Low Gravity -- Question?

So, I was brewing an Oatmeal Porter yesterday, and was having some issues with my thermometer(s).

First, I did a protein rest at 125 for the oatmeal, and then I ramped up to 156 for sacch. rest. At the last second, I remembered I had a digital thermometer that was much more accurate. When I put that in the mash, it read 152. I pulled some wort off...heated it on a separate burner and put it back in...now at 156 (almost on the nose).

Anyway, I mashed for an hour and "sparged" (I have to top off my keggle anyway). Then, I squeezed the bag and went on my merry way. Haha.

I had let my plate chiller sit with StarSan in it up until transferring to fermenter. When I transferred, I dumped the StarSan out and then took the first bit of wort out and set it to the side to cool.

When I took the reading, I was 6 points low. I've never had this issue since moving to BIAB, and I was wondering something. What is the possibility that the first wort through the chiller pushed out some StarSan and "diluted" my wort enough for the reading to be low?

The first thing I'd consider is that pulling some wort off and heating would denature the enzymes, so if there was quite a bit of wort pulled off then the mash conversion would end there. That seems to me to be the most likely thing that happened, without knowing more. It seems sort of odd to do a protein rest with a porter- but that is probably not the cause of a slightly lower efficiency.

I pulled off about 1/2-3/4 of a gallon out of 12.5 gallons of mash water/wort.

I read that there was benefit doing protein rest for Oats, but after researching all morning...not going to do that again.

I would be surprised if the StarSan and your first wort didn't mix at least a little bit. So it's a least possible the measurement was altered by that. Without knowing how much you ran off, it would be nearly impossible to know if you got a reading of 100% wort or not. If you took a preboil measurement and your volumes are correct you could easily calculate out your SG.

There's a good chance there was a sizeable amount of Starsan left in your plate chiller. I have to shake it well every which way to get most of the water/Starsan out.

I would take a sample out of the kettle, earlier on, to see where it stands. A refractometer beats the time it takes to read or monitor gravity during the boil.

Maybe your crush was not as fine as usual. I gather you don't sparge? Squeeze? You can reclaim a few more point from that.
The protein rest doesn't harm anything. How did you ramp up from there to 156?

Yeah...I took about 200 ml of wort straight out of the chiller. I should have done a Pre-Boil gravity, but with cold weather, I had to do a lot more work...transferring water from house to garage, etc. Next time, I'll do that. Depending on how this turns out, I'll do it again. I'm probably also going to get a PID to calibrate and use a thermometer.

Put partial amount of (30 qts) water in at 133*F to get to 125*F. Then, I took 20 quarts of boiling water to get to 156*F. Reading was low with a better thermometer, and then I pulled off about 1/2 to 3/4 gallon of mash water/wort (was only in kettle for 3 minutes or so), heated it to 170*F, poured back in, and then had almost 156*F dead nuts.