Mercury Battery Replacement in Cameras and Accessories
By far, the scourge of classic photography equipment is Mercury batteries.
The problem with different kinds of batteries is the voltage. Just about all the mercury batteries that were used in camera gear were rated at 1.35 volts. Unfortunately, you can't get mercury batteries anymore (mercury being extremely toxic), so you have to find an equivalent.
Varous battery stores are happy to sell you the "equivalent," except that they're really 1.5 volts. For transistor radios and a lot of other things, that makes no difference. But old exposure meters did not have voltage regulators in them, and the extra voltage throws off the calibration and they tend to read on the high side. This goes for both hand-held meters and in-camera meters, too.
These are your options:
The best is to get the meter professionally recalibrated for a modern equivalent battery. This is also the most expensive, so you really need to love that meter for it to be worth the expense.
Buy another size battery that will fit in the chamber, but has the same voltage. Some hearing-aid batteries have the equivalent voltage, but they're too small to fit. Some people have had luck making spacers (like a rubber o-ring) that hold the battery in place. How well this works depends on the battery chamber and whether the contacts can still touch the battery. Note that some people complain that certain battery compositions (like alkaline) don't produce the proper voltages for very long—they drop off quickly and run at a lower level, and that throws the meter calibration off again.
Buy a WeinCell, which is an "equivalent" battery. They're more expensive than other batteries, but they do give the proper voltage. Their shelf lives are short; so if you use the meter only here-and-there, they're not a good solution. If you want to use the meter a lot, then the shelf life isn't such a problem.
Change the electronics in your meter circuit to cut down the battery's voltage. There are articles floating around the net on how to do this with different components, but they all require you to open up your meter and do some soldering. (See Links below.)
Check your meter against a known-good meter under various conditions and see how far off it is, then make notes of allowances. It's the most work for you but it is the least expensive.
If your meter takes two PX13s (or one PX14) you might be able to use Gossen's adapter kit. Supposedly, the kit is two modern SR44 batteries and a little adapter cylinder that also cuts the voltage down to the proper level. I don't know if it works yet, but it works in the Luna Pro, it ought to work in the Weston Ranger 9 and similarly-configured meters.
I have not tried any of these solutions, but I found on them on the net and you may find them interesting or informative. A simple Google search will get you a lot more.