As soon as the weather changes, the first sign of cold, viral fever, flu or any internal infection appears in the form of increasing body temperature. In such a situation, before running to the doctor or starting first aid at home, the first medical device that comes to hand is a thermometer. In medical science, a thermometer is considered the primary diagnostic tool of the human body, which measures how hot our immune system is getting while fighting a foreign virus or bacteria. In ordinary days, people think of it as just a small stick to measure fever, but complex physics and sensor technology work behind the different types of thermometers available in the market. In today's time when healthcare awareness has increased, it has become essential to have an accurate and reliable thermometer in the home medicine cabinet. This question arises in the minds of many people that what is the real value of a good thermometer, which thermometer gives the most accurate results and after all, what technology is hidden inside the glass tube or digital screen which converts our body heat into degrees Fahrenheit or Celsius in a few seconds. Major types of thermometers in the market and their current prices There are mainly three types of clinical thermometers available in the Indian market and local medical stores, the prices of which are decided based on their technology, brand and features. The most traditional category is the mercury glass thermometer, which costs around ₹50 to ₹150. However, due to the toxicity of mercury and the danger of glass breaking, its use has now been limited in many hospitals. The second and most popular category is that of digital electronic thermometer, which has become the first choice of every middle class family today. The starting price of a basic digital thermometer starts from ₹100 to ₹150 and goes up to ₹250 to ₹500 for brands like Doctor Morepen, Hicks or Citizen. These have features like flexible tip, waterproof body, liquid crystal display and beep sound. The third modern category is non-contact infrared thermometer (thermal gun), which measures the temperature from the forehead without touching the patient. This technique is considered suitable for newborns, children and clinics. Infrared guns from good brands like Omron or Dr. Trust usually cost between ₹800 to ₹2,500. Apart from this, tympanic (ear) thermometers which measure temperature through ears are also available in the market in the bracket of ₹ 1,200 to ₹ 3,000. How a Mercury Thermometer Works: The Simple Science of Thermal Expansion The working of a traditional glass clinical thermometer is based purely on the law of 'thermal expansion' of physics. At the lower end of this thermometer there is a thin metal or glass bulb, inside which liquid mercury is filled. Mercury is the only metal which remains in liquid state even at normal room temperature and expands or contracts very rapidly on slight fluctuations of temperature. When the bulb of the thermometer is placed under the tongue or in the armpit of the patient, the body's internal heat is transferred through the bulb to the mercury. On receiving heat, the kinetic energy in the mercury molecules increases and they start expanding. Since there is only a very fine glass capillary tube in front of the bulb, the mercury has no other way to spread except by climbing up. A narrow constriction is made at the base of the capillary tube, which prevents the mercury from falling down automatically when the thermometer is taken out of the mouth after measuring the temperature. The doctor determines to what mark the mercury has risen by looking at the 95°F to 108°F (or 35°C to 42°C) markings on the glass, and then shaking it forcefully to send the mercury back down into the bulb. Internal mechanism of digital thermometer: The game of thermistor and microprocessor. The digital thermometers used in homes today do not contain any kind of liquid or mercury, rather it works completely on electronic circuits. Just behind its sharp end (metal tip) there is a special type of electronic component called thermistor (thermal resistor). The thermistor is made of semiconductor metal oxide, whose electrical resistance changes very sensitively as the temperature changes. It usually features Negative Temperature Coefficient (NTC) technology, which means that as the tip is heated by the patient's body heat, the electrical resistance of the thermistor decreases rapidly. A small button cell inside the thermometer passes a small current into the battery circuit. When the resistance changes, the current flow also changes in the same proportion. A microprocessor chip inside calculates this current change in milliseconds and converts it into a precise numerical temperature via an analog-to-digital converter (ADC). As soon as the temperature rise becomes stable, a beeper fitted in the circuit sounds and the final temperature is recorded on the LCD screen. Non-contact measurement technology of infrared thermometers: Thermopile and infrared rays Non-contact infrared thermometers have become very popular in public places, hospitals and homes since the Covid pandemic. The principle of this thermometer works on the natural scientific truth that every object and living body in the universe existing above absolute zero temperature (-273.15°C) emits invisible 'thermal radiation' i.e. infrared rays. The warmer a person's body is, the greater the intensity of infrared rays emitted from his skin. An optical lens is fitted in front of the infrared thermometer, which collects the thermal rays emanating from the patient's forehead or temple and focuses them on the thermopile sensor installed inside. A thermopile is actually a group of many small thermocouples, which convert the heat of infrared radiation directly into microvolt electricity. The device's internal computer program decodes this voltage into Fahrenheit or Celsius temperature and displays the result on the screen within just one to two seconds. There is no need to touch the patient, due to which the risk of spreading the infection from one person to another becomes zero. Basic rules and correct method of measuring body temperature: No matter how expensive a thermometer is, if it is used incorrectly, it can give wrong readings, which can divert the direction of treatment. The normal body temperature of a healthy adult is considered to be around 98.6°F (37°C), although this can normally fluctuate from 97.8°F to 99°F depending on the time of day and physical activity. If you are taking a reading in the mouth (oral) with a digital thermometer, place the tip at the back just below the tongue and keep the mouth closed; Keep in mind that the patient should not eat or drink anything cold or very hot for at least 15 minutes before measuring the temperature. If measuring temperature in the axilla, keep the tip dry from sweat and against the skin; Axillary readings are usually about 0.5°F to 1°F lower than oral readings, so doctors often add 1 degree to estimate total fever. In case of children, while using the infrared gun, wipe the sweat from the forehead and keep the device at a prescribed distance of 1 to 3 cm from the forehead. Thermometer Cleaning, Maintenance and Best Choice for Home Proper maintenance of the thermometer is very important to maintain its accuracy. The tip of the digital thermometer should be thoroughly cleaned with 70% isopropyl alcohol or a sanitizing wipe after each use; Never wash it in boiling water as this may damage the electronic circuits or battery. When the battery indicator on the display appears slow or blinking, insert a new cell immediately as weak batteries sometimes cause the thermometer to give incorrect readings. If you have small children or elderly people in your house, then a waterproof digital thermometer with flexible soft tip is the most suitable and safe option. Whereas for newborn babies, infrared gun which gives reading in seconds without touching proves to be the most convenient. One should avoid keeping a thermometer containing mercury in the house, because if it accidentally falls on the floor and breaks, the poisonous vapors of mercury can cause severe damage to the lungs and nervous system through inhalation.
