SENSORS Prepared by Dr Mohamed Ahmed Awad Observable

SENSORS Prepared by Dr. Mohamed Ahmed Awad

Observable effects of temperature change • • • Change in physical state Change in chemical state Change in dimension Change in electrical properties Change in radiation properties Prepared by Dr. Mohamed Ahmed Awad

• Change in physical state (boiling and freezing point) • Last three type of change are used for the temperature measurement. Prepared by Dr. Mohamed Ahmed Awad

Various temperature measuring devices • Mechanical Methods • Electrical methods • Based on radiation properties Prepared by Dr. Mohamed Ahmed Awad

Mechanical Methods • Expansion of solids – Solid Rod thermometer – Bimetallic thermometer • Expansion of liquids – Liquid in glass thermometer – Liquid in metal thermometer • Expansion of gases – Gas thermometer Prepared by Dr. Mohamed Ahmed Awad

Electrical Methods • • Electrical resistance thermometer Thermistors Thermocouples Thermopile Prepared by Dr. Mohamed Ahmed Awad

Radiation Methods • Total radiation pyrometer • Optical pyrometer • Infra red pyrometer Prepared by Dr. Mohamed Ahmed Awad

Solid rod thermometer • The change in the length due to change in temperature change is • Δl = l *α *Δt The expansion of a single metal is widely used in the rod type thermostat as switch But not used for indicating thermometer Prepared by Dr. Mohamed Ahmed Awad

Thermostat • Metals expand at different rates/ The major element is two strips of different metals bonded together. As the bimetal strip is heated, the two metal strips begin to expand. Because of expanding difference, the strip bends away from the metal with the higher coefficient of expansion. • The strip is used as a part of the switching circuit, which contact if the strip bends Prepared by Dr. Mohamed Ahmed Awad

Prepared by Dr. Mohamed Ahmed Awad

Bimetal thermometer Prepared by Dr. Mohamed Ahmed Awad

When two metal strips having different coefficient expansion are bonded together, an increase of temperature causes the deflection of the free end of the strip. The defection is nearly linear. One metal with low thermal expansion is used and the other one with high expansion like brass is used Prepared by Dr. Mohamed Ahmed Awad

Bimetal thermometer Prepared by Dr. Mohamed Ahmed Awad

Liquid in Glass thermometer It works on the principle of volume expansion It consists of Alcohol and mercury are two Liquids most commonly used Prepared by Dr. Mohamed Ahmed Awad

Electrical methods Thermo resistive or variable resistance Thermo electric or e. m. f. generating Prepared by Dr. Mohamed Ahmed Awad

Resistance Temperature Detector (RTD) • The Resistance Temperature Detector (RTD) or resistance thermometer uses the fact that the resistance of metals increases with temperature. Examples are RTD's are shown schematically below. Prepared by Dr. Mohamed Ahmed Awad

Thermometer – typical devices use platinum wire (such a device is called a platinum resistance thermometers or PRT) – linear but has poor sensitivity Prepared by Dr. Mohamed Ahmed Awad

RTD • Pros – Stable and accurate – Linearity is better than thermocouples – Higher signal to noise ratio • Cons – More Expensive – Self heating – Requires a current source Prepared by Dr. Mohamed Ahmed Awad

Bulk Semiconductor Sensor (Thermistor) • Similar to Resistance Temperature Detectors (RTD), the Thermistor (Bulk Semiconductor Sensor) uses resistance to detect temperature. However, unlike an RTD's metal probe where the resistance increases with temperature, thermistor uses ceramic semi conducting materials which respond inversely with temperature. Examples of thermistors are shown in the following schematic Prepared by Dr. Mohamed Ahmed Awad

Bulk Semiconductor Sensor Thermistors • use materials with a high thermal coefficient of – resistance sensitive but highly non-linear – A threaded thermistor Prepared by Dr. Mohamed Ahmed Awad

Pros and Cons • Pros – High accuracy – High sensitivity – Small size compared to thermocouple – Response time shorter than RTD • Cons – Limited temperature range (100 -150) – Non linear resistance temperature relationship Prepared by Dr. Mohamed Ahmed Awad

Generating e m f (Thermocouples • Thermocouples operate on the principle that two wires made of dissimilar materials connected at either end will generate a potential (a difference in voltage) between the two ends that is a function of the materials and the temperature difference between the two ends. Prepared by Dr. Mohamed Ahmed Awad

i s a t e m p e r a t u r e d i f f e r e n c Prepared by Dr. e. Mohamed Ahmed Awad m Thermocouple • See beck effect: current will flow through a junction of dissimilar metals if there is a temperature difference • mill volts and non linear • varies from 6 u V/C to 90 u V/C

Prepared by Dr. Mohamed Ahmed Awad

• The temperature difference between the two junctions is detected by measuring the change in voltage (electromotive force, EMF) across the dissimilar metals at the temperature measurement junction Prepared by Dr. Mohamed Ahmed Awad

Thermocouples Prepared by Dr. Mohamed Ahmed Awad

Thermocouples circuit Prepared by Dr. Mohamed Ahmed Awad

Prepared by Dr. Mohamed Ahmed Awad

Thermocouple compensation Prepared by Dr. Mohamed Ahmed Awad

Pros and Cons Pros – – – Low cost No moving parts, less likely to be broken Wide temperature range Reasonable short response time Reasonable repeatability and accuracy Cons – Low sensitivity – Requires a known temperature reference, usually 0°C (32°F) ice water – Nonlinearity could be bothersome Prepared by Dr. Mohamed Ahmed Awad

Radiation Pyrometer • A Pyrometer, or radiation thermometer, is a non-contact instrument that detects an object's surface temperature by measuring the temperature of the electromagnetic radiation (infrared or visible) emitted from the object. Prepared by Dr. Mohamed Ahmed Awad

Pyrometer Prepared by Dr. Mohamed Ahmed Awad

The Electromagnetic Radiation Spectrum Prepared by Dr. Mohamed Ahmed Awad

Pros and Cons • Pros – Non contact Measurement – Fast response time – Good stability • Cons – Expensive – Accuracy may be affected by surrounding – ( dust, smoke, thermal radiation) Prepared by Dr. Mohamed Ahmed Awad