Thermometer

Thermometer

To measure temperature accurately, we have to use a thermometer, which makes use of thermometric substances.

Thermometric substances: substances (solid, liquid or gas) that have physical properties that vary continuously and linearly with temperature.
Thermometric properties Examples of thermometer
Volume of a fixed mass of liquid Mercury-in-glass thermometer,
Alcohol-in-glass thermometer
Electrical resistance of a piece of metal Resistance thermometer
Voltage or electromotive force (e.m.f.) Thermocouple thermometer
Pressure of a fixed mass of gas at constant volume Constant-volume gas thermometer

Features of a good thermometer

  • An easy-to-read scale
  • Safe to use
  • Responsive to temperature changes
  • Sensitive to small temperature changes
  • Able to measure a wide range of temperatures

Calibration of Thermometer

Step 1: Determine two fixed points on an unmarked mercury thermometer
Lower fixed point
(Ice Point at 0⁰C)		 Upper fixed point
(Steam Point at 100⁰C)
Diagram of apparatus

            		 Diagram of apparatus

   
Description

Immerse the bulb of the thermometer into a funnel containing pure melting ice at one atmospheric pressure.

When the mercury level in the thermometer stem is steady, make a mark l0 at that level on the stem. This is the ice point.		 Description

Insert the thermometer into the apparatus such that the bulb should be just above the boiling water at one atmospheric pressure.

When the mercury level in the thermometer stem remains steady, make a mark l100 at that level on the stem. This is the steam point.
Definition
  • Fixed point is the standard degree of hotness or coldness such as a melting or boiling point of a substance.
  • Ice point is the temperature of pure melting ice at one atmospheric pressure and has a value of 0⁰C.
  • Steam point is the temperature of steam from pure water boiling at one atmospheric pressure and has a value of 100⁰C.

Step 2: Divide the interval between ice point and steam point into a 100 equal parts. Each interval is 1⁰C.
Temperature of unknown substance

Method 1: 

Once the Celsius scale is determined from above, we can simply place the calibrated thermometer into the unknown substance and get the reading.

Method 2:

After getting l0 and l100, we can again find the height of the mercury level lθ when the thermometer is placed in the unknown substance. Then the temperature of unknown substance is found using this equation

   θoC = (lθ - l0)⁄(l100 - l0) X 100oC

Note:

This equation can be applied in any thermometric properties discussed previously.
where X = any thermometric properties
    upper = upper fixed point
     lower = lower fixed point
            T = temperature

Note that Tupper  - Tlower is not always 100⁰C.
Example

A thermocouple indicates 0.1 mV at ice point and 2.6 mV at 500⁰C. What will be the temperature when the thermocouple indicates 3.0 mV? 

temp = (3 - 0.1) ⁄(2.6 - 0.1) X (500 - 0)= 580oC
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