Thermometer calibration - my maths...please check!

[morethanworts]

It would seem I have been mashing lower than I thought.

My (glass) thermometer reads just under 1C (let's say 0.75) in ice water. It shows 102.5C in boiling water. We're at 46m above sea level, so it should be 99.85C I think. I've watched demos of how to take the measurements and have been as careful as I can.

So if it's +0.75C out at 0C and +2.65C out at 99.85C, by how much is it likely to be over at 66C? I'm going to have to make a very dubious assumption that the increase in inaccuracy is linear over the range, but I'm fairly happy that it will be close enough!

The difference in inaccuracy is 1.9C (from 0.75C to 2.65C) between the readings taken at 0C and 99.85C.

66C is 66.1% of the way towards 99.85 (from zero).

66.1% of 1.9C is 1.26C

1.26C plus the 0.75C it was already out at zero is +2.01C

So I guess, in the absence a thermometer that I know to be accurate at 66C, I must assume that my thermometer is about 2C over when taking mash temperatures.

Please point out any errors in the logic or execution!

 

[pittsy]

I think it should be around 1.2 c out , that's an estimate based on the difference of approx 2c and half between 0 and 100 is 50c which should be approx half of the 2c out which is 1c and then 66c is a further 1/4 of 50c so approx 1/4 of a degree which is 0.25c therefore 1.2c approx . Does that make sense . :shock:

 

[marksa222]

I've got a thermometer built into my pot. It was out when I got it so I used a screw driver to correct it. I put a digi thermometer and checked the temp of a glass of water at room temp and adjusted the other one to match its temp. Then I poured a glass of warm and did the same, expecting it them to marry up now... they didn't... I adjusted it again so they matched, then a third time, I did it with warmer water, again, they we misaligned... At boiling, they both measure about 100c, anything under boiling, they are upto 4c out from each other... I don't get it. I've been following the digital one though as it was only the last brew day that I noticed the one built into the pot now reads 100c at boiling. Before I tried "calibrating it", it measure about 104c at boiling. Pain in the arse... Maybe I broke it or something.

 

[andyd]

I think your maths is good, based on a linear scale

y=101.75/99.85*x+0.75

101.75 comes from the 102.5 less the 0.75 error at zero

y is the thermometer reading, x is the correct temperature, so:

x=0, y=0.75
x=99.85, y=102.5
x=66, y=68.01 etc.

 

[morethanworts]

I think your maths is good, based on a linear scale

y=101.75/99.85*x+0.75

101.75 comes from the 102.5 less the 0.75 error at zero

y is the thermometer reading, x is the correct temperature, so:

x=0, y=0.75
x=99.85, y=102.5
x=66, y=68.01 etc.

Thanks all. :thumb:

That's a handy formula for me to work from andyd, thanks. I will get a few key temperatures worked out in advance, around the mash to strike temperature range and the pitching/grain temperature up to rehydration ranges. The rest I can work quickly if needed from that.

I've had high attenuation with the yeasts I've used in my first 5 AGs (though two were tripels with abbey yeast and 13% sugar), which ties in with the theory I may have been mashing low. My last brew, an ESB, did finish a little dry, as I've discussed on other threads, so for anything like that, I think I may give it a full 2C extra on that thermometer to see what difference it makes.

I haven't checked it against my STC1000 probe yet, which is currently in action. That may also be an interesting test. My old glass thermometer (since broken) used to match it pretty exactly.