I just replaced the door on my OBS - but that's another story.
In doing so, I had to move the thermometer from the old door to the new one. What better time to check the accuracy?
Boiled water in a small pot. Then, everybody into the pool; the Bradley, my wired probe and my instant read.
The Bradley was as close to 100oC (212oF as I could read it. The other two were reading 210 and 211.
Looks good to me!
I don't think the issue is the accuracy, its the placement.
Especially when you have a cabinet full of cold meat.
Arnie, you will glad you did it, just one less thing to worry about.
And that means alot when you get to our age! ;D
How do you check with water? Since water STARTS to boil at 212 F and climbs in temperature. And that's only at sea level. Your water could be 230, 240, or even 205 F. Depending on where your at. Too many variables.
You can only guestamate.
The Variables:
If the heat of vaporization and the vapor pressure of a liquid at a certain temperature is known, the normal boiling point can be calculated by using the Clausius-Clapeyron equation thus:
(http://upload.wikimedia.org/math/2/8/4/284198a003c58bf4d93d4e869bd24529.png)
where:
TB = the normal boiling point, K
R = the ideal gas constant, 8.314 J · K-1 · mol-1
P0 = is the vapor pressure at a given temperature, atm
ΔHvap = the heat of vaporization of the liquid, J/mol
T0 = the given temperature, K
ln = the natural logarithm to the base e
;D ;D ;D ;D ;D
At least with the water test, you can verify that each thermometer reads the same reference source.
You won't know for sure that _your_ boiling water is 212F, but at least you can tell if one reads higher or lower than the other.
Maybe I'll just remove the post.
Quote from: ArnieM on January 26, 2010, 04:25:05 PM
Maybe I'll just remove the post.
No, don't you make a good point. FLBentRider is correct. If one is off by 10F then toss it. You just can't check one. You'll even get different readings if you do the test in glass then do the test in a cheap metal pot. So many variables.
A professor in college drove us wacko with this exact thing.
I've done this before with other thermometers (not the Bradley door one), and it seems accurate enough for me. If you got three or 4 of em in there, and they are all reading the same thing - probably pretty accurate.
One thing that checking the thermometer with boiling water does not address is linearity. My door thermometer is accurate over a limited range but non-linear and hence inaccurate outside of that range.
My understanding of the boiling point of water is that once it reaches it's boiling point it remains at that temperature and does not continue to climb. For home use, in an uncovered container water boils at 212°F at sea level, will maintain that temperature, and will not get hotter. Elevation and barometric pressure also have a bearing on the temperature, and for that you can easily find a calculator on line. To get water to boil at a higher temperature you need to make it denser, such as by adding salt, or to pressurize it. So for home use I feel that the boiling point test is the best way to calibrate or check the accuracy of your thermometer.
Such as this one; Boiling Point Calculator (http://www.virtualweberbullet.com/boilingpoint.html)
QuoteMy understanding of the boiling point of water is that once it reaches it's boiling point it remains at that temperature and does not continue to climb.
That is quite
true for pure water or any other pure liquid. However, if the water contains a nonvolatile solute then the boiling point can increase as water boils off increasing the solute concentration. That is kinda nit picking though for the ball park measurements made with the door thermometer. The altitude correction should be all that's needed for our purposes.
Quote from: 3rensho on January 26, 2010, 09:48:17 PM
One thing that checking the thermometer with boiling water does not address is linearity. My door thermometer is accurate over a limited range but non-linear and hence inaccurate outside of that range.
I picked boiling water because it's pretty close to my normal cooking temps. If it goes non-linear outside of that range, I'm not too worried about it. If I start doing smoked sausage or fish, I'll get a PID.
Here's a nice site to find out your altitude. EarthTools (http://www.earthtools.org/)
Just type in your address for the results.
I use Weather.com for the barometric pressure.
The last time I checked my location the boiling point of water was 210.97
Quote from: 3rensho on January 27, 2010, 03:35:48 AM
QuoteMy understanding of the boiling point of water is that once it reaches it's boiling point it remains at that temperature and does not continue to climb.
That is quite true for pure water or any other pure liquid. However, if the water contains a nonvolatile solute then the boiling point can increase as water boils off increasing the solute concentration. That is kinda nit picking though for the ball park measurements made with the door thermometer. The altitude correction should be all that's needed for our purposes.
True, the boiling point will increase with increase density. Most sites recommend using distilled water. The boiling water method is very accurate for home use with just the elevation. I feel since I have access to the calculator and I know both values, I may as well plug them in.
Its been at least 2 years sense I checked my boilin point and it is 210.59 deg.
You think I need to check it again? ;D