Sensors/Transducers

Sensors/Transducers& types of Temperature and Pressure transducers

Shaheera MaH e Muneer 11-MCT-34Khush Bakhat Rana 11-MCT-67

Sensor: As the term suggests, it is a body whichreactsto a physical, chemical or biological condition. It can be considered as a detector. It converts into another analogue quantity that can be measured electrically such as voltage, ohmic resistance, inductance or capacitance. Transducer: The conversion ofenergy from one form to another is known as Transduction. A transducer serves for this purpose. Transducer is a device that is connected to sensor so as to convert the measured quantity into standard analog electrical signal such as 0 to10 VDC, -10 to +10 VDC,4 to20 mA,0 to20 mA,0 to 25mA etc. The output can be used directly by the system designerBlock Diagram

Temperature Transducer:Temperature transducers can be divided into four main categoriesResistance temperature detectors (RTD) ThermocouplesThermistorsUltrasonic thermometer

1. Resistance temperature detectors (RTD) RTD commonly employ platinum, nickel, or resistance wire elements, whose resistance variation with temperature has a high intrinsic accuracy. They are available in many configurations and sizes. They are shielded or open units for both immersion and surface applications.

Resistance temperature detectors (RTD): The relationship between temperature and resistance of conductors can be calculated from the equation. Where:R = The resistance of the conductor at temperature t (C) Ro= The resistance at the reference temperature, usually 20C = The temperature coefficient of resistanceT= The difference between the operating and the reference temperature

72. Thermocouples:Thermocouples are made from a number of difference metals or metal alloys covering a wide range of temperatures from as low as -270C(-418F) to as high as 2700`C (about 5000F).

8Internal circuitry of Thermocouple: When a pair of wires made of different metals are joined together at one end, temperature difference between this end and the other end of the wires produces a voltage between the wires. The magnitude of this voltage depends on the materials used for the wires and the amount of temperature difference between the joined ends and the other ends. The other ends of the wire are connected to the voltage measuring equipment. Junction of dissimilar metals will produce some thermocouple voltage so the wires between the sensing junction and the rest of the equipment must be carefully controlled.

3. Thermistors: A thermistor is a semiconductor made by sintering mixtures of metallic oxide, such as oxides of manganese, nickel, cobalt, copper, and uranium. Thermistors have a negative temperature coefficient. That is why their resistance decreases as their temperature rises. The electrical resistance of most materials changes with the temperature. By choosing materials that are very sensitive to temperature.

Ultrasonic Temperature Transducers: Ultrasonic's, which are sound vibrations above 20,000 Hz. can be useful when we are concerned with rapid temperature fluctuations. Temperature extremes, limited access, nuclear and other severe environmental conditions or when we must measure the temperature distribution inside solid bodies. The need to measure simultaneously the distribution of parameters other than temperature (e.g., flow)

Applications: Thermistors can be connected in series parallel arrangements for applications requiring greater power handling capability. High resistance find application in measurements that employ wires or cables with small quantities of lead. Thermistors are chemically stable and can be used in nuclear environments. Their wide range of characteristics also permits them to be used in limiting and regulation circuits, as time delays.

Pressure Transducers:

An electronic device that converts pressure into electrical signals or mechanical movement. Pressure is a key factor in almost every fluid-power circuit. Pressure transducers can interface that factor with the control system.Generally, a pressure transducer consists of two parts. An elastic material which deforms under the application of pressure. An electrical part which detects this deformation.

Types of Pressure Transducers:

On the basis of measuring fluid type.Static measurement Dynamic measurementOn the basis of mounting.Standard Top mountSide mountHigh accuracy High temperature.On the basis of working:Mechanical pressure transducersElectrical pressure transducers

On the basis of working:

Mechanical pressure transducers:Mechanical movement is produced with the following elements. Bourdon Tube. Spring and Piston. Bellows and capsules. Diaphragm.Electrical Pressure Transducers: Strain Gauge types. Piezo electric types. Other electric effects.(capacitive and inductive)BOURDON TUBE:

Hollow tube with an elliptical cross section. When a pressure difference exists between the inside and outside, the tube tends to straighten out and the end moves. The movement is usually coupled to a needle on a dial to make a complete gauge.

CAPSULES AND BELLOWS

A bellows is made of several capsules. hollow flattened structures made from thin metal plate. pressure is applied , bellows expand and produce mechanical movement.

PISTON TYPE:

The pressure acts directly on the piston and compresses the spring. The position of the piston is directly related to the pressure. A window in the outer case allows the pressure to be indicated. Used in hydraulics where the ability to withstand shock, vibration and sudden pressure changes is needed (shock proof gauge).

Electric Pressure Transducers:Strain gauge type:

The change in length of the element produces changes in the electrical resistance. This is processed and converted into a voltage. A typical pressure transducer would contain a metal diaphragm which bends under pressure.

PIEZO ELECTRIC TYPES:

A piece of crystalline material that produces an electric charge on its surface when it is mechanically stressed. The electric charge may be converted into voltage. Used in the pick up crystal of a record player In microphones To generate a spark in a gas igniter.OTHER ELECTRIC EFFECTS:

Other electric effects commonly used in transducers are CAPACITIVE and INDUCTIVE. In the capacitance type pressure transducers, change in pressure is measured change in capacitance between two capacitance plates. One plate bonded to the deformable side of the elastic material while other one is bonded to the unpressurized surface. In inductive pressure transducer the deformation of the elastic material is used to provide linear movement of a ferromagnetic core. This linear movement will vary the induced AC current.