I do not know what exactly you are trying to do but there is a lot of work done on grip strength. In a quick search it looks like the standard tool for measuring this is a Jamar dynamometer.
Jamar hand dynamometer (Hydraulic Hand Dynamometer, 5030J1, Sammons Preston, Bolingbrook, IL, USA). This is the adress
And here are presented normal values
Bohannon, R.W., et al., Reference values for adult grip strength measured with a Jamar dynamometer: a descriptive meta-analysis. Physiotherapy, 2006. 92: p. 11-15