Chapter 4 :
Process
Instrumentation
Outline
• General Principles
• Functions of Instruments
• Types of Instruments
• Temperature Measurements
• Pressure Measurements
• Flow measurements
The thermocouple working principle is based on the Seeback Effect. This effect states that when a closed circuit is
formed by jointing two dissimilar metals at two junctions, and junctions are maintained at different temperatures
then an electromotive force (e.m.f.) is induced in this closed circuit.
• Temperature
Control
• Level Control
Functions:
1. Transmitting: Instrument conveys information concerning the measured quality over some
distance to remote point.
Ex: Telephone
2. Signaling: Instrument indicates the general value or range of values of its measured quantity.
Ex: Grocer’s scale
3. Registering: Instrument merely indicates by numbers or some other symbols of discrete
increments, the value of some quantity.
Ex: Water meter and Counting Machine
4.Indicating: Instrument indicates the value of the measured quantity using a calibrated pointer
and scale.
Ex: Pressure gauge and Rotameter
5. Recording: Instrument keeps a written record of the measured quantity usually against time.
Ex: Data logger (Temperature)
• The basic principle of the pyrometer is, it measures the
object’s temperature by sensing the heat/radiation emitted from
the object without making contact with the object. It records the
temperature level depending upon the intensity of radiation
emitted. The pyrometer has two basic components like optical
system and detectors that are used to measure the surface
temperature of the object.
• When any object is taken whose surface temperature is to be
measured with the pyrometer, the optical system will capture
the energy emitted from the object. Then the radiation is sent to
the detector, which is very sensitive to the waves of radiation.
The output of the detector refers to the temperature level of the
object due to the radiation. Note that, the temperature of the
detector analyzed by using the level of radiation is directly
proportional to the object’s temperature.
Pirani Guage
Mcleod Guage
Thermal Conductivty Guage
The thermocouple gauge is one of a group of gauges
working on the thermal conductivity principle. At low
pressure, the kinematic theory of gases predicts a
linear relationship between pressure and thermal
conductivity. Thus measurement of thermal
conductivity gives an indication of pressure.
Thank You
