High vacuummeasurement

Process ControlProcess Control

andandandand

InstrumentatInstrumentation

Dr. Debasis SarkarD t t f Ch i l E i iDepartment of Chemical EngineeringIndian Institute of Technology Kharagpur

Pressure MeasurementPressure Measurement

HighHigh--Vacuum MeasurementVacuum Measurement

1. McLeod Gage

2 Ionization gage2. Ionization gage

3. Thermocouple gage

4. Pirani gageThermal conductivity gage

5. Knudsen gage

HighHigh--Vacuum MeasurementVacuum MeasurementMcLeod Gage:McLeod Gage:

To vacuum

Sealed capillaryReference capillary

Zero line

McLeod Gage

HighHigh--Vacuum MeasurementVacuum MeasurementMcLeod Gage:c eod Gage

Example:

A M L d h V 100 3

To vacuum, p

A McLeod gage has VB = 100 cm3

and a capillary diameter of 1 mm. Calculate the pressure indicated by the reading of 4 cm. by e ead g o c

What error would result if we use :Reading: 4 cm

Instead of

2

B

aypV

VB = 100 cm3B

2cyVayp Mercury

iB B

pV ay V ay reservoir


Advantages of the McLeod Gage: It is independent of the gas composition It is independent of the gas composition

Used as standard to calibrate other low pressure gages

No need to apply any corrections to the McLeod Gage readingspp y y g g

Limitations of McLeod Gage/Precautions to be Taken: The gas must obey the Boyle's lawg y y

Presence of condensable vapor causes pressure readings to be low, so moisture

traps should be provided

The measuring tube should have small diameter, but capillary effect can produce

significant uncertainty

It cannot give continuous readings thus steady-state condition must prevail for It cannot give continuous readings, thus steady-state condition must prevail for

useful measurement


Ionization Gage:An electron passing through a p g gpotential difference will acquire a kinetic energy that is proportional to the potential difference=> difference

If an electron strikes a gas molecule, the electron may knock out an electron from the gas molecule leaving it positively charged

Gas moleculeIon

Number of positive ions formed is dependent on number of gas molecules per

- +Ion Electron

unit volume pressure

HighHigh--Vacuum MeasurementVacuum MeasurementIonization Gage:

Working Principle:Working Principle:

Ionization gage measure vacuum by measuring the current produced by ionized gas molecules. The gas molecules are ionized as a stream of electrons collide with them.

An electron passing through a potential difference will acquire a kinetic energy that is proportional to the potential difference If this energy is large and thethat is proportional to the potential difference. If this energy is large and the electron collides with a gas molecule, the electron may knock out a secondary electron from the gas molecule. Thus the gas molecule will be a positively charged ion.

Number of positive ions (ion current) depends on electron current (no. of electrons emitted by the cathode) and number of gas molecules. For a given gas and a constant electron current, ion current become a direct measure of g ,number of gas molecules per unit volume that is pressure.


Three types of Ionization gages:Three types of Ionization gages:

Hot cathode ionization gage: consists of a heated filament (cathode), a grid (upto 10-10 torr) with negative potential (ion collector), and an

anode (electron collector).

Cold cathode ionization gage: No heated filament to produce electrons. (upto 10-5 torr) Uses a high electric field (~4kV) between(upto 10 torr) Uses a high electric field ( 4kV) between

cathode and anode to draw electrons out. High (Also known as magnetic field (~1500 gauss) causes the

Penning gage) electrons to move towards the anode. E iExpensive gage.

Alphatron vacuum gage: Relatively less expensive. Uses suitable alpha-emitter p

HighHigh--Vacuum MeasurementVacuum MeasurementCold Cathode Ionization Gage (Penning gage):

HighHigh--Vacuum MeasurementVacuum MeasurementTh l C d ti it GThermal Conductivity Gage: Thermocouple Gage & Pirani gage

Working Principle:

Thermal conductivity of a gas is independent of pressure at normal pressure. But at low pressure, thermal conductivity of a gas depends on pressure (decreases with pressure)

Heat loss from a heated conducting wire (or hot thin metal surface) is dependent on thermal conductivity of the surrounding gas. Thus, equilibrium temperature of a heated conducting wire (or hot thin metal surface) is a function of pressureheated conducting wire (or hot thin metal surface) is a function of pressure.

Range: 10-4 to 1 Torr

C lib ti d d b i dFilament

Calibration depends on gas being used

Tovacuum

Cold surface(glass tube)

HighHigh--Vacuum MeasurementVacuum MeasurementTh l GThermocouple Gage:

For a given gas and heating current, the temperature assumed by the hot surface depends on pressure. Temperature of the surface is measured by a thermocouple.

Cold surface(glass tube) Hot surface

(thin metal strip)

Range: 10-4

to 1 Torr(thin metal strip)

Adjust heating current

(to heat the hot

Measurement may be affected by ambient

High i d

mV

the hot surface)

ambient temperature

Surface of

impedence microvolt meter

Thermocouple

low emissivity is used to reduce

Pressure

reduce effect of radiation

HighHigh--Vacuum MeasurementVacuum MeasurementTh l GThermocouple Gage:

For a given gas and heating current, the temperature assumed by the hot surface depends on pressure. Temperature of the surface is measured by a thermocouple.

HighHigh--Vacuum MeasurementVacuum MeasurementPirani Gage:

Uses same principle as Thermocouple gage

Function of heating and measuring temperature bi d i i l l tare combined in a single element

Generally more accurate and expensive than Thermocouple gage

Range: 10-4 to 1 Torr

Gage has to be calibrated for individual gas

Temperature compensation is provided

HighHigh--Vacuum MeasurementVacuum MeasurementPi i GPirani Gage:

Pirani gage also uses the same principle as Thermocouple gage. But unlike thermocouple gages, they don’t measure the wire temperature directly. Instead p g g , y p ythey use the fact that the resistance of a conducting wire changes with the wire’s temperature.

The heated wire (measuring element: Pirani gage) is connected as one leg of aThe heated wire (measuring element: Pirani gage) is connected as one leg of a Wheatstone bridge. An exactly identical element is connected as another leg which is exposed to ambient temperature and works as a compensator.

If the circuit is initially balanced, then application of pressure to the measuring element will cause unbalance in the circuit.

This is because the sensor wire changes its resistance with change in pressure that changes the wire’s temperature.


Compensating element

Two modes of operation:

1. Output current is function of pressure

Sealed and evacuated

2. Keep the temperature of resistance constant

To

by adjusting the current passing through the element. Then, change in

vacuum

Measuring element

current is function of the pressure.

HighHigh--Vacuum MeasurementVacuum MeasurementPirani Gage: • The Pirani is a dedicated

low vacuum gauge device

• The resistance of the hot wire changes with the rate of heat loss (conduction)of heat loss (conduction) to the gas

• The Wheatstone bridgeThe Wheatstone bridge then measures the change in resistance of the hot wire

• Pirani’s are rugged and generally reliable and rarely need attentionrarely need attention

HighHigh--Vacuum MeasurementVacuum MeasurementKnudsen Gage:

It is an absolute gage in the range of 10-8 to 10-3 torr

Independent of gas composition

More suitable for laboratories

Depends on momentum transfer principle


Knudsen Gage:The gap between the fixed plates and the movable To vacuumFixed heated plates (TF)vanes must be less than mean free path of the gas molecule

Gas molecules rebound

To vacuumFixed heated plates (TF)

Range:10 8 to 10 3 torr Gas molecules rebound

from the heated plates with greater momentum than from the cooler movable vane – this gives a net f th i

M

10-8 to 10-3 torr

force on the spring restricted movable vane.

The force can be measured by the deflection of the ymovable vane

Movable vane (TV)

( / ) 1F V

KFpT T

Scale

Temperature of fixed plates: TFTemperature of movable vane: TVTF > TVLight source

SolidSolid--State Pressure SensorState Pressure SensorSolid-state pressure sensor is based on integrated circuit technology

Finds extensive application in the range of 0 to 100 kPa (0 to 14.7 psi)

The basic sensing element is small wafer of silicon that acts as a diaphragm. The deflection of the wafer is sensed by a strain gage grown directly on the silicon wafer. Even signal condition circuitry (for temperature compensation and linear g y ( p ppressure-voltage output) is also grown directly on the wafer.

To pressure source, P1

Connections to wafer

Can measure: G

External connections

Housing Gage pressure Absolute pressure Differential pressure

P2 = atmospheric

SolidSolid--State Pressure SensorState Pressure SensorImportant Characteristics:

Hi h iti it 10 100 V/kP High sensitivity: 10 – 100 mV/kPa

High accuracy and reliability

Fast response: response time on the order of 10 msp p

Linear voltage versus pressure within the specified operating range

Temperature compensation for broad range of temperature: 0°C to 60°C

Easy to use: often with only three connections: DC power supply (~ 5V), ground,

and the sensor output voltage

Range of Vacuum SensorRange of Vacuum Sensor

Selection of Pressure SensorSelection of Pressure Sensor

Strongly influenced by intended application

High vacuummeasurement

Engineering

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