The cheap Hanna meter broke! I wanted to be able to run experiments this weekend, so I built my own based on this web site that describes a simple DIY meter. Its heart is a TL082 op amp. After spending a few USDs at Radio Shack, I built this (inside view):
Here it is taking a measurement of a developer. Connect a voltmeter to it, and then calculate pH based on millivolts.
You calibrate it any way you'd like, which gives you a slope and offset. Having those, the actual pH calculations are quick; much quicker than the time it takes to measure and mix the chemicals and wait for the probe to settle.
I want a better probe than the one that came in the Hanna (pictured above). Do you suggest single- or double-junction? Refillable versus gel? I've heard that refillable probes are faster than gel-electrodes. Mine takes about 5 minutes for the tenths of millivolts to stabilise -- that's slow! jochen pointed out that good probes are important. Any opinions about these two: