Sudden drop in mash temp after 40 mins

[DoctorVenkman]

I recently bought a new stainless steel mash tun with a digital thermometer mounted on the outside, so I was eager to put it to use. After vigorously stirring my mash I hit 152.5F, with a target of 152F. So good so far!

I monitored the temp every 5-10 minutes and noticed minimal temperature loss. After 30 minutes the temp was still slightly above 151F. However, at the 40 minute mark, something weird happened: the temp dropped to 147F. As soon as I saw that I started to boil some water, during which the temp dropped further to 145F. Once I added the boiling water and gave a quick stir I finished the mash around 150F.

I don't think this temp loss can be explained purely from physics since heat transfer happens at a quicker rate the hotter something is, so it wouldn't make sense for the heat loss accelerate after 40 minutes. I can think few things that could have caused this:

1) There was a cold "pocket" of grain/dough somewhere that was insulated and took a while to absorb the heat around it. I think this is somewhat unlikely since I stirred the mash pretty hard before putting the lid on.
2) The thermometer bugged out. Also unlikely since it is brand new, and the temp increased when I put boiling water in so it wasn't like it was producing nonsense numbers.
3) Someone dosed my homebrew with LSD and I hallucinated the whole thing. Most plausible of the three options, IMO

Has anyone else experienced this? Did you ever find out what the cause was? It's a bit disconcerting since I bought this SS mash tun to increase my consistency!

Cheers

edit: I forgot to mention that I preheated the mash tun with boiling water as well.

 

[dmtaylor]

Yet another reason to only mash for 40 minutes. It's been good enough for me and my process for the past 10 years.

 

[kh54s10]

I use a water cooler mash tun without a temperature gauge. I only take a measurement when stirring in the grains. In the beginning I measured at the end to see what I had lost. It was usually 2-4 degrees in the summer and 2-6 degrees in the winter.

I now only measure until I am close when starting the mash then I close the tun and wait until done. Sometimes I will give it a stir sometime during the mash.

So, no answer, other than to not worry too much about it. It is often said that 90% of conversion is done in the first 20 minutes.

 

[mongoose33]

How is the tun insulated? I agree, that temp drop is very strange, but at 40 minutes you're likely done anyway.

 

[day_trippr]

I think I would do some testing of that thermometer...

Cheers!

 

[Aristotelian]

I would guess #1. The temperature probably wasn't evenly distributed and you were in a warm spot earlier and/or hit a cool spot later. I am sure your mash will turn out OK. Next time give it a stir when you take your measurements.