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TRANSMITTER

OBJECTIVES :

On proper learning of this chapter you should be able to :

1. Identify  all components of transmitter

2. Calibrate the  differential pressure transmitter.

3. Perform isolation and putting in service of transmitter for flow measurement.

4. Perform the isolation and putting  in service of transmitter for level measurement.

1. INTRODUCTION

A transducer is a device that receives information in one form and generates an output in response to it. A transmitter is a transducer that responds to a measurement variable (called the measurand) and converts that input in to a standardized transmission signal. The standard output signal may be a variable dc current (4 to 20 ma / 10 to 50 ma) or a pneumatic pressure signal(usually 3 to 15 psig). The most common industrial transmitter is a combination of a transducer and signal conditioning circuit that produces an output current proportional to the measurand. The process control industry’s standard current signal is 4 to 20 milliamperes (mA); 4 mA DC represents zero, and 20 mA DC represents 100 % or full scale. The current is used because it is not affected by wire impedance and noise as are voltage  signals when transmitted over long distance.

Transmitters are classified as

Electronic transmitters  with 4-20 ma DC output

Pneumatic transmitters with 3-15 psig output

Transmitters are also available as

Pressure transmitters (for measurement of pressure & open tank level) Differential pressure transmitters (for measurement of closed tank level and flow )

The differential pressure transmitters are having two input pressure connections i.e. high & low pressure connections where as the pressure transmitters are having only one pressure input connection (no Low Pressure connection). The differential pressure transmitter also can be used as a pressure transmitter with its low  pressure side open to atmosphere.

2. Desirable Transmitter Features:

The characteristics most desirable for an electronic transmitter are

high resolution

accuracy

rangeability

reliability

 low cost.

A transmitter should be designed to have the following features:

Small size and weight for easy installation and maintenance.

Rugged design to withstand industrial environment.

Minimum dependence on environmental condition for accuracy.

This requires good temperature stability

3. Differential Pressure Transmitter (Electronic Transmitter)

In process control systems, it is required to measure the differential pressure between two points in a fluid system and to transmit a proportional signal to a remote location. The electronic differential pressure transmitter converts the differential pressure signal to a standard electrical signal, usually 4mA to 20mA. In general it measures differential pressure and can be used in pressure, level & flow control loops in series with other instruments like indicator, Recorder, Controller or digital control systems. The transmitter is located in field, close to the detecting element and the output signal is transmitted to the remote control room/control equipment room.

3.1. Working Principle

This transmitter works on change in capacitance principle. The sensing element is a capsule, which senses the differential pressure signal, and converts into change in capacitance. The converted signals are further conditioned in to a standard electrical signal of 4mA DC to 20ma DC with the help of electronic signal conditioning circuits. Fig. 1 gives the input-output relation of the Electronic Transmitter.

Pressure is applied to opposite sides of the silicon oil filled diaphragm capsule through the high and low-pressure connections. Any difference between these pressures exerts a force on the isolating diaphragm, which is rigidly connected with the sensing diaphragm.

The sensing element consists of a sensing diaphragm and two capacitor plates. The sensing diaphragm is flexible and is located in the centre. The sensing module is filled with silicon oil, which works as a di-electric for C1 & C2, the capacitance between the sensing diaphragm and the capacitor plates. When the pressures, high pressure and low pressure are equal both the capacitance (C1 & C2) will be equal. When differential pressure is applied the force is transmitted to the central sensing diaphragm plate which in turn reduces the distance between the low pressure diaphragm plate and increases the distance between high pressure side diaphragm plate which changes the capacitance values of C1 & C2. The differential capacitance between the sensing diaphragm and the capacitor plates is electronically converted in to 4 to 20mA DC signals, which can be used for remote indication.

This approach is based on the following concepts.

P is the differential pressure,

K is the constant,

C1 is the capacitance between the high pressure side and the sensing diaphragm (central plate).

C2 is the capacitance between low pressure side and the sensing diaphragm (central plate)

where ε= Dielectric constant,

 A = area of the plate

D = distance between the plates.

4. Transmitter Calibration Setup

In general the process’s differential pressure is set to desired pressure with the help of PRV and a pressure gauge is used for indication purpose. All this is done with the instrument known as the pneumatic calibrator. With the help of

transmetter calibration set up

This instrument air pressure is applied to transmitter to be calibrated.

For the operation of electronic circuit of transmitter and current converter it requires a DC voltage in the range of 12 to 45 V DC (because circuit has a zener voltage regulator for voltage regulation purpose). This power supply is connected in series with indicator, recorder and controller. For calibration purposes only indicator is required.

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