LIMIT SWITCHS COMBINATION

Switch combinations in measuring a process variable



In measuring a process variable 2 conditions
are measured one is high limit of process variable and other is low limit of process variable

In normal condition of a process variable both the high or low limit switchs are used in closed condition

Because during the normal behavior of a process variable if there is any abnormality (i.e. breakup in the wire of switch) then that state must be detected

Therefore the high limit switch’s combination is used in normally closed combination. While low limit switch is used in normally open combination

INTERFACE LEVEL MEASUREMENT

Interface level Measurement


What is interface level measurement?
Ans: The level measurement between two liquids having different specific gravity

How interface level is measured?
There are different methods of interface level measurement
1. interface level measurement using level troll
2. interface level measurement using capacitance probe
3. interface level measurement using DP transmitter

Interface level measurement by using level troll

There are two methods of interface level measurement with level troll
a) using service fluids (both)
b) using the water


a) Using the service fluids

First of all the the level troll is 100% filled with the liquid having low specific gravity.
Then out put of transmitter is set to 0% (4ma/3 psig).

After that the level troll is 100% filled with the liquid having heavier specific gravity and the out put of transmitter is set to 100% (20ma/15psig)
By this transmitter would measure the interface level

b) using the water

In this method level calculation is made as follows


Water level for low density liquid= S.G of liquid*total height of level troll
Fill up the level troll about the level obtained by the formula then set the out put of transmitter for 0% (4ma/3psig)

After that obtain the level of water for the high density liquid

After level for high density liquid= S.G of liquid*total height of level troll
Fill up the level troll about the level obtained by formula and set the transmitter out put for 100 %( 20ma/15psig)


Interface level measurement by using capacitance probe

How capacitance probe measure the level?
Ans:
To know about capacitance probe u must know about the working of a capacitor
A capacitor consist of two parallel plates separated by a dielectric (insulator)
The capacitance of capacitor depends upon the height, distance of the plates and amount of dielectric between the plates
You can vary the capacitance of capacitor by varying dielectric between the plates
This method is used in level measurement
A probe is inserted into the tank containing dielectric liquid and the tank wall is referenced as the 2nd probe
As well as liquid level increase or decrease the capacitance (between probe and wall of the tank) will also increase or decrease

Interface level with the capacitance probe can be also measured
Two liquids having different specific gravity will also have a different dielectric constant therefore by using capacitance probe the probe will measure a interface between dielectric liquid’s level (capacitance) which can be denoted as interface level

NOTE: that capacitance probe must always be 100% submerged with either high, low density liquid or with the both liquids other wise it will measure two interface levels one between air and low density liquid and the other between low density and high density liquid

Types of Compressors