Ok, I've said this a few times in other posts but thought I would start a new thread to make sure people understand.
YOU HAVE TO TAKE A SAMPLE. You can't put the meter directly in the milk. I just got off the phone with Hanna Instruments for the second time and this guy knew what he was talking about. The electrode PH meters use leak electrolites into the sample. He said it's a very minute amount but shouldn't be used in the actual batch. You have to take a sample out, test it and then throw the sample away.
Just for a little more info, although I have a really expensive PH meter it doesn't go down to .01 decimal places, so I was going to get the one Wayne and Dave have, but the guy at Hanna said it doesn't have temperature correct. This means if the sample is anything other than 69.8 the PH will be off a little. Here is the temperature correction chart.
If your sample is 86F subtract .01, so if it reads 7.01 on the meter it is really 7.00.
86 = -.01
95 = -.02
104 = -.03
113 = -.03
122 = -.03
131 = -.03