One of the most commonly used medical devices which every household has is an Electronic thermometer. It is the ‘go-to’ device when one feels feverish and the body is hotter than the normal temperature which varies from 97° Fahrenheit (36.1°Celsius) to 99° Fahrenheit (37.2° Celsius).
The electronic thermometer measures the body’s temperature if it is normal or more than the normal. A temperature over 100.4° Fahrenheit (38°Celsius) most often means that one is having fever which may be caused by an illness or infection.
In the olden days, this increase in temperature was measured using the old-fashioned mercury thermometers.
These mercury thermometers had mercury inside a glass tube. When the thermometer is put inside the mouth or under the armpit to measure the body temperature, the mercury expands by an amount that relates directly to the temperature increase.
But gone are the days when people used to measure temperature using these simple thermometers.
Nowadays, electronic thermometers are available in the market whose functions are based on a different scientific principle and have a wide range of advantages which mercury thermometers do not have.
These electronic thermometers are also called digital thermometers, as the measured temperature is displayed digitally. In the digital world in which we all are living, we have used or at least seen an electronic thermometer.
An electronic thermometer has many advantages. The first one is of course that they have a Liquid Crystal Display (LCD). So one does not need extra efforts to read the temperature as it is conveniently displayed.
Also, the temperature can be read accurately after the decimal point. Mostly the electronic thermometers are equipped with a buzzer which beeps an alarm when the measurement is complete.
Some other advanced features include memory functions which can record and store multiple readings.
The electronic thermometers are fast and mostly the temperature shows up within a minute. These thermometers are made of fibres or plastic body except for the tip so, they are easy to handle than the glass-based mercury thermometers.
Finally and most importantly the electronic thermometers do not have mercury which is a highly toxic element hence they are safe to use.
So let us find out how an electronic thermometer functions?
The main part of any electronic thermometer is the sensor. Most of the sensors are either Resistance Temperature Detectors (RTD), Thermistors, Thermocouples or Solid-State Sensors.
All of these sensors produce either a voltage, current, or resistance change when there is a change of temperature.
The voltage or current are measured in analog signals and are changed into a digital signal using an electronic circuit called the analog to digital converters.
RTD based thermometers are also known as the resistance thermometers and are one of the common types of electronic thermometers.
They are based on the principle that the electrical resistance of metal changes with temperature. So, when a very small current is given (through a battery) to the sensor, with a change in the metal resistance (due to temperature) the voltage will also change. This change is calculated and indicates the body temperature.
A thermistor or a “Thermal Resistor” is a resistance thermometer, whose resistance is also dependent on temperature. It is made of metallic oxides, pressed into a bead, disk, or cylindrical shape and then encapsulated with epoxy or glass.
The main difference between RTDs and thermistors is the material by which they are made. While RTD resistors are pure metal, thermistors are made of polymer or ceramic materials. Thermistors are more temperature-sensitive than RTDs.
Thermocouple based thermometers are based on the principle that sensors composed of two metals that generate voltages when there are temperature differences between them. The amount of voltage produced is dependent on these differences in the temperature. This is called the Seebeck Effect.
A solid-state temperature sensor-based electronic thermometer uses the PN junction of a diode to measure temperature. It is similar to the thermocouple based thermometer where a voltage is developed across this junction. This voltage changes according to the temperature of the junction.
In all the above cases, an electronic chip converts the change in the value of voltage or resistance or current into an extremely accurate reading of temperature and display it numerically on a digital screen.
All digital thermometers work on button-sized batteries. The battery is made up of metal, is small so that it can be housed at the back of the electronic thermometer casing.
The type of battery used to power the thermometer is called button cell. It supplies a voltage of 1.5 V and has excellent resistance to leakage therefore it can be stored for a longer period of time.
Infrared (IR) based electronic thermometer
The advantage of IR based thermometer is that they are handheld and the thermometer can read the temperature of a person from some distance hence they are suitable for places where the temperature of hundreds of people are to be taken without any physical contact.
The IR thermometer finds application in places like hospitals, during some pandemic or medical camps etc. Since it is contactless, it needs no sterilization so it is the best choice when it comes to hygiene.
IR thermometers are based on the principle that every object emits some amount of energy, and that energy is emitted in the form of heat.
Since heat is emitted by the human body also, an IR thermometer can use the difference between the IR rays coming off the human body and the surrounding environment to determine the temperature of the human body.
The difference in temperature is changed to the current which is finally displayed on the electronic thermometer. The reading of an IR thermometer is generated in a matter of seconds as compared to the regular electronic thermometer.
So as we see the scientific principles on which an electronic thermometer is based might be different but the measured parameter is finally changed to a current which is displayed as a digital display.
Maybe the next time you measure your body temperature using an electronic thermometer you will have a bit of extra information on how this common device is functioning.