Blichmann Brewmometer problem?

Hey all-
BIAB. ~16 gal liquor, 22lbs grain. sacc rest 151F. Blichmann 30gal boilermaker with Brewmometer. direct fired KAB4 banjo burner 30PSI. ambient 70F.

I made up a 12 gallon batch last night. My 3rd propane burn went awry. About 40 minutes in, I needed another (3rd) maintenance burn to bring up from 148F. I did a 90 second burn at about 3/8 turn needle. I expected that to raise the temp about 3 or 4 degrees F. But after I cut it off and stirred it real well, the Brewmometer read 161!!! I plunked another thermometer in there and confirmed around 162 ish.

Any speculation as to what went on here? Perhaps a micro meteor landed in the kettle?

PS-
I do have the provided SS heat shield wired to the bottom of the valve.

Unless you are circulating the wort while direct firing the mash tun, it will be very difficult to heat the mash uniformly and accurately hit your target temp. It appears that you are simply guessing at how long to fire the burner. That will surely be a hit or miss method. It's likely not a problem with your thermometer. Was this your first batch using this procedure? It kinda sounds like it might be.

Not recirculating. It is the 3rd batch on this system. About batch 48 or so for me.

Before my first brew, I did perform a heat-time test and graphed it etc for this very purpose.

The prior (2) maintenance burns were only 60 seconds and raised just a hair shy of 2 degrees.

OK, let's take a closer look. Based on 16 gallons of strike water and 22 lbs of grain, you would have a water to grain ratio of 2.91 quarts per pound. That's a really thin mash. The total mash volume would be about 18.75 gallons by my calcs. That's a relatively large mash volume. I'm having a difficult time visualizing two 60 second burns raising the 18.75 gallon mash volume by nearly two degrees. That just seems very unlikely based on my experience with direct firing. I would think that it would take much longer than two minutes for the mash temp to equalize after applying any heat at all. I'm thinking that there's something amiss with the way you are measuring the grain bed temps. Also, you should be able to dough in with properly heated strike water and hit your target temp within a couple of degrees. With the huge thermal mass of nearly 19 gallons of grain and water I wouldn't expect the mash temp to drop as much as you report. What did the heat-time test graph look like? Can you post a copy? That might reveal something.

I'm thinking that raising the temperature of water and raising the temperature of water+grain are two very different things. I can say that I have never used the BIAB method, so I might have a few off assumptions, but how can the thermometer get an accurate reading of the internal grain bed if it is hitting the edge of a bag? Granted its been a few years since I have taken a thermal/fluid dynamics class, and I am far from being an engineer of that type, but it would seem to me that the liquid outside the grain bag heats and circulates faster than the internal grain bed which could explain your temperature discrepancy. Call me dumb, call me drunk, but like Catt said, without constant recirculation it is very difficult to maintain and get an accurate reading of what the mash temp actually is.

Yeah RCB, we are generally thinking alike on this. I agree on the water vs. water+grain mostly because one is very free flowing and the other not which would slow down the diffusion of any added heat. I suppose it would be even worse for a bag of saturated grain surrounded by liquid. I have zero experience with BIAB, so this is only based on what I think would be going on intuitively.

For the OP, you said this was your third batch on this system. How did it go with the previous two? Did they behave similarly or not?

Maybe a cooler would work better for the BIAB method. Then it would be very much like batch sparging only without a manifold, braid, screen or false bottom to deal with. The cooler should minimize the heat losses and eliminate the need to direct fire the MT.

Thanks for the interest.

The process is this. Stir. Wait until temp drops below rest temp. Apply heat for a certain time based on estimation from heating test. Then mix (see below) and observe if the target temp was hit. I will wait 5-10 minutes before heating again even if it initially reads low.

To mix, I stir with a 24" potato masher comically rapid up and down motion so I can see the grain moving around quickly. About 15 seconds worth of that. I Dunk the bag up and down some too.

The graph was linear 2 degrees F / minute at 20 gallons of tap water.

I am upgrading from a 5 gal capacity cooler-based eRIMS in favor of 15 gallon capacity. The kettle alone was 500$ so it will be a while before I have a full blown 15 gallon eRIMS.