Thermocouple gauge & pirani gauge

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PRESSURE MEASUREMENT DEVICES Submitted by, Navaneeth Krishnan A R B130592CH B Batch S5 CHED

Transcript of Thermocouple gauge & pirani gauge

Page 1: Thermocouple gauge & pirani gauge

PRESSUREMEASUREMEN

TDEVICES

Submitted by,Navaneeth Krishnan A RB130592CHB BatchS5 CHED

Page 2: Thermocouple gauge & pirani gauge

Thermocouple Gauge&

Pirani Gauge•  Both works on the same principle of detecting

thermal conductivity of the gas by a change in temperature.

• In the thermocouple gauge, the temperature is sensed by a thermocouple  and in Pirani gauge by the change in resistance of the heated wire.

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THERMOCOUPLE GAUGE

• A Thermocouple is kept in contact with the heated wire and the temperature of the wire is directly measured as a measure of pressure.

•  For different pressures, the temperature is measured by the fine-wire thermocouple, the hating current being initially fixed by the resistance as shown in the figure. This device is usually used for comparison purposes. The sensitivity of such an instrument depends on the pressure and the wire current.

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WORKING• Two sets of thermocouples are used to measure temperatures

of heater wires in the two chambers and oppose each other. When there is a difference in pressures, there occurs an unbalance which is measured by a potentiometer circuit. Instead of a single thermocouple per wire, a thermopile is often chosen to increase sensitivity. The thermocouple gauge is also composition dependent and needs empirical calibration for the high vacuum range.

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PRINCIPLE• The T/C gauge contains two elements: a heater (filament) and a

thermocouple junction which contacts the filament. With the filament current held constant, as the pressure within the tube is decreased the filament will become hotter because of the improved thermal insulation provided by the increasingly rarefied gas. This temperature is sensed by the thermocouple junction. Measurement is accomplished by reading the thermocouple junction voltage on a sensitive meter which has previously been calibrated against a manometer

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CALIBRATION• Thermocouple gauges are calibrated such that the

wire’s temperature is displayed as a pressure reading. This allows such problems as variations in heat flow through the supporting electrodes to be taken into account.

•  T/C gauges are not particularly accurate instruments. Most often they are used only as rough indicators of pressure where 10 to 20 percent accuracy is acceptable.

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PIRANI GAUGE• The Pirani gauge is a robust thermal conductivity

 gauge used for the measurement of the pressures in vacuum systems.

• The thermocouple (or T/C) gauge is one of the more common and cost effective gauges for vacuum pressure measurement in the 1 Torr to 1 milliTorr range

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STRUCTURE

• The Pirani gauge consists of a metal filament (usually platinum) suspended in a tube which is connected to the system whose vacuum is to be measured. Connection is usually made either by a ground glass joint or a flanged metal connector, sealed with an o-ring. The filament is connected to an electrical circuit from which, after calibration, a pressure reading may be taken

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PRINCIPLE

• A conducting wire gets heated when electric current flows through it. The rate at which heat is dissipated from this wire depends on the conductivity of the surrounding media. The conductivity of the surrounding media in turn depends on the density of the surrounding media (that is, lower pressure of the surrounding media, lower will be its density). If the density of the surrounding media is low, its conductivity also will be low causing the wire to become hotter for a given current flow, and vice versa.

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DESCRIPTION

The main parts of the arrangement are:• A pirani gauge chamber which encloses a platinum

filament.• A compensating cell to minimize variation caused due to

ambient temperature changes.• The pirani gauge chamber and the compensating cell is

housed on a wheat stone bridge circuit as shown in diagram.

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OPERATION• A constant current is passed through the filament in the pirani

gauge chamber. Due to this current, the filament gets heated and assumes a resistance which is measured using the bridge.

• Now the pressure to be measured (applied pressure) is connected to the pirani gauge chamber. Due to the applied pressure the density of the surrounding of the pirani gauge filament changes. Due to this change in density of the surrounding of the filament its conductivity changes causing the temperature of the filament to change.

• When the temperature of the filament changes, the resistance of the filament also changes.

• Now the change in resistance of the filament is determined using the bridge.

• This change in resistance of the pirani gauge filament becomes a measure of the applied pressure when calibrated.

• Note: higher pressure – higher density – higher conductivity – reduced filament temperature – less resistance of filament & vise versa

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• The gauge may be used for pressures between 0.5 Torr to 10−4 Torr. The thermal conductivity and heat capacity of the gas may affect the readout from the meter, and therefore the apparatus may need calibrating before accurate readings are obtainable. For lower pressure measurement other instruments such as a Penning gauge are used.

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Applications of Pirani gauge• Used to measure low vacuum and ultra high vacuum

pressures.Advantages of Pirani gauge• They are rugged and inexpensive• Give accurate results• Good response to pressure changes.• Relation between pressure and resistance is linear for

the range of use.• Readings can be taken from a distance.Limitations of Pirani gauge• Pirani gauge must be checked frequently.• Pirani gauge must be calibrated from different gases.• Electric power is a must for its operation.

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WHICH IS PREFFERED

PIRANI GAUGE OR

THERMOCOUPLE GAUGE

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ANSWER :• PIRANI GAUGE IS PREFFERED OVER

THERMOCOUPLE GAUGE BECAUSE:o ITS TEN TIMES FASTER THAN THEROCOUPLE

GAUGEo WIDER RANGE OF PRESSURES CAN BE

MEASURED WITH A PIRANI GAUGE.

 Well designed Pirani gauges offer better accuracy and response time than do thermocouple gauges (often tens of milliseconds vs. several seconds).