TEMPERATURE

Temperature is measure of hotness or coldness of an object. Temperature is measured by using thermometer. The units of temperature are: Kelvin (K), degree Celsius (°C) and Fahrenheit (F). There are different types of thermometers Liquid in glass thermometers Laboratory thermometer Clinical thermometer Thermocouple thermometer

Laboratory thermometer Laboratory thermometer is used to measure the temperature different things while doing experiments. The laboratory thermometer shown below, the liquid in the bulb expands when the temperature rises. The tube is made narrow so that a small increase in volume of liquid produces a large movement along the tube. So which means in laboratory thermometer the physical property varies with temperature is volume of the liquid.

Purpose or working principle Main design features Purpose or working principle 1 The mercury or alcohol is contain in a thin wall glass bulb Thin wall of the bulb allow s conduction of heat quickly through the glass (a poor conductor of heat) to the liquid. 2 Bulb is made small to contain a small amount of liquid. Small amount of liquid will be more responsive to heat 3 The bore of capillary tube is fine and uniform The fine tube allows noticeable movement of the liquid for small change in temperature, (i.e. good sensitivity). The uniform tube ensures even expansion of the liquid. 4 The walls of the long tube above the bulb are made thick and circular shape. This acts as a magnifying glass for easy reading of the mercury thread in the stem. 5 The size of the thermometer is relatively small. The small size allows it to be portable and also cheap to produce.

2. Clinical thermometer Clinical thermometers are used to measure the temperature of human body in hospitals. In clinical thermometer also the physical property varies with temperature is volume of the liquid.

Purpose of working principles Main design features Purpose of working principles 1 The glass wall of the bulb is thin Thin wall of the bulb allow s conduction of heat quickly through the glass (a poor conductor of heat) to the liquid. 2 Bulb is made small to contain a small amount of liquid. Small amount of liquid will be more responsive to heat 3 The bore of capillary tube is fine and uniform The fine tube allows noticeable movement of the liquid for small change in temperature, (i.e. good sensitivity). The uniform tube ensures even expansion of the liquid. 4 The walls of the long tube above the bulb are made thick and circular shape or pear-shaped. This acts as a magnifying glass for easy reading of the mercury thread in the stem. 5 The scale is limited to small range between 35°C to 42°C. Since the normal body temperature is 37°C, the short range allows for greater accuracy and the stem can be made reasonably short. 6 A narrow constriction After taking temperature, it prevents contracting mercury from flowing back into the bulb by breaking the mercury threat at constriction.

The span of temperature that the thermometer can measure. Sensitivity, linearity and range of liquid-glass-thermometers Sensitivity Sensitivity of the thermometer is the change in reading of the thermometer for a small change in temperature. If the reading of the thermometer has a noticeable change for a small change in temperature, the sensitivity is HIGH. If the reading of the thermometer shows small difference change in temperature, the sensitivity is LOW. Sensitivity of the thermometer can be increased by narrowing the bore tube and using thin glass bulb. Range The span of temperature that the thermometer can measure. Example: For laboratory thermometer, −10°C to 110°C For clinical thermometer, 35°C to 42 °C

3. Thermocouple thermometer Sensitivity, linearity and range of liquid-glass-thermometers Linearity The liquid of the thermometer expands linearly with increase in temperature. For the same increase in temperature, the length of mercury thread increased by the same amount. 3. Thermocouple thermometer Thermocouple thermometer is used to measure very high temperatures in industries. It make use of physical property varies with temperature is electromotive force (e.m.f). Thermocouple has two junctions hot and cold junctions. To use thermocouple its ends are connected to a sensitive voltmeter. The one junction is place in melting ice at 0°C while other end is placed in object whose temperature is measured. The voltmeter shows the reading. The greater the voltage produce, the bigger difference in temperatures between two junctions. The thermocouple must be calibrate so that the temperature can be deduce from voltage.

Calibrating thermometer scale First put the unmarked thermometer into the ice and wait until the liquid of the thermometer becomes steady. When it becomes steady by using marker mark that point, which is ice point or lower fixed point.

After that put the unmarked thermometer into the boiling water and wait until the liquid of the thermometer becomes steady. When it becomes steady by using marker mark that point, which is steam point or upper fixed point. Now calibrate the scale of the thermometer between two fixed points by using ruler. Measure the distance between two fixed points and divide the divisions.