Wednesday, February 23, 2011


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

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?
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

Sunday, December 5, 2010


Cascade control system

In some process two or more process variables effect equally on the behavior of each other where on is controlled with the control combination of another

For these kind of process the cascade control is used

Cascade control is a type of control terminology in which a process variable is

controlled by control combination of two process variables

In cascade control two process variables are controlled with a single final control element

In this control system one controller is recognized as master controller and the other as slave controller

The master controller is given a setpoint by operator and its out put becomes the setpoint of the slave controller and output of the slave controller goes to the final control element

Therefore any change of any process variable creates the change in final control element position therefore process is controlled by control combination of both process variables

Following a type of cascade control is given

(in this system temperature is controlled by controlling the flow)

Here burner temperature controller is used as master controller and fuel flow controller is used as slave controller where temperature controller’s out put becomes setpoint of the flow controller and finally out put of flow controller goes on the fuel flow control valve

Hence controlling the flow of fuel the burner’s temprature is controlled

Saturday, November 13, 2010

Hart communicator's function

Hart communicator’s mostly used functions in calibration of a device

LSV= lower set value:

The minimum range value of device, it can measure

USV=upper set value:

The maximum value of device , it can measure

LRV=lower range value:

The minimum value to which device has to read it as 0%

USV=upper range value

The maximum value to which device has to read it as100%

Sensor zero trim:

To zero the sensor value (means which output is generating sensor to zero it)

D/A trim=digital to analog trim:

To zero the out put of transmitter,to generate the 0% out put at that state of the device

Wednesday, November 10, 2010

calibration of transmitter

Calibration of a pressure transmitter

Calibration with analog apparatus


Pressure Transmitter range (0 to 200 psig )

Pressure source range (0 to 250 or more than)

Multi meter

24vdc power supply


Set range of transmitter to required range (0 to 200 psig)

Connect multi meter in series of power supply to the transmitter

Make graduation of the range of transmitter as follows

(for 0 to 200 psig range)

Applied pressure

Multi meter output


0 psig

4 ma


50 psig

8 ma


100 psig

12 ma


150 psig

16 ma


200 psig

20 ma


First adjust the zero (with zero screw) that multi meter must read 4ma

If multi meter is reading less then 4 ma then increase the out put by clockwise moving to the zero set screw

And if multi meter is reading more than 4 ma then decrease the out put by moving the anticlockwise to zero set screw

After adjusting the zero apply 50 psig pressure (as u made graduation) to the transmitter where multimeter must show 8ma if not then adjust it from span screw

After adjusting check

100 psig = 12ma.

150 psig = 16ma

200 psig = 20ma respectively

after adjusting all the readings check two or more time to all readings if no changes occur then your transmitter is calibrated

Calibration with HART communicator (Highway addressable remote transducer)


Pressure Transmitter range (0 to 300 psig ) “must be Hart enabled”

Pressure source range (0 to 350 or more than)

Hart communicator (275/375)

24vdc power supply


Connect the Hart communicator with the transmitter

Connect its terminals across the 250 Ohms resistor connected in series of power supply to the transmitter

When Hart communicator detects your device then go into its setup

Adjust the range through the option lower/upper range value

Set lower range value to zero (0) and upper range value to 300 psig

Set the zero by zero trim option (given pressure must be zero)

Apply 300 psig pressure to the transmitter and set it the 100% range value of transmitter by span trim option

After this setting check the 0 and 100% out put by connecting also multi meter in the series of the power supply

At 0 psig pressure multi meter must read 4ma

Apply the 300 psig where multi meter must read 20ma

Repeat this practice two or three times your transmitter will be calibrated when it matches its output reading match to the input

Sunday, November 7, 2010

What is Fusible Loop

What is a fusible loop charge panel? A fusible loop charge panel is a pneumatic panel that monitors a fusible loop system for the detection of fire. Fusible loop system are installed on offshore platforms to detect fire. The installation of the fusible loop are in accordance with API RP 14C recommendation.

The charge panel will regulate and meter a fix amount of instrument air to the fusible loop. During a fire, the heat generated by the fire will melt one or more of the fusible plug located in or near the fire. The 'melting' of the plug will cause a release of air from fusible loop through the plug. Since the release of this regulated air exceeds the amount of regulated/metered air supply, the pressure in the loop will decrease significantly. This decrease in pressure is monitored by a pressure switch on the charge panel. The signal from the switch is then utilized to signal an alarm to the Fire and Gas System.  In addition, an additional pneumatic output is tied to the deluge valve pilot valve where it is use to open the deluge valve.