Note: Difference between the two models is the reference cell electrolyte. 65-IP is with solution form and can be refilled. 65-IIP is in gel form which last life time, not need to refill. Difference between the two models is the shanpe of the tip. 65-IA is sharper, 65-IIA is a little round Model ; 65-IP 65-IIP 65-IA 65-IIA Measuring Range: 0 - 14 pH Temperature Range: 0 - 60癈 Zero point value: 7 +/- 1 pH Junction: Ceramic Reference Cell: Ag/Agcl, KCL solution refillable Ag/Agcl, KCL gel Ag/Agcl, KCL solution refillable Size: dia.12 x length 140 mm dia.9 x length 140 mm Plug: BNC + 1 m cable