Possible reason why cheap pool thermometers give wrong temps? (tube glued into holder at wrong reference temp)

Dan8899

Bronze Supporter
Nov 4, 2017
37
Apache Junction, AZ
Pool Size
15000
Surface
Plaster
Chlorine
Liquid Chlorine
I just purchased 2 inexpensive alcohol filled LIG(liquid-in-glass) thermometers for the pool. Sitting side by side 1 is consistently reading 3deg higher than the other and then I noticed both have a black horizontal line marked on the glass, one lining up about 70Fdeg(~21C) and one lining up about 67deg(~20C) on the plastic housing temp scales. So there's the reason for the 3deg diff in temp.

After some searching, I learned the black line is a reference temperature line put on thermometers when temp scale is printed on a separate housing and meant to align with a certain degree on the scale, but what is the correct 'reference' scale temp alignment value? I also came across something called an alcohol temperature correction chart that seems to show 20C deg as the baseline temp (0 correction needed).

For $10, I realize temp reading varying by 2-3deg is probably to be expected, but I was just curious if anyone here may know if the reference mark on an alcohol filled thermometer is supposed to be lined up with the 20degC mark on the plastic temp scale?
 

Bperry

Gold Supporter
Aug 20, 2020
1,001
Knoxville, TN
Pool Size
27000
Surface
Plaster
Chlorine
Salt Water Generator
SWG Type
CircuPool RJ-60
I just purchased 2 inexpensive alcohol filled LIG(liquid-in-glass) thermometers for the pool. Sitting side by side 1 is consistently reading 3deg higher than the other and then I noticed both have a black horizontal line marked on the glass, one lining up about 70Fdeg(~21C) and one lining up about 67deg(~20C) on the plastic housing temp scales. So there's the reason for the 3deg diff in temp.

After some searching, I learned the black line is a reference temperature line put on thermometers when temp scale is printed on a separate housing and meant to align with a certain degree on the scale, but what is the correct 'reference' scale temp alignment value? I also came across something called an alcohol temperature correction chart that seems to show 20C deg as the baseline temp (0 correction needed).

For $10, I realize temp reading varying by 2-3deg is probably to be expected, but I was just curious if anyone here may know if the reference mark on an alcohol filled thermometer is supposed to be lined up with the 20degC mark on the plastic temp scale?
I’ve had three of those dumb thermometers and they all read different and had different marks where they lined up. I’d be surprised if there was a universal mark they all used, but would enjoy knowing. I just switched to an electric BBQ meat thermometer when needed.
 

Dan8899

Bronze Supporter
Nov 4, 2017
37
Apache Junction, AZ
Pool Size
15000
Surface
Plaster
Chlorine
Liquid Chlorine
Same here, just really posted/asked out of curiosity. Many of the documents I searched on the US nist.gov standards and measures site regarding themometers mention the reference mark, but none seem provide a standard reference temp, other than saying its 'often the freeze point', which def not what this one is using. Maybe it's different for each type of liquid used in the tube.
 

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