Thermometers - adjusting for errors

[MSBrewer76]

Just wondering if anyone has any thoughts on this. I have been using a glass thermometer purchased from Austin Home Brew for my first four all grain batches. Started wondering if it might be out of calibration based on all four of these beers (two pale ales, one English brown, and one English Mild recipe) finishing out at lower than normal finish gravities, and the beers (particularly the English beers) being thinner bodied than the same recipes done using liquid extract.

I ordered a "Big Daddy" dial thermometer from AHS this week along with two all grain kits. I tested both this thermometer and the Model 21 glass thermometer I purchased from AHS and have been using up till now. In a cup full of ice and water, the dial thermometer reads 36 degrees F. The glass thermometer reads 30 degrees. Boiling point (with distilled water) for the glass thermometer reads 216 degrees F. The dial thermometer reads only 209 degrees F.

It appears both thermometers are inaccurate, however I cannot make adjustments for the discrepancies while mashing since the inaccuracy range for the two thermometers is reversed at freezing and boiling levels. If I could determine how much either or both of the thermometers are off, I could then make adjustments while mashing.

I mashed the previous four batches at approximately 152 - 153 degrees F (using the glass thermometer). I am guessing that the actual temperatures probably did not exceed 150 degrees. Today I brewed an AHS Nut Brown Ale, and mashed at a temperature reading of 156 degrees on the glass thermometer, and 154 degrees on the dial thermometer. Finished the mash about 1 degree lower than this (one hour mash). Basically experimenting and seeing how this turns out.

Any advice would be appreciated.

 

[Wynne-R]

Usually a thermometer will be high or low, not both. I’m guessing your glass thermometer is a lab type. Is it this one? http://www.austinhomebrew.com/product_info.php?cPath=178_67_65&products_id=999

These are generally ±1 ºC 76mm partial immersion. It should read within a couple of degrees Fahrenheit if you put it in the liquid up to the line, about three inches from the tip. If it’s deeper than that, it’ll read low on the bottom and high on the top.

I bought one of these at the LHBS for eight bucks. It looks identical to the AHS one. It was 1 ºC low at freezing and nuts on at boiling. That’s pretty typical and actually not bad for a cheap thermometer.

A chemist friend wrote out this procedure for me:

If you can be assured of linearity the water melting point, boiling point calibration is pretty damned accurate. The tricks are:

The water/ice must be pure, as in distilled.

Use slush (crushed ice) with the water level slightly below the ice. When determining melting point remember that water is densest at 4C so the ice/water mixture must be continuously well stirred or the water at the bottom will be 4C and the rest of it somewhere between 0C and 4C (magnetic stirrer works best). A steady well-stirred temperature is 0.0C.

For the boiling point, use glass beads (small stream pebbles would work) to prevent bumping of the ultrapure water. Use an erlenmeyer flask (or something else with a narrow neck) with the thermometer clamped to a stand and the thermometer mounted ABOVE the water in the steam space, arranged so that it doesn't get splashed with water. Record the steady steam temperature at a rolling boil. Record the barometric pressure.

You can find charts of boiling point vs atmospheric pressure (http://www.hi-tm.com/Documents/Calib-boil.html) on the Web or use the steam tables (look up saturation temperature for corresponding atmospheric pressure.http://www.efunda.com/materials/water/steamtable_sat.cfm ).

 

[SpeedYellow]

Ha ha, WynneR, your buddy is correct about adding beads/pebbles/etc to the boiling water. I was freaking out because my lab thermometer was reading 216F at a boil. Then, on a hunch, I tossed in a tiny pinch of chalk, and VOILA suddenly it's boiling at 212F. grrrrrrr

 

[MSBrewer76]

Hey, thanks for the feedback. Actually the thermometer is this one: http://www.austinhomebrew.com/product_info.php?cPath=178_67_65&products_id=997

I don't have the flask or other equipment to check the thermometer. I think I will just see how this batch of beer turns out. If mashing at the higher temperatures works out to the right body in my beer and the alcohol content is closer to correct, I think I can then assume that my thermometer is indeed off calibration. Does this sound reasonable?

I adjusted my Blichmann thermometer using boiling distilled water in a saucepan, hope it is close enough. Maybe I should take it out of my pot and check mash temperature next to the other two thermometers and see how close it is.

 

[mjohnson]

For our purposes, I think its more important to use the same thermometer and adjust based on results. Accuracy is less important than consistency. Assuming that the thermometer is at least consistent between uses, doing what you did (bumping the mash up to 156) is exactly right. Its about knowing your system. The worst is when you have a bunch of thermometers and they're all different!

 

[TheZer]

So how do i calibrate my barometer?