range?

 

This shunt is about 135.2mV well that is as far as my meter can read, I need to read like 135.200mV so I can accurately adjust it.  There is a multiply factor in the gauge circuit which at lower amperage its easy to get a perfect reading, but as you go up in amps it reads too low so I need to spend more time getting it perfect.  I have wasted way too much copper tinkering so time to upgrade my equipment. 

[AquaticsLive]

So far I am leaning towards the OWON XDM1041, there are some flaws but for the task at hand should work.  For now though I figured out i can probe it after the op amp so I don't need to spend any money right now to complete this task. 

Now that I know how this one works, I plan to open up the case on some of mine and see if I can do the same thing.  Also I guess its obvious to keep the wires from the shunt to the meter as short as possible.  There is no load on the sensor wires so it should be pretty good, but just slight differences can change the accuracy at higher currents quite a bit.  This is important for my sensors because I am using those readings to calculate used Ah and charged Ah.  It really wasn't that big of deal for this dash gauge that was just nice to get it reading correctly.