There are two standard units used for measuring temperature, viz. Celsius and Fahrenheit. When we think of a thermometer, a mercury-filled glass tube comes to our mind. Galileo invented a basic thermometer in 1593, but it was not approved for many years. Later on, Gabriel Fahrenheit invented the mercury-filled thermometer. Along with mercury, alcohol was also used earlier to measure temperature in a thermometer. Today, there are different kinds of thermometers which are more accurate in terms of temperature measurement and are very easy to use.
Measurement Units of Thermometer: Celsius and Fahrenheit
The Celsius thermometers use the Celsius (°C) as the measurement unit and Fahrenheit thermometers use Fahrenheit (°F) as the temperature measurement unit. Some thermometers have the measuring scales for both Celsius and Fahrenheit. These thermometers give the reading in Celsius as well as Fahrenheit.
Applications of Different Thermometers
➥ Clinical Thermometers
The clinical thermometers are used to measure the body temperature of the patient. There are again three types of clinical thermometers depending on the body part used to measure the temperature.
● Ear (Tympanic) Thermometers
Human ear is located near the brain. This makes it an accurate point to measure the body temperature. The temperature of the eardrum is measured by the ear thermometers. However, the eardrum is most fragile and delicate body part. Therefore, the body temperature cannot be measured by touching the eardrum. For temperature measurement, infrared sensors are used to remotely sense the temperature of the eardrum. Thermopile, an infrared sensor, is commonly used in ear thermometer.
● Pacifier Thermometers
The pacifier thermometers are used to check the body temperature of babies or infants. They help measure the body temperature without irritating the baby. The thermometer is held in the mouth of the baby and the baby's natural sucking instinct is used to check its body temperature. The pacifier thermometers are very safe for checking the body temperature of babies.
● Underarm or Oral Thermometers
The underarm thermometers are kept in the underarms to measure the body temperature of the patient. Likewise, the oral thermometers are held in the mouth for temperature measurement. Rectal thermometer also works on the principal of other oral thermometers. It is however considered one of the most accurate method to find the body's temperature.
➥ Food Thermometers
Following are some of the most commonly used food thermometers:
● Dial oven-safe thermometers
The dial oven-safe thermometers are used for thick foods and can be placed in the food while you are cooking the food. However, the dial oven-safe thermometers are not suitable for thin and watery foodstuffs.
● Digital instant thermometers
These thermometers read the temperature within 10 seconds but cannot be kept in the food while cooking.
● Pop-up thermometers
The pop-up thermometers are used to measure the temperatures of turkey and chicken.
● Disposable thermometers
The disposable thermometer strips are used to measure the temperature of food after they are cooked and they change the color according to the temperature. The color of these thermometer strips can be matched to a chart, which gives the corresponding temperature of food.
➥ Outdoor Thermometers
Also known as weather thermometer, the outdoor thermometers are used to measure the temperature of the surrounding air. These thermometers are usually filled with mercury and the recorded temperature can be seen as the rise/drop of the mercury level in the bulb. You can also get spring based weather thermometers which shows the temperature according to the expansion and contraction of the heat sensitive spring.
Mechanism of Different Kinds of Thermometers
➥ Mercury and Alcohol Thermometers
Mercury thermometers have mercury filled in a glass tube and has a glass bulb at the bottom. As the temperature increases, the mercury rises in the glass tube. The glass tube is calibrated in Celsius, Fahrenheit or both. The rise in mercury determines the temperature according to the calibration. Alcohol thermometers have ethanol or toluene instead of mercury in the glass tube. All the other mechanism of the thermometer are same as that of the mercury thermometer. They are used as clinical thermometers and outdoor thermometers.
➥ Bimetal Thermometer
A bimetal thermometer has a strip usually in the form of spiral rod. This strip has two fused strips of different metals or alloys. When there is a change in temperature the strip beds or unwinds which causes the pointer attached to the end of the strip to move which indicates the temperature.
➥ Resistance Thermometer
In this thermometer the temperature is recorded by a thermistor, which is a resistor made up of semiconductor material. The change in temperature is indicated on a meter or a digital display.
➥ Infrared Thermometer
Infrared thermometers use the infrared sensors which measure the heat radiations emitted by the object to determine the temperature. One of the best examples of infrared thermometer is ear thermometer which records the temperature of the tympanic membrane. Laser thermometer also works on the principle of infrared thermometer. It is just facilitated with a laser beam which helps you to target the area whose temperature needs to be determined. The temperature can be seen on a digital display. The best thing about this thermometer is it can measure the temperature of a very hot object without coming in contact with the object.
➥ Gas Thermometers
Gas thermometer can be of mainly two types, constant volume and constant pressure gas thermometer. The constant volume thermometer determines the temperature by measuring the pressure required to keep the volume of the gas constant. Whereas constant pressure thermometer determines the temperature on the basis of thermal expansion of the gases at constant pressure.
Various types of thermometers are available today, instead of the traditional mercury thermometers. So, you can use the best suited thermometer depending upon your application. Calibrate your instrument before use and follow proper guidelines to get accurate readings. External and internal factors can also affect the readings so consider taking at least three readings.