low pressure measurement

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LOW-PRESSURE MEASUREMENT

SUBMITTED BY:PRODIP KUMAR SARKERDEPARTMENT OF ME,RUET, BANGLADESH

2Introduction

measurement of low pressures which are not usually

accessible to the conventional gages.

absolute pressures below 1 torr ( 1 mmHg, 133 Pa)

3Measuring Vacuum Method

MEASUREMENT BELOW ATMOSPHERIC PRESSURE 1- 1 TORR.

MECHANICAL TYPE – MCLEOD GAUGE.

THERMAL TYPE – PIRANI GAUGE AND THERMOCOUPLE.

Knudsen GaugeIONIZATION TYPE – HOT CATHODE AND COLD CATHODERADIATION VACUUM GAUGE – ALPHATRON , QUARTZ REFERENCE

4THE MCLEOD GAUGE vacuum gauge with same principle as manometer. Range: 10-4 Torr. multiple compression technique.  P1 = ah^2/(V1-ah)

whereP1 = Pressure of gas at initial conditionV1 = Volume of gas at initial Condition. a = cross – sectional area of measuringcapillary tube h = height of the compressed gas in the measuring capillary tube

5Operation of McLeod Vacuum gauge:

The pressure to be measured (P1) is applied to the top of the reference column. The mercury level in the gauge is raised by operating the piston. When this is the case(condition – 1), the applied pressure fills the bulb and the capillary.Now again the piston is operated so that the mercury level in the gauge increases. 

When mercury level reaches the cutoff point, a known volume of gas (V1) is trapped in bulb and measuring capillary tube. The mercury level is further raised by operating the piston so the trapped are compressed. This is done until the mercury level reaches the “Zero reference Point” marked on the reference capillary (condition – 2). In this condition, the volume of the gas in the measuring capillary tube is read directly by a scale besides it. That is, the difference in height ‘H’ of the measuring capillary and the reference capillary becomes a measure of the volume (V2) and pressure (P2) of the trapped gas

6PIRANI THERMAL-CONDUCTIVITY GAUGE

fine wire of tungsten or platinum about 0.002 cm in diameter.  temperature range (7-400)▫C heating current is between (10-100) mA. Range: Torr – 1 Torr

Pirani gauge

Fig: schemetic of pirani gauge

7Working principle 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.

8KNUDSEN GAUGE An absolute gauge in the range of - torr Independent of gas composition More suitable for laboratories Depends on momentum transfer principle P= 4FWhereT= temp. Of heated plates = temp. Of vens at the temp of gas

9WORKING PRINCIPLE

Two vanes along with a mirror are mounted on a thin-filament suspension. Near these vanes are plates. Heaters are installed so that temperature of plates is higher than that of the surrounding gas. The molecules striking the vanes from the hot plates have a higher velocity than those leaving the vanes because of the difference in temperature. Thus, there is a net momentum imparted to the vanes which may be measured by observing the angular displacement of the mirror. Total momentum exchange is a function of molecular density, which is related to  pressure and temperature of the gas.

10THE IONIZATION GAUGE

measure pressure down to about 2 torr p=WhereS = sensitivity of gauge ( a typical value for nitrogen is S= 20 ) = plate current = grid curent

11WORKING PROCEDURE

An ionization gage is setup to have a heated cathod that emits electrons, a grid that accelerates the electrons, and a plate that collects positive ions.As the electrons move toward the grid, they produce ionization of the gas molecule. It is found that the pressure of the gas is proportional to the ratio of plate current to grid current.

12THE ALPHATRON The Alphatron is a radioactive ionization gage. A small radium source serves as an alpha-particle emitter. These particles ionize the gas inside the gage enclosure no of ions directly proportional to gas pressure the degree of ionization is determined by measuring the voltage output FIG: schemetic of alphatron Range: - torr

13Summary:

Fig: summary of applicable range of pressure gauges

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