Basic Principle of Pirani Gauge

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Basic principle of Pirani gauge 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 inturn depends on the densisty 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. Description of Pirani gauge The main parts of the arrangement are: 1. A pirani gauge chamber which encloses a platinum filament. 2. A compensating cell to minimize variation caused due to ambient temperature changes. 3. The pirani gauge chamber and the compensating cell is housed on a wheat stone bridge circuit as shown in diagram. Operation of Pirani gauge 1. A constant current is passed through the filament in the pirani gauge chamber. Due to this current, the filament

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Transcript of Basic Principle of Pirani Gauge

Page 1: Basic Principle of Pirani Gauge

Basic principle of Pirani gauge

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 inturn depends on the densisty 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.

Description of Pirani gauge

The main parts of the arrangement are:1. A pirani gauge chamber which encloses a platinum filament.2. A compensating cell to minimize variation caused due to ambient temperature

changes.

3. The pirani gauge chamber and the compensating cell is housed on a wheat stone bridge circuit as shown in diagram.

Operation of Pirani gauge

1. 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.

2. 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.

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

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

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5. 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] and vice versa.

Applications of Pirani gauge

Used to measure low vacuum and ultra high vacuum pressures.

Advantages of Pirani gauge

1. They are rugged and inexpensive2. Give accurate results

3. Good response to pressure changes.

4. Relation between pressure and resistance is linear for the range of use.

5. Readings can be taken from a distance.

Limitations of Pirani gauge

1. Pirani gauge must be checked frequently.2. Pirani gauge must be calibrated from different gases.

3. Electric power is a must for its operation.

A Simple Pirani Gauge reading to 0.001 mB

Commercially available Pirani gauges seem to be unnecessarily expensive. The basic design consists of a simple coil of wire exposed to the vacuum to be measured and connected as an arm of a Wheatstone bridge, with a meter reading the out-of-balance voltage of the bridge. With suitably chosen components, the out-of-balance voltage can vary with the pressure from about 10mB down to 0.001 millibars. Although commercial versions, albeit with some extra sophistication, can cost over $2000, the instrument described here can be constructed for one hundredth that sum and prove as useful in the laboratory as its more expensive cousin.

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Basic Principles of Operation

If a wire, surrounded by a gas, is heated electrically, heat is lost from it by three processes: radiation, conduction and convection. The first of these is independent of the pressure of the gas and cannot therefore contribute to its measurement although it gives rise to a constant loss of heat. Convection contributes significantly at high pressures, but the heat loss caused by it is not proportional to the pressure and in the ideal case, is independent of it. Conduction of heat by an ideal gas is proportional to the pressure over a range approximately lying between 10-5 mB and 10-2 mB. In practice, the variation of heat loss from a hot wire with pressure can be exploited between 0.001 mB and 10 mB. The instrument described here operates in this range.

The Pirani Gauge Head

The construction of the Pirani gauge head is shown on the right. A glass envelope of diameter 28mm is provided with two 1 mm bore capillary inlets with wide openings as shown. A filament structure comprising 0.7mm diameter stainless steel wire supports F, welded to a stainless steel wire coil E ( 0.1mm diameter wire, 56cm in length, wound in an open helix diameter 8mm, resistance at 20 C of 24.6 ohms) was supported by a short length of glass tube to which the 0.7mm wires were cemented using Ceramabond 569 (Aremco Products Inc). The 0.7mm SS wires were cemented into the capillary tubes using Araldite standard Epoxy. ( Separate experiments were conducted to examine the quality of seal produced in this way. By applying vacuum, the resin could be drawn down the capillaries for about 20 mm prior to hardening. It was found that cured seals made in this way were sound at least to 10-5 mB pressure. A high vacuum epoxy is available which is capable of sustaining a vacuum of 10 -9 mB, but this was considered un-necessary in the present application.) After hardening the epoxy, the 28mm tube was drawn down as shown and fitted with an inlet tube of 6mm OD.

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The Pirani Gauge Controller

There are three modes of operation for a Pirani gauge: (i)constant current, (ii)constant resistance and (iii)constant voltage. The first is termed the Pirani-Hall Gauge and involves feeding the wire filament with a constant current and measuring the voltage across it, which increases with falling pressure as the temperature of the wire increases. Option (ii) requires the incorporation of the filament in a micro-calorimetric bridge such that the current is varied automatically to keep the bridge balanced and the product of the voltage and filament current are used to compute the power dissipation which is approximately proportional to the pressure over the usable range. This measurement mode is capable of the best precision, but involves more complicated electronics and some computation to exploit it. Option (iii) is the best for general purpose use and requires only very simple electrical circuitry as the diagram shows. R1 & R2 are 1 watt 100 ohm resistors which form the reference arm of the bridge. Rs & R4 complete the bridge and R4 is adjusted to give 0.66V out of balance voltage when the gauge is at 1 atmosphere pressure. This represents the lowest wire temperature. As the pressure is reduced, the filament temperature rises to about 500 degrees C at 0.001 mB, and the out of balance voltage indicated by the meter falls to about 0.07V.

Operational Data

With the values of components shown in the schematic, Out of balance voltages were measured against pressure using a Vacuum Generators Pirani gauge type PIR 1A. The data are shown in the table:

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If identical values of components are used and the specifications of the Pirani gauge head are carefully followed, the data in the above table can be used to produce a calibrated scale for the 1mA meter. Below is a drawing of such a scale In use the adjustable resistor R4 is set to give the meter reading as FSD at 1 atm pressure before pumping down the gauge.

PRESSURE

Vbridgepercent FSD

log pressure

mB mV    

1000 660 100 3

7 570 86.36 0.8451

2 400 60.60 0.3010

0.4 340 51.51 -0.3979

0.2 260 39.39 -0.6989

0.1 210 31.82 -1

0.09 195 29.545 -1.045

0.08 185 28.03 -1.0969

0.06 165 25 -1.2218

0.059 157 23.788 -1.2291

0.027 118 17.87 -1.5686

0.036 125 18.93 -1.443

0.010 110 16.7 -2

0.005 105 15.9 -2.3010

0.003 90 13.636 -2.522

0.001 70 10.606 -3

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Comparison with the commercial Vacuum Generators Pirani Gauge and with a McLeod gauge showed that the instrument constructed indicated a pressure within a factor of 2 of the pressures measured with the other instruments across the range.