1.0 Introduction
A sensor is a device that measures a physical quantity and converts it into a signal which can be read by an observer or by and instrument. For example, a mercury-in-glass thermometer converts the measured temperature into expansion and contraction of a liquid which can be read on a calibrated glass tube.
A thermocouple converts temperature to an output voltage which can be read by a voltmeter. For accuracy, all sensors need to be calibrated against known standards.
A sensor is a device which receives and responds to a signal or stimulus. Here the term “stimulus” means a property or a quantity that needs to be converted into electrical form. Hence, sensor can be defined as a device which receives a signal and converts it into electrical form which can be further used for electronic devices.
Sensor sensitivity indicates how much the sensor's output mercury in a thermometer moves 1 cm when the temperature changes by 1°C, the sensitivity is 1cm/°C. Sensors that measures very small changes by changes must have very high sensitivities. Sensors also have impact on what they measure; for instant, a room temperature thermometer inserted into a got cup of liquid cools the liquid while the liquid heats thermometer.
Sensors need to the designed to have a small effect on what is measured; making the sensor smaller often improves this and may introduce other advantages. Classification of Measurement Errors
A good sensor obeys the following rules:
- It's sensitive to the measure property;
- It's insensitive to any other property;
- It does not influence the measured property.
Ideal sensors are designed to be linear. The output signal of such a sensor is linearly proportional to the value of the measured property. The sensitivity is then defined as the ratio between the output signal and measured property. For example if a sensor measures temperature and has a voltage output, the sensitivity is a constant with the unit [V/K]. This sensor is linear because the ratio is constant at all points of measurement.
Types of Temperature Measurement Sensors
1) Thermocuple Temperature Measurement Sensors:
Thermocouple consists essential of two strings or wire made of different metals and joined at one end. Changes in the temperature at the juncture induce a change in electromotive force (emf) between the other ends. As temperature goes up, this output emf of thermocouple rises, though not necessarily linear.
2) Resistance Temparature Devices (Rtd)
Resistive temperature devices capitalize on the fact that the electrical resistance of a material changes as its temperature changes. Two key types are the metallic devices (commonly referred to as RTD) and thermistor.
As their name indicates, RTD rely on resistance change in a metal with the resistance rising more or less linearly with temperature. Thermistors are based on resistance change in a ceramic semi conductor. The resistance drops nonlinearly with temperature rise.
3) Infrared Temperature Measurement Devices
Infrared sensors are non-contacting devices. They infer temperature by measuring thermal radiation emitted by a material.
4) Bimetallic Temperature Measurement Device
Bimetallic device takes advantage of the difference, if rate of thermal expansion between different metals strips of two metals are bonded together. When heated one side will expand more than the other and the resulting bonding is translated into a temperature reading by mechanical linkage to a pointer.
5) Fluid Expansion Temperature Measurement Devices
Fluid-expansion devices, typified by the household thermometer, generally come in two main classifications: the measuring type and the organic-liquid type. Versions employing gas instead of liquid are also available. Mercury is considered an environmental hazard, so there are regulations governing the shipment of devices that contain it. Fluid-expansion sensors do not require electric power. They do pose explosion hazards, and are stable even after repeater cycling.
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