Digital Thermometers and Its Applications


A thermometer is a device used to measure the temperature of any particular device or living body and displays the reading. A thermometer scale can be in Fahrenheit or Celsius.

2 Kinds of Conventional Thermometers used earlier

1. Bulb or Mercury Thermometers: These thermometers consisting of a sealed glass tube with a bulb-like glass container at the end. It works on the principle that liquids expand on getting heated. However, a disadvantage of these thermometers is that they can measure temperature only up to a certain extent. Also, the thermometers are based on the expansion of matter with the rise in temperature and the readings were made from the scale readings. This often leads to erroneous results. Also, in case of the tube breaking accidentally or deliberately, the leaked out mercury can be very dangerous. Hence these thermometers have to be very carefully handled. Also, mercury has a low freezing point and cannot be used at places with low ambient temperature.

2. Bimetallic Thermometers: These thermometers consist of two metals joined together and as these metals get heated, they get expanded at different rates causing the bending of either of the metal. This bimetallic strip is attached to dial with a calibrated temperature scale to indicate the readings. These thermometers can be connected to a switch on the other end and the change in temperature can cause the switch to open and close. These thermometers can be used to control the temperature. They can be installed inside a refrigerator or an oven. However, these systems are also easily prone to breakage. The calibration is not accurate and can change easily. Also, these thermometers cannot be used at low temperatures.

Having read the above-written stuff, you must have by now had an idea about thermometers and the need to shift to a different approach to thermometers. In both the above two kinds of thermometers, the major problem lies in the principle and in the display technique used. Thus a basic solution is in replacing the whole principle and the display method.

Defining Digital Thermometer:

It consists of a thermistor to sense the temperature and an electronic display of the temperature. Digital thermometers are used orally, rectally, or under the arm. It can read the temperature from 94⁰F to 105⁰F.

Digital Thermometer components:

  • Battery: It is a button cell LR41battery made up of metal and provides around 1.5V supply to the thermometer.
LR41 (LR736) cell By Lead Holder
LR41 (LR736) cell By Lead Holder
  • Body: The body of the thermometer is made up of hard plastic and is 100.5mm long and its width varies from bottom to top, with the bottom being thinner.
Digital Medical Thermometer by rambergmedia
Digital Medical Thermometer by rambergmedia
  • Thermister: It is a semiconductor material made up of ceramic and is used to sense the temperature. It is placed at the tip of the thermometer by binding with epoxy and is enclosed inside a cap made up of stainless steel.
NTC bead type thermistor by Ansgar Hellwig
NTC bead type thermistor by Ansgar Hellwig
  • LCD: It is the display of the thermometer and measures around 15.5mm long and 6.5mm wide. It displays the reading for 3 seconds and then starts flashing indicating the next temperature to be measured.
  • Circuit: It consists of an ADC and a microcontroller along with some passive components.
Digital Thermometer Circuit by GXTI
Digital Thermometer Circuit by GXTI

Digital thermometer Principle of Working

The digital thermometer basically consists of a sensor that measures the change in resistance due to heat and converts this change in resistance to temperature.

Digital Thermometer Circuit:

Digital Thermometer Circuit
Digital Thermometer Circuit

The thermistor is a resistor whose resistance value varies with temperature.  As the thermistor gets heated, its resistance increases or decreases (depending on whether it is a negative temperature coefficient or positive temperature coefficient). The analog output from the thermistor is supplied to the ADC through the wires, where it is converted to a digital signal and is then given to the microcontroller for further processing and the output in form of temperature reading is displayed on the LCD interfaced to the microcontroller.

A  Digital Thermometer kit using Temperature Sensor DS1620 and a control application

A Digital Temperature sensor is used which provides 9-bit temperature reading and is interfaced to the microcontroller. The microcontroller receives this digital input and displays it on the LCD interfaced to it.

Digital Thermometer circuit diagram
Digital Thermometer circuit diagram

The above system consists of temperature sensor IC DS1620 which is an 8 pin IC and can measure temperature from -55 degrees Celsius to +125 degrees Celsius. It contains two pins which indicate if the measured temperature exceeds the user-defined temperature. Thus this device can also be used to control the switching of loads in case of any temperature fluctuations.

In the above system, the temperature IC first measures the ambient temperature and converts this temperature to digital data and feeds it to the microcontroller which displays the temperature reading on the display. Using the push button switches the user-defined temperature can be set. When the ambient temperature increases or decreases then the user-defined temperature, the microcontroller accordingly controls the switching of the relay and so the load.

Modern Available Digital Thermometers:

Digital Thermometer ref ECT-1: It measures temperature from 32⁰C to 42⁰C with an accuracy of 0.1⁰C. It is used mainly in medical applications.

Digital ThermometerModel Number: EFT-3: It measures temperature from 50⁰C to 125⁰C. It is used to measure the temperature of solid and liquid food

Thermolab digital thermometer IP65: It measures temperature range from 50 to 200 ⁰C with an accuracy of +/-1⁰C.

Advantages of Digital Thermometers:

  • Accuracy: The temperature reading doesn’t depend on scale reading and instead shown directly on the display. Hence temperature can be read exactly and accurately.
  • Speed: Digital thermometers can reach a final temperature in 5 to 10 seconds compared to conventional thermometers.
  • Safety: Digital thermometers don’t use mercury, hence the hazards of the mercury is eliminated in case the thermometer breaks.
  • Strong: The thermometer doesn’t need to be shaken for the proper mercury level, hence the risk of the tube getting broken is eliminated.

Applications of Digital Thermometer:

Medical Applications: The digital thermometers are used to measure human body temperature around 37⁰C. These thermometers are mostly probe type or ear type. It measures oral, rectal, and armpit body temperature.

Marine Applications: Digital thermometers with a high-temperature exhaust gas sensor as the temperature sensor can be used in marine applications for measuring the local temperature.

Industrial Applications: Digital thermometers are also used in power plants, nuclear power plants, blast furnaces, shipbuilding industries, etc. They can measure temperature from -220⁰C to +850⁰C.

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So now let me know more about the practical applications of Digital Thermometer?

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