Sensors and Transducers Flashcards
What are the two basic requirements of a sensor?
- To indicate the values of the measured variables
- To transmit the signals to the controller
What type of circuits should the signals be transmitted through?
Pneumatic pipeline, electric circuit
Four examples of pressure transducers
- manometer
- strain meter
- piezo-electric sensor
- traditional transducer
What is present in most pressure transducers?
There is a diaphragm which contracts the fluids
What is the advantage of the diaphragm?
To protect the measuring setup isolated from the measured fluids, most of which may be corrosive
Describe the strain meter.
In a strain meter, under the diaphragm there is a spring type of resistance wire. Under pressure, the spring will press together and cause a short circuit in some regions and this reduces the resistance of the wire. The resistance will be inversely proportional to the strain on it. The changing resistance may be measured by being included into an electric circuit.
Describe the piezo-electric sensor
There is a quartz crystal between a set of parallel plates. The electromotive force across the plates will be directly proportional to the pressure being applied. By measuring the electromotive force, the applied pressure can be indicated and transmitted.
Describe the traditional transducer
The pressure on a diaphragm is arranged to control the flow of air into, or out of, a chamber on the opposite side of the diaphragm until a balance is obtained. The balancing pressure is an indication of the measured pressure.
Four examples of level sensors
- Pressure-operated sensor
- Float operated sensor
- Capacity bridge sensor
- Optical sensor
Describe the pressure operated sensor
There is a unique quantitative relationship between the liquid level and the pressure at the bottom of the tank
Describe the float operated sensor
There is a float on the surface of liquid. The change in liquid level will cause movements of this float. Monitoring the movement of a signal of the level is generated and transmitted.
Describe the capacity bridge sensor
Electrode, electronic unit, indicator. One arm of the bridge is formed by the capacitance between the level sensing electrode and the earth. A change owing to the rise and fall of the material around the electrode produces an out of balance current flow from the bridge which is measured and transmitted.
Describe the optical sensor
When there is no liquid around, the receiver can detect the reflected signal and this light signal is converted to an electrical signal which is transmitted. With the liquid around the sensor, the receiver can’t get the same amount for light reflected. The change in the light received is converted into an electrical signal.
Two methods of measuring flow
- Differential pressure method
- Rotating vane meter
Describe the differential pressure method
The net cross-section area of the stream is reduced, causing an increase in the velocity and an increase in the kinetic energy. The increase in the kinetic energy comes at the expense of the pressure energy. e.g. orifice, Venturi tube
Describe the rotating vane meter
Liquid passing through the meter is directed in to the rings of the vane and rotates at a rate which depends on the velocity of the liquid. The rotation can be arranged to drive some electric transducers and give a signal.
Three methods of measuring temperature
- Resistance thermometer and thermistor
- Thermocouple
- Silicon semiconductor
Describe the resistance thermometer and thermistor
Electrical resistance depends on temperature. The change in resistance can be converted to a transmittable electrical signal.
A thermistor is made of a semiconductor, a mixture of metal oxide. Semiconductors have a negative resistance coefficient unlike a thermometer.
Describe the thermocouple
If electric circuit consists of all metallic conductors and all parts of circuit are at same temperature, no electric force in circuit and no current. If junctions between two metals are at different temperature, then there will be an emf and a current will flow. The junction between the two metals is the thermocouple. The emf depends on the temperature difference between the two junction.
Describe the silicon semiconductor
Below a voltage, a diode will not pass current. Above the voltage, the diode will pass current/ The pass required voltage of silicon diodes depend uniquely on the temperature and the voltage signal can be used to indicate temperature
