Accurate Thermometer

[tanker]

Following up on a previous person's thread on thermometer accuracy I decided to "check" my thermometers. I got a cup of ice water (mostly ice) and put my thermometers in. Think that ice water is 32 degrees whatever difference would be needed to either added or subtracted to get an accurate temp--Am I right??

The 3 thermometers I put in all measured 40 degrees. So are all my thermometer all off by 8 degrees? So when my tank measures 76 it is really 68 degrees??

Is my science wrong?

 

[SnakeIce]

Could be, or it could be that heat is flowing into the cup fast enough to raise the water temp. I also know that alcohol thermometers take time to reach equilibrium, and the last few degrees can take a bit. So could be a collection of things giving you that result.

 

[biondoa]

Water freezes at 32 degrees. If there was still liquid water in the cup, then it probably was not yet at 32. Even after ice freezes, it may get colder than 32. Most freezing compartments in modern refrigerators are generally at about zero degrees. The ice that comes out is probably about at that temp until it begins to thaw.

 

[Rbishop]

and, all thermometers accuracy varies a bit over their range. A thermometer that covers 0 to 120 may not be as accurate at the high and low ends, as it is in mid-range.

 

[Slappy*McFish]

When purchasing thermometers at the store, compare all of them for differences in temp readings. If they vary quiet a bit, buy one of the ones that all read the same temperature or within the middle of the highest and lowest readings. Since all of yours read the same 40°, I'd assume they are fairly accurate.

 

[Duckie]

The temperature of water (let's assume for simplicity's sake that we have pure water/ice without impurities and normal barometric pressure) at its densest is 39°F (4°C) and melts at 32°. While the water/ice mix is equalizing its temperature (water is warmer than 32°, ice is colder) there will be 39° water that will sink to the bottom, as it should since it is densest. You cannot measure right at the bottom or you measure the glass wall as well which is affected by room temperature. Getting 40° is pretty close to what a melting ice in water should read when measuring water temperature only.

 

[Duckie]

To add, ice water is only 32° when it is perfectly at equilibrium in temperature between ice and water. Impossible to keep it there unless the room temperature is also 32°. At that point ice melts as fast as water freezes, keeping the total amount of ice constant. That would be impossible (or rather impractical) to archive at home.