Overview
Digital Thermal scanner is a device that enables to measure temperature or temperature gradient without making any physical contact. This task is being accomplished by using electronic heat sensors rather than mercury like conventional thermometer.
Depending on the use and the situation, there are different types of digital thermometers. The most common type is that, used for checking body, air and food temperature and is very affordable.
The most important part of this device is the sensor. These sensors work either producing a voltage, current, or resistance change when there is a change of temperature. Each of these sensors gives a different analog output signal. The thermometer should measure the “signal” from the sensor, that will result is an electrical signal proportional to temperature.
The accuracy of the digital thermometer depends on both the analog and the digital environments.
Mainly 4 types of sensors being used:
Thermocouples:
It has a self-generated mV signal being proportional to the temperature difference between its two ends.
Resistance Temperature Detectors (RTD or PT100):
A RTD changes its resistance linearly with temperature.
Thermistors:
It is a resistor that changes its resistance, non-linear way with temperature.
Solid State Sensors:
It requires external power .It emits a small linear voltage proportional to temperature.
Different types of thermometer:
Digital Thermocouple Thermometer:
It needs two measurements to determine the temperature. Here the sensor measures the temperature at the junction where the thermocouple connects to it .This is known as “cold-junction compensation (CJC). It also measures the mV signal coming from the thermocouple. It subtracts the CJC temperature from the received hot end signal, followed by converting that voltage to temperature.
Digital RTD Thermometer:
It is basically an ohmmeter and work on the principle of measuring the resistance of the sensor. It applies either a very small excitation voltage or current to its sensor, followed by measuring the voltage across the sensor. Here a 4-wire connection is generally used to minimize measurement reading errors, where 2-wires carry the excitation and other 2-wires measures the voltage. It uses the Callendar Van Dusen equation in its functionality to describe the relationship between the resistance of a platinum resistance sensor (RTD) and temperature.
Digital Thermistor Thermometer:
It provides the thermistor with an excitation voltage or current. Here the thermistor change in resistance is comparatively large than the resistance of the leads. The thermistor generally has 2-wire connection. Here the excitation is converted to a voltage signal by the thermistor followed by the conversion of the measured voltage to temperature by device. It uses the tables or polynomials.
Digital Solid State Thermometer:
It provides an excitation current to the sensor in order to measure the linear (mV/°) signal from the sensor. Here the thermometer conditions the sensor followed by making an electrical measurement of its output. This signal, voltage or resistance, is basically the analog signal. In this thermometer this signal goes to an Analog to Digital converter that converts the analog signal into a series of digital pulses.
Now this digital signal is being matched against the resistance curves of the sensor. Then the CJC has been added for a thermocouple .The digital signal is being sent to a display drive that in turn converts the temperature signal turns on certain segments of the display so that and displays the temperature reading. It gets a linear mV/ᵒ signal, thus no additional digital conditioning is required. The accuracy it dependents upon three factors, that is precision of the excitation, the Analog to Digital converter and the inherent error of the sensor.
Application:
Business houses
Defense area
Healthcare industry
