Applications of DP transmitters

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The DP transmitter is a very versatile pressure-measuring device. This one instrument may be used to measure pressure differences, positive (gauge) pressures, negative (vacuum) pressures, and even absolute pressures, just by connecting the “high” and “low” sensing ports differently. 

In every DP transmitter application, there are means of connecting the transmitter’s pressure-sensing ports to the points in a process. Metal or plastic tubes (or pipes) are the means used for this purpose, and are commonly called impulse lines or sensing lines.

Let us now look at a few of the several applications using the versatile DP transmitter:

4 - 20mA Transmitter Wiring Types: 2 -Wire, 3 - Wire & 4 - Wire

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Today’s electronic process transmitters - pressure, temperature, flow and level are connected in different wire types or configurations. These connection methods are of great concern to the instrument engineer/technician. The 2 - Wire, 3 - Wire and 4 - Wire types are often used to describe the method of connection of electronic transmitters. However in today's rapidly evolving technological world, the 2 - Wire type transmitter is by far the most common. Evidently so because of the huge savings in wiring and other advantages it possess over the other transmitter wire configurations.

Two wire transmitters:

These are the simplest and most economical and should be used wherever load conditions will permit. They are often called loop powered instruments. In a 2 -wire system, the only source of power to the transmitter is from the signal loop. The 4 mA zero-end current is sufficient to drive the internal circuitry of the transmitter and the current from 4 to 20 mA represents the range of the measured process variable. The power supply and the instruments are usually mounted in the control room. The schematic diagram below shows the wire transmitter configuration:

An Introduction to DP Transmitters

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Differential pressure (DP) transmitters are one of the most common, versatile and most useful pressure measuring instruments in industrial instrumentation systems. A DP transmitter senses the difference in pressure between two ports and outputs a signal representing that pressure in relation to a calibrated range.

DP transmitters currently in use in most instrumentation systems are based on any of the following technologies: 

a) Force-balance principle

b) Strain gauge 

c) Differential capacitance 

d) Vibrating wire or mechanical resonance

The force-balance principle is utilized in pneumatic pressure transmitters while most of today’s electronic pressure transmitters that have practically replaced the pneumatic pressure transmitters, use the technologies (b) – (d).

Linear Variable Differential Transformer (LVDT)

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A pressure sensor can be created using the motion of a high permeability core in a magnetic field created by the coils of a transformer. This principle is what is used in a Linear variable differential transformer. The movement of the core is transferred from the process medium to the core by the use of a diaphragm, bellows or bourdon tube.

The LVDT operates on the inductance ratio between the transformer coils.

Vibrating Wire Sensors

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It is a well known fact that the natural frequency of a tensioned string increases with tension. Mathematically, the relationship between the resonant frequency of a string and the tension applied on the string is given by:

Where,

F = Fundamental resonant frequency of string (Hertz)

L = String length (meters)

T = String tension (newtons)

μ = Unit mass of string (kilograms per meter).

This implies that a string can be used as a force sensor. In this type of sensor design, an electronic oscillator circuit, is used to keep a wire vibrating at its natural frequency when under tension. The principle is similar to that of a guitar string.

Differential Capacitance Pressure Sensors

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Like the strain gauge, differential capacitance sensors use a change in electrical characteristics to infer pressure. Here a change in capacitance is used to infer pressure measurement. The capacitor is a device that stores electrical charge. It consists of two metal plates separated by an electrical insulator. The metal plates are connected to an external electrical circuit through which electrical charge can be transferred from one metal plate to the other.

The capacitance of a capacitor is a measure of its ability to store charge. The capacitance of a capacitor is directly proportional to the area of the metal plates and inversely proportional to the distance between them. It also depends on a characteristic of the insulating material between them. This characteristic, called permittivity is a measure of how well the insulating material increases the ability of the capacitor to store charge. Mathematically this can be put as:

C = ε A/d, 

here C = capacitance, A = area of plates, d = distance between plates of capacitor. ε = is the permittivity of the insulator between capacitor plates.

How a Strain Gauge Works

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Several different technologies exist for the conversion of fluid pressure into an electrical signal response. These technologies form the basis of today’s electronic pressure transmitters. One of such technology is the strain gauge discussed here.

A strain gauge is what may be described as a ‘’piezoresistive element’’. This means its resistance changes with changes in applied pressure. Basically, a strain gauge uses the change of electrical resistance of a material (wire, foil or film), under strain to measure pressure.

The electrical resistance of any conductor is proportional to the ratio of length over cross-sectional area (R ∝ L/A), which means that tensile deformation (stretching) will increase electrical resistance by simultaneously increasing length and decreasing cross-sectional area while compressive deformation will decrease electrical resistance by simultaneously decreasing length and increasing cross-sectional area.

The complete strain gauge pressure-measuring device includes:

Mechanical Pressure Sensors

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Pressure:

Pressure is defined as a force per unit area, and can be measured in units such as psi (pounds per square inch), inches of water, millimeters of mercury, pascal (Pa, or N/m²) or bar. Until the introduction of SI units, the 'bar' was quite common.

The bar is equivalent to 100,000 N/m², which were the SI units for measurement. To simplify the units, the N/m² was adopted with the name of Pascal, abbreviated to Pa.

Pressure is quite commonly measured in kilo pascals (kPa), which is 1000 Pascal and equivalent to 0.145psi.

Absolute, Gauge and Differential Pressure:

Pressure varies depending on altitude above sea level, weather pressure fronts and other conditions. The measure of pressure is, therefore, relative and pressure measurements are stated as either gauge or absolute.

Gauge pressure is the unit we encounter in everyday work (e.g., tire ratings are in gauge pressure). A gauge pressure device will indicate zero pressure when bled down to atmospheric pressure (i.e., gauge pressure is referenced to atmospheric pressure). Gauge pressure is denoted by a (g) at the end of the pressure unit , e.g., kPa (g)