Ever since I started brewing All-Grain I was only using calculators (either BrunWater and/or Kai's calculator on Brewer's Friend) to determine my water adjustments and never measured pH. The only beer that has had a water issue was my first porter, which was too acidic. I used a higher mash pH for a rebrew (again - based solely on what the calculators were telling me), and nailed it. Every other beer seems to be doing exactly what I want it to do from a pH standpoint.
I recently picked up some colorpHast strips and started using them a few batches ago to check my mash pH. I'm not trying to diagnose any problems (I don't feel like I have any right now), but I just want a quality control check. I shoot for pH in the 5.3-5.4 range for most of my brews, but I have been getting readings between 4.8-5.1. I know the colorpHast strips read a bit low, but when I'm reading 4.8 on a beer that I'm targeting 5.4 something seems off. All the beers have tasted fine, so I think the issue is in my use of the strips rather than in my actual pH.
I have been waiting until about 20-30 minutes into the mash, then drain about 1/2 an ounce into a Red Solo Cup. I let it sit until it comes to room temp, then dip my strip for 10 seconds or so. My first thought is that I may be getting enough evaporation to concentrate my wort, and therefore decreasing the pH a bit. Any other thoughts or suggestions?