Confused about Mash Ph measurement

I'm getting ready to brew a Stout and am paying attention to pH for the first time.

The thing I'm most confused about is whether mash water pH should be measured before grains are added, or after the mash is complete.

The tap water, which I've been using for about a year and with which I've not had many difficulties, is around 7.2 pH. I plan on checking the various levels when I make this next batch and am wondering when I should be checking and what numbers are appropriate. I read John Palmer's information, but am still a little confused.

Any help is appreciated.

Measuring the pH of the water before you add grains means little. The mash pH is what you're interested in. Shoot for around 5.2. Measure during the mash, near the beginning to be able to make adjustments if necessary. And take a small sample out of the mash tun - don't put the probe in the mash tun. And cool the sample to room temp before you run it.

A good way to cool your sample is keep a few shot glasses in the fridge and transfer
the sample back and forth. Cools it down quik.

You're shooting for a mash pH of approx 5.5 at "room temp". It would be approx 5.2 at "mash temp", but measuring it at that temperature shortens the life of your pH meter electrodes. Check out the Brew Science forum for more info.