Types of Thermometer

1. The liquid-in-glass thermometer

The liquid-in-glass thermometer uses liquid(mercury) as thermometric substance. In which the change in volume of the liquid measures the change in temperature.  Any liquid that will be used as a thermometric liquid must be a good conductor of heat, be easily seen in glass, have a high boiling point, have a low freezing point, have a low specific heat capacity, must not wet glass and must expand uniformly. The liquid-in-glass consists of the school thermometer, clinical thermometer, maximum and minimum thermometer.

(i) The clinical thermometer has a constriction and it has a short range (35⁰C – 43⁰C).  The narrow constriction prevents the mercury from flowing back into the bulb immediately after the thermometer has been removed from the patient’s body.

(ii) The school thermometer is used in school laboratory, it ranges from 0⁰C – 100⁰C. It has no constriction.

(iii) The Maximum and minimum thermometer is used to measure the maximum and minimum temperatures of the day. It contains alcohol at two upper part of the bent tube and mercury below.  A steel index is seen in the two tubes. When the temperature rises, since alcohol expands more than mercury, The alcohol expands and flows in clockwise direction and the mercury pushes steel index x upwards. The maximum temperature is read from the lower end of x. When the temperature falls the alcohol contracts and moves in anticlockwise direction, the mercury pushes steel index Y up. The lower end of Y indicates the minimum temperature.

EVALUATION

1. State three desirable properties of a thermometric liquid.
2. List four advantages of mercury over alcohol as a thermometric thermometer.

2. Constant – Volume Gas Thermometer

The constant – volume gas thermometer depends on the variation in the pressure of a gas at constant volume with changes in the temperature of the gas. On heating the bulb to a specific temperature, the gas expands and pushes the mercury down to tube A and consequently mercury level at C rises. The right side of AC of the manometer is moved up and down in order to bring the mercury level on the other side to its original position D to ensure that the volume of gas is constant. The pressure of gas is read from the manometer.

When the level of mercury in A is higher than that of D, then

P=h+H, where H is the atmospheric pressure.

But when the level of mercury is lower, then

P=H−h

To use the thermometer it must be calibrated by getting the gas pressure at 0⁰C , when the bulb containing is placed in pure melting ice. The pressure at 100⁰C is also gotten by placing it over stem.

θ−0100−0=Pθ−P0P100−P0

P_θ – resistance at temp θ⁰c

P₀ – resistance at O⁰C

P­₁₀₀ – resistance at 100⁰C

Example 1:

A constant volume gas thermometer records pressure of a body as 250mm of Hg at 0⁰C and 350mmHg at 100⁰C. Calculate the temperature of the body when the gas pressure reads 300mmHg.

Solution:

p_θ = 300mmHg,   p₀ = 250mmHg,  P­₁₀₀ = 350mmHg, θ = ?

θ=Pθ−P0P100−P0×100θ=300−250350−250×100θ=50100×100=50o

EVALUATION

1. A constant volume gas thermometer records pressure of a body as 300mm of Hg at 0⁰C and 400mmHg at 100⁰ Calculate the temperature of the body when the gas pressure reads 350mmHg.

3. Platinum resistance thermometer

This thermometer depends on the variation in the electrical resistance of a conductor with temperature. The higher the temperature the greater the resistance. The ends of the wire are connected to a wheatstone bridge which measures the resistance at 0⁰C, at 100⁰C and the resistance at the desired temperature. The temperature can be calculated using the equation below.

θ−0100−0=Rθ−R0R100−R0θ=Rθ−R0R100−R0×100

R_θ – resistance at temp θ⁰c

R₀ – resistance at O⁰C

R­₁₀₀ – resistance at 100⁰C

Example 2:

The electrical resistance of the element in a platinum resistance thermometer at 100⁰C, 0⁰C and room temperature are 75.000Ω, 63.000Ω and 64.992Ω respectively. Determine the room temperature.

R_θ = 64.992Ω,      R₀ = 63.000Ω,        R­₁₀₀ = 75.000Ω,        θ = ?

θ=64.992−63.00075.000−63.000×100θ=1.9921.200×100=166.0oC

4. The Thermocouple

A thermocouple consists of two different metals joined together by a circuit containing a galvanometer.  The working principle of a thermocouple depends on the variation of the electromotive force (e.m.f) between junctions.  The equation of the relationship is

E=a+bt+ct2, (a, b, c are constants)

GENERAL EVALUATION ( Post your answer in the comment box for rating and discussion)

1. Describe the action of resistance thermometer.
2. (a) Draw a labeled diagram of a clinical thermometer and explain how it works.
3. (b) Give the reasons for the following features of the thermometer

(i) a bulb of thin glass.

(ii) the constriction on the bulb.

(iii) a tube of fine bore.

3. State one advantage which a constant-volume gas thermometer has over other thermometers and one reason why it is seldom used as an everyday laboratory instrument.
4. (a) State three physical properties of  substances which may be used to measure temperature.

(b) State three desirable properties of a thermometric liquid.

(c) List four advantages and four disadvantages of mercury as thermometric liquid.

(d) Why is water considered as an unsuitable liquid for a thermometer?

(e) The resistance in the element in a platinum resistance thermometer is 6.750Ω at 0⁰c, 7.750Ω at 100⁰c and 6.900Ω at room temperature.