Monthly Archives: September 2016

About Pressure Transducer

pressure transducer is used to convert a certain value of pressure into its corresponding mechanical or electrical output. Measurement if pressure is of considerable importance in process industries.

Types

The types of pressure sensors are differentiated according to the amount of differential pressure they are able to measure.
For low differential pressure measurement Liquid Column Manometers are used. Elastic type pressure gauges are also used for pressure measurement up to 700 MPa. Some of the common elastic/mechanical types are:

  • Bourdon Tubes
  • Diaphragm
  • Piston Type Pressure Transducer
  • Bellows

Other Types:

  • Electrical Type Pressure Sensors
  • Bell Gauge
  • Manometers

 Before going into further details regarding pressure measurement, it is important to know the different terms related to pressure.

 Basic Terms Related To Pressure Measurement

  • Pressure

Pressure is known to be the force that is exerted due to the weights of different gases and liquids. Some common examples are atmospheric pressure and the pressure implied by liquids inside the walls and underside of a container.

Pressure can be measured as the force exerted over a certain area.

Pressure = Force/Area

Pressure is not an independent variable as it is derived from force and area and it is not ideal as it depends on other factors like elevation, fluid density, temperature, flow velocity, and so on.

In instrumentation analysis, pressure is commonly expressed in pounds per square inch (psi). It can also be expressed in pounds per square feet (psf) and Pascals (Pa). Pascal is the SI unit if pressure. In many cases, pressure is expressed in terms of atmosphere which is the height of the barometric column at zero degrees Celsius, being equal to 76 cm of mercury or equivalent to 14.696 pounds per square inch absolute, 1 kg/cm2. Most of the pressures range from a little below atmosphere to hundreds of atmospheres.

  • Density is the mass per unit vlume of the material. It can be expressed as kilogram per cubic meter. (kg/m3).
  • Specific Weight is the weight per unit volume of the material. It can be expressed as Newon per cubic meter (N/m3).
  • Specific Gravity is basically a non-dimensional value as it is the ratio of two measurements in the same unit. It can be the ratio between the density of a material and the density of water or even the ratio between the specific weight of a material to the specific weight of water.
  • Static Pressure is the fluid or gas pressure that does not move.
  • Dynamic Pressure is the gas or fluid pressure that is obtained when it impacts with a surface or an object due to its motion or flow.
  • Impact Pressure is the total pressure or the addition of both the static and dynamic pressures.
  • Atmospheric Pressure is the surface pressure of earth and is available due to the weight if the gases in the earth’s atmosphere.
  • Another important aspect of pressure measurement is the measurement of very low pressure or what is known as vacuum. With the advancement of scientific research and industrial application of the results, pressure is as low as 10-6 mm of mercury is often required to be measured in some systems. Measurement of pressure, therefore, consists of two parts – that of pressure and vacuum. The force exerted by the fluid per unit area of the wall of the container is called the absolute pressure, whereas the gauge pressure is the difference between the absolute and local atmospheric pressure, and when gauge pressure is negative, it is known as vacuum.

Basic methods of pressure measurement are same as those of force measurement. For high vacuum, however, some special techniques are necessary. Primary sensors are mostly, mechanical which through secondary sensing means provide electrical outputs. Manometers and elastic element sensors are used as primary pressure sensors while secondary sensing, often called transducing here, involves resistive, inductive and capacitive changes for deriving electrical outputs.

About Vacuum Gauges

About Vacuum Gauges

For pressure measurement below atmosphere or vacuum  gauge, different gauges are available. Manometers and bell gauges can go up to 0.1 Torr. Diaphragm gauges are usable up to a pressure of 10-3 Torr. For pressure below this value, electrical gauges like Pirani or Ionization Gauges are used. Vacuum measurement is broadly classified into Mechanical Type, Thermal Type, Ionization Type, and Radiation Vacuum Gauge.

  • A standard manometric type mechanical gauge that is used down to 10-4 Torr is the McLeod Gauge.
  • Thermal type vacuum gauge can be further divided into Pirani Gauge and Thermocouple Type Vacuum Gauge.
  • Ionization Gauge can be divided into Hot Cathode Type and Cold Cathode Type.
  • One type of radiation gauge called Alphatron is explained.
  • Other type of vacuum gauge includes the Quartz Reference Vacuum Gauge.

Thermal Types

The heat conductivity of gases is independent of its pressure, at normal pressure. But, heat conductivity starts falling as the pressure is lowered t 10 Torr and below. The reason behind this is less collision between gas molecules within the wall and also their small number in a specific volume. The energy is carried to the walls of the container due to this collision. Thus, lesser number of molecules will be available to take the heat away from the source.

At low pressures, the heat loss that occurs from a hot wire mounted in a glass or metal tube is due to the following factors.

  • Convection
  • Conduction through the lead mines
  • Radiation
  • Conduction in the gas

Out of these, convection is comparatively negligible. A new clear wire has a small surface emissivity and is god for producing high temperature at low gas pressures. Due to oxidation and carbonization, the surface tends to deteriorate. This causes error of the device at low pressure ranges.

Absolute Pressure Gauge

Absolute Pressure– This is a combination of gauge pressure of the media with the pressure of the atmosphere. When the location changes, say as per the change of altitude, the atmospheric pressure also changes which compel the reference point to differ as well. If an absolute pressure sensor is used for eliminating the reference of varying atmospheric pressure.

About Pirani Gauges?

A basic pirani gauge consists of a fine wire of tungsten or platinum of about 0.002 cm in diameter. This wire is mounted in a tube and then connected to the system whose vacuum is to be measured. The temperature range is around (7-400) degree Celsius and the heating current is between (10-100) mA. A bridge circuit is also used for greater accuracy. The pirani gauge is connected as one arm of the bridge circuit. The figure is shown below. Vacuum measurement is usually taken in three ways.

When the pressure changes, there will be a change in current. For this, the voltage V has to be kept constant.
The resistance R2 of the gauge is measured, by keeping the gauge current constant.
The null balance of the bridge circuit is maintained by adjusting the voltage or current. This change is made with the help of a potentiometer and the change brought will be a measure of the pressure produced.

Pirani Gauge

Pirani Gauge

An additional reference gauge can also be used in the adjacent arm of another pirani gauge, in the bridge circuit. The additional gauge is evacuated and sealed, which helps in the compensaton for variation in ambient temperature. For commercial use, the range of the Pirani Gauge instrument can be extended from 10-3 Torr to 1 Torr.