I use an Aqualyser from Nico2000.net out of UK. The nice thing about this unit is it allows you to not only connect pH probe, but also dissolved oxygen, temperature, and three other ion specific electrodes of your choice (eg. sodium, potassium, calcium, nitrate, chloride, etc.), and it monitors and records all data over time (you set measuring time interval). Very useful for fermentations where you want to see the decrease in pH over time as the fermentation progresses. Fairly reasonably priced at around $1000-2000 USD depending on which probes you select.