16. Liquid-in-glass thermometer properties


How does the structure of a liquid-in-glass thermometer affect its sensitivity, range and linearity?

A liquid-in-glass thermometer comes in 2 basic types: mercury or alcohol.

The bulb is where the liquid is stored. When this heats up, the liquid expands and pushes the liquid up the thin tube and a reading can be taken from how high the fluid has risen.

How can you increase the sensitivity of a liquid-in-glass thermometer?

A more sensitive thermometer can be made by decreasing the diameter of the tube. If you make the tube bore thinner, then for a 1 degree temperature rise, the same volume of liquid will need to rise higher (a long thin cylinder of the liquid vs a short fat cylinder). So we will see a larger rise in the liquid level for the same temperature change.

We can also use alcohol rather than mercury. This is because alcohol expands more than mercury for the same temperature rise.

Finally, we can increase the volume of the bulb. There is quite a lot of conflicting ideas about this on the web, so let's clear up the confusion.

One web source says, "Some of the factors which increase sensitivity are: Using a thermometer with a smaller glass bulb, as a smaller bulb contains less liquid and therefore, absorbs heat in a shorter time".
However, another source says: "...more volume in the bulb means more liquid in the bulb to expand which means a given temperature change will result in more fluid being squeezed down the tube to travel farther which helps increase sensitivity in face of a larger bore (this will also reduce the range for a given tube length).

So which of these ideas is correct?

The correct answer is that a LARGER bulb will increase the sensitivity of the thermometer. A smaller bulb will react FASTER to a change in temperature because there is less 'thermal mass' which takes less time to warm up. However, the sensitivity of a thermometer is defined as the smallest temperature change that can be detected or measured, NOT how quickly that temperature change can be detected.

So if you want to increase the sensitivity of a liquid-in-glass thermometer (resulting in a larger rise in the column of liquid for each degree Celsius), increase the volume of the bulb. Note however, that a larger bulb will decrease the range of temperatures that can be measured.

How can you increase the RANGE of a liquid-in-glass thermometer?

Let's say we want to measure a wider range of temperatures. We will need to make the thermometer less sensitive, so that we don't get as much movement in the thread of liquid for a certain temperature rise. To do this, we need to:

  • Decrease the volume of the bulb
  • Increase the diameter of the tube
  • Use mercury, which doesn't expand as much as alcohol. Mercury also has a much wider range between its melting and boiling points, so higher and lower temperatures can be measured.

So sensitivity and range are opposite characteristics of a liquid-in-glass thermometer. An example of this is in a clinical thermometer where we need a high sensitivity. However, because we are only measuring temperatures close to the body's temperature, we don't need a very large range.

How do we get better linearity of a liquid-in-glass thermometer?

Linearity means that the thread of liquid rises and falls by a uniform amount for similar temperature increases or decreases, i.e. it doesn't rise more for hotter temperatures than for cooler temperatures.

Firstly we need to make sure that the diameter of the tube is constant along its length. If its diameter decreased significantly, then the liquid would rise faster for a given temperature rise.

We can also use mercury to get a better linearity. This is because mercury expands much more uniformly than alcohol, which tends to have different expansion coefficients at different temperatures.

How do we increase the responsivity of a liquid-in-glass thermometer (how fast it reacts to a temperature change)?

If you have thicker glass walls in the bulb and tube, then thermal energy will take longer to reach the liquid and so the thermometer's response time will be slower. A thin walled thermometer will have a faster response time compared to a thicker walled thermometer. However, it will be more fragile.

GCSE Keywords: Bulb, Thread diameter, Liquid, Glass thickness

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