The document discusses the design and functioning of differential pressure transmitters, which utilize a taut metal diaphragm to measure pressure differences through differential capacitance. It highlights the importance of built-in square root extraction for accurately relating flow rates to differential pressures, simplifying system design and ensuring real-time monitoring. Additionally, it outlines advantages such as smart transmitters with remote calibration and digital communication capabilities.

1. Differential Pressure Transmitter
Presented by
Dr.T.Mrunalini

2. Differential Pressure Transmitter:
• Electrical pressure sensor design works on the principle of differential capacitance
• Sensing element: taut metal diaphragm located equidistant between two stationary metal
surfaces, comprising three plates for a complementary pair of capacitors.
• An electrically insulating fill fluid (usually a liquid silicone compound) transfers motion
from the isolating diaphragms to the sensing diaphragm

3. • Any difference of pressure across the cell causes the diaphragm
to flex in the direction of least pressure
• The applied force in this case can only be a function of
differential pressure acting against the surface area of the
diaphragm in accordance with the standard force-pressure-area
equation F = PA.

4. Built-in Square root extractor
• A differential pressure (DP) transmitter with a built-in square root
extractor is designed to measure flow rates accurately
• The relationship between flow rate and differential pressure is non-
linear
• the flow rate is proportional to the square root of the differential
pressure
• This means that if you double the flow rate, the differential pressure
increases by a factor of four
• To convert the differential pressure reading into a linear flow rate, the
transmitter uses a square root extraction function.

5. • Having the square root extraction built into the transmitter simplifies
the system design and reduces the need for additional components
• It also ensures that the flow rate is accurately represented in real-time,
which is crucial for process control and monitoring
Square root extraction formula
To convert a a linear 4-20mA signal to a square root signal use the following
formula:
[Output Sq Rt] = 4mA + (4 x Square Root of ([Output Linear] – 4mA))
And to convert a square root signal to a linear 4-20mA signal use this formula:
[Output Linear] = 4mA + (([Output Sq Rt] - 4mA)^2 / 16)

8. Advantages:
• Improved version of transmitters are called smart transmitters (with
computer compatibility)
• HART (Highway Addressable Remote Transducer) – smart digital
communication
• Remote calibration, span changing, zero adjustmrnt also possible.