**2. Contact sensors**

When positioned close to an object to be detected for heat or cold, contact sensors are used to measure the object's temperature. These sensors can determine the concentration of liquids, solids or gases throughout a wide temperature range.

Thermocouples and thermistors are good examples of contact temperature sensors.

Thermocouples are inexpensive, and it is easy to find the basic materials needed to manufacture thermocouples [15, 16].

Contact Sensors are devices that measure temperature by placing it in direct contact with the object being measured or the desired measurement environment. They can be used to detect temperature changes in gases, liquids or solids in a range of temperature measurements. Thermocouples and thermistors are two contact sensor types. Its model and fundamental components are straightforward, and thermocouples are frequently inexpensive.

Additionally, thermocouples have the broadest temperature range of any temperature sensor, ranging from well below -200°C to well over 2000°C [16].

Thermocouples are thermoelectric sensors that are essentially made of two welded or crimped junctions of dissimilar metals, such as copper and constantan. The reference (cold) junction and the measuring (hot) junction are the two junctions that are maintained at the same temperature. As illustrated below, a voltage is created across the junction when the two junctions are at different temperatures. This voltage is used to measure the temperature sensor [16].

## **2.1 Construction of a thermocouple**

**Figure 3** shows how a thermocouple is constructed by joining two metals of iron and constantan.

**Figure 3.** *Construction of a thermocouple [16].*

## **2.2 Working principle of a thermocouple**

The thermocouple's working principle is quite straightforward and fundamental. When two different metals, such as copper and constantan, are fused together, a "thermoelectric" effect results, producing a constant potential difference between the two materials of only a few millivolts (mV). The "Seebeck effect" refers to the voltage differential between the two junctions because an electromagnetic field (emf) is created when a temperature gradient develops between the conducting wires. The output voltage of a thermocouple is then dependent on temperature variations [17].

If both junctions in **Figure 3** are at the same temperature (zero potential difference across the junctions), and there is no voltage output because V1 = V2. But when the junctions are linked together in a circuit and operate at different temperatures, a voltage output, V1 - V2, corresponding to the temperature differential between the two junctions, will be noticed. This is because the characteristics of the two different metals employed influence how much of a voltage difference will increase with temperature until the junction reaches its maximum voltage level [17].

Extreme temperatures between 200°C and over +2000°C can be recorded using thermocouples, which can be constructed from various materials. Internationally recognised standards have been created with thermocouple colour codes to help users select the best thermocouple sensor for a given application due to the wide variety of materials and temperature ranges available. Below is a list of the standard thermocouple colours used in Britain [17].

**Figure 4**. shows the thermocouple colour codes that were used in the manufacturing of different types of thermocouples. Thermistor contacts are the second kind of contact temperature sensor. The resistance of thermistors is dependent on temperature change, as opposed to other types of resistors whose value is determined by the colour code [18].

Thermistors are available in two types which are:


A PTC thermistor's resistance rises with temperature, but an NTC thermistor's resistance falls with temperature. Therefore, an NTC thermistor is the most common type of thermistor.

Temperature sensors include thermocouples. They can be found in common appliances, including ovens, refrigerators and fire alarms. Thermometers and numerous other vehicle appliances also include them [18].

**Figures 5** and **6** show PTC (left) and NTC (right) thermistor electrical symbols and a typical NTC thermistor.

#### **2.3 Advantages of a thermistor**



**Figure 4.** *Thermocouple colour codes [17].*
