Why the alcohol thermometer is less precise than the mercury one?

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Discussion Overview

The discussion revolves around the precision differences between alcohol and mercury thermometers. Participants explore various factors that may contribute to the perceived lower precision of alcohol thermometers, including thermal expansion properties, the behavior of the liquid in the thermometer, and human reading errors.

Discussion Character

  • Debate/contested
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • Some participants suggest that the thermal expansion coefficient of alcohol is not constant, which may affect precision.
  • Another viewpoint is that alcohol wets the walls of the thermometer, potentially leading to less accurate readings.
  • One participant initially thought that the coefficient of expansion was the reason for the difference but later corrected themselves, noting that alcohol expands more than mercury.
  • There is a question raised about whether the use of mercury is due to practical size considerations rather than precision alone.
  • Concerns are expressed regarding the human factor in reading the thermometers, particularly the variability of the meniscus in alcohol thermometers.
  • A participant questions the existence of a deterministic equation relating temperature to liquid volume in a sealed glass tube, suggesting that the relationship might not be linear and inquiring about potential error sources.

Areas of Agreement / Disagreement

Participants express differing opinions on the reasons for the precision differences between alcohol and mercury thermometers, indicating that there is no consensus on the primary factors involved.

Contextual Notes

Participants mention various assumptions about thermal expansion, human error in reading, and the physical properties of the liquids involved, but these assumptions remain unresolved within the discussion.

ORF
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Hi,

Why the alcohol thermometer is less precise than the mercury one?

If I remember correctly, it is because the thermal expansion coefficient of alcohol is not constant, but I am not sure.

Thank you for your time.

Regards,
ORF
 
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I think it's because alcohol wets the walls of the thermometer making the reading less accurate.
 
My first thought was coefficient of expansion; maybe mercury is higher so a bigger change in volume per change in temperature. But this is wrong. A little internet searching shows volumetric coefficients for "ethyl alcohol" as 1100 (x10-6 per degree C), compared to 180 for mercury. So the alcohol expands ~6 times as much.
 
ORF said:
Why the alcohol thermometer is less precise than the mercury one?
Do you know for sure that it is? I thought the reason mercury is used is because it allows for reasonable sized thermometers whereas alcohol ones would be awkwardly large to get an error range similar to a mercury one.
 
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kuruman said:
I think it's because alcohol wets the walls of the thermometer making the reading less accurate.
Do you mean that the meniscus is variable, or that it's more prone to humans misreading it consistently?
 
Mostly the meniscus.
 
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Since both have to be read by a human (I assume), then I'm not sure your accuracy question is complete. You are not addressing the biggest error source, IMO. This is one of many reasons everyone uses electronics to collect data these days.

Anyway, why would there not be a deterministic equation that relates temperature to liquid volume in a sealed glass tube? Granted, it might not be linear, but does that matter?

A question for those who know materials science better than I (a pretty low bar, actually). What can you do to change the volume of a liquid at a fixed thermal equilibrium and constant pressure (because the glass is thick)? Those would be the error sources, right?
 

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