hey try here,
http://www.matrixorbital.com/products.htm
I don't know how reliable this company is but it may be of some use. (download the pdf'z and take a look at them)
or you could get a digidoc 5, I think that might alow you to make voltage readings, but it displays a ton of other stuff and I don't know how customizable it is. Although, I think it's around 50-60 bucks for it and it takes up a drive bay.
Davis