Process Instrument

RTD Probe

RTD probe is the most rugged form of RTD. A probe consists of an RTD element mounted inside a metal tube, also known as a sheath. The sheath protects the element from the environment.
RTD Types / Commonly used Material

PT 100 is commonly used RTD. At 0 degrees Celsius, its Resistance is 100 ohms. Element used is Platinum.

Used for measuring temperature. They consist of two dissimilar meals wires, joined together at one end. When this junction of the two metals is heated or cooled a voltage is produced that can be correlated back to the temperature.
They are positive temperature coefficient devices. Their effective resistance increases with the increase in temperature.

Beaded wire thermocouples consist of two pieces of thermocouple wire joined together with a welded bead. Beaded wire thermocouples are a good choice for the measurement of gas and air temperature.
Since they can be made very small, they also provide very fast response time.
Types / Temperature range:-

Proximity sensors are discrete sensors that sense when an object comes near to the sensor face.
There are basically 4 types of proximity sensors:-
1. 1. 1. Inductive
    2. Capacitive
    3. Ultrasonic
    4. Optical

Just inside the face of the sensor, is a coil that is the part of the oscillator. When the oscillator oscillates there is an alternating magnetic field produced by the coil. This magnetic field radiates through the face of the sensor.
When any magnetic material (steel, iron, aluminum, tin, copper) comes near the sensor, the output amplitude of the oscillator decreases. The trigger circuit senses the loss of oscillator output and causes the output switching device to switch on. These sensors are used for inductive material/Magnetic material.

An integral oscillator oscillates when a target material moves close to the sensor face. The target material varies the capacitance of the capacitor in the face of the sensor that is the part of the oscillator circuit.

Dielectric type capacitive sensor, there are 2 side by side capacitor plates in the sensor face. The external target acts as dielectric. As the target moves closer to the sensor face, the change in dielectric increases the capacitance of the capacitor, making the oscillator amplitude to increase. This causes the sensor to output an ON signal.
Conductive type capacitive sensor has only one capacitor plate in the sensor face. The target becomes the other plate. Capacitive proximity sensors are suitable for non-magnetic materials such as foam, paper, plastics etc.

Inductive & Capacitive sensors have distance limitations. When the distance between the object and the sensor is more ultrasonic proximity sensors are used.

Photoelectric Sensors are electronic devices that utilize the principles of light emission and constitute a family of sensors used in a wide variety of industrial automation sectors for object detection and counting, contrast reading, measuring etc. generally a photoelectric sensor consists of a light source or emitter, a receiver, an amplifier / demodulator and an output stage. When the light beam generated by the photoelectric sensor is interrupted, the output stage changes its logic state.
Type 1: - The emitter and receiver are housed in two separate devices, typically mounted one in front of the other. Each object that comes between the two devices interrupts the beam and is detected. This type of sensor is used for long range applications or in dirty working environments.
Type 2: - Photoelectric sensors of this type are devices in which the emission and reception photo elements are both contained in the same mechanical housing. This type of sensor has a good operational distance and is easy to align even in environments where space limitation is criteria.

The cable color code is shown in the fig. The brown and blue cable are supply cables for the sensor and the black (NO) and white (NC) cable are for output signal from the sensor.
The black cable is in NO condition it means, when the sensor is in normal position (Nothing to sense) then no signal comes from black cable and when sensor is sensing any object then the signal from black cable is 24V for PNP and 0V for NPN.
The white cable is in NC condition it means, when the sensor is in normal position (Nothing to sense) then the signal from white cable is 24V for PNP and 0V for NPN, and when the sensor is sensing any object then no signal comes from white cable.
If the sensor type is PNP then the output signal from the output cable (Black/White) of the sensor is 24VDC.
If the sensor type is NPNthen the output signal from the output cable (Black/White) of the sensor is 0VDC.

A limit switch is an electromechanical device that consists of an actuator mechanically linked to a set of contacts. When an object comes into contact with the actuator, the device operates the contacts to make or break an electrical connection.

An encoder is a device, circuit, transducer, software program, algorithm or person that converts information from one format or code to another, for the purposes of standardization, speed or compression.
It is a feedback device which converts mechanical motion into electronic signals. Usually an encoder is a rotary device that outputs digital pulses which correspond to incremental angular motion.
The encoder consists of a glass or metal wheel with alternating clear and opaque stripes that are detected by optical sensors to produce the digital output.