I could not find anything looks like something for calibration in the
voltmeter. Thanks for the tip though.

I do not know about the meter you have. But most analog meters
including analog multimeters have a screw at the pivot point of the
needle.

Look at the enlarged picture of the meter.

http://www.go2marine.com/g2m/action/...1&WT.mc_id=fr1

That looks like much more sophisticated than the simple analog
voltmeter on the dashboard of my boat. Mine is just a round thing with
a needle and a couple screws at the back with backlight. OK, I will go
looking for some thing similar to the voltmeter in my boat and see if I
can find an instruction manual and a wiring diagram for it.

Thanks for taking the time to find a sample marine voltmeter for me to
take a look.

Jay Chan