Rosey, the hearing aid batteries are zinc-air types, their voltage is close enough to 1.35v for them to replace mercuries and they have the same nice nearly constant voltage until they die. I use 'em in my Minolta Flash Meter (original model, no #), and the only problem is that they don't last long.
Chris, I did have problems with hearing aid batteries, though, the 675s I bought are a bit small and had to be wrapped with stuff so they'd stay in place and make good contact. In addition, they take some time to come up to voltage after their seals are removed. Try again.
FWIW, a similar question came up yesterday. The current price of the adapter for using a pair of SR44s in a LunaPro is around $38. I think that C.R.I.S.' price for their adapter that holds a single SR44 and is the same size as a 625 is $30. Get the good 'un and don't look back. I use one in my LunaPro and it works well.
Cheers,
Dan