GB2255828A - Vacuum gauges - Google Patents

Abstract

A vacuum gauge of the Pirani type having a head which comprises an envelope within which is supported an electrically conductive coiled filament 30 and having means 21, 22 to allow current to be passed through the filament, wherein the coiled filament 30 is formed in a non-linear configuration (eg. V, W or U shaped). <IMAGE>

Description

VACUUH GAUGES This invention relates to vacuum gauges and, more particularly, to gauges of the "Pirani" type.

Pirani vacuum gauges date from about 1906 and are well known and widely used. The gauge has a head which comprises a glass (or metal) envelope containing an electrically heated filament of a metal having a high temperature coefficient of resistance, for example nickel, platinum or tungsten. The gauge also has a control circuit which commonly is a Wheatstone bridge in which one of the four legs is the filament of the gauge head.

As the pressure in the gauge head increases, the temperature of the filament, and therefore its electrical resistance, tend to decrease.

The gauge may be operated in a constant temperature mode in which the electrical energy supplied to keep the filament at a constant temperature is measured and the rate of change acts as a measure of the pressure or the degree of vacuum.

Alternatively, the gauge may be operated in a constant voltage or constant current mode in which the total bridge voltage or current is kept constant and the out-of-balance bridge current/voltage meter is read to determine the pressure or the degree of vacuum.

Calibration of the gauges can be effected in a variety of ways which are well known end means may be employed to compensate for changes to the ambient temperature and bridge voltage.

The gauge head is clearly a critical part of any Pirani gauge. As stated above, it generally comprises a metal filament wire, for example 0.005 to 0.1 mm in diameter, which is wound in the form of a helix, for example 0.5 to 2 mm diameter with a helix pitch of at least 10 wire diameters in size (and probably greater) so as to prevent any one turn of the-helix shielding the adjacent turns. The filament is held in that configuration within the glass envelope in linear form by being fastened in slight tension between two supports, for example 6 to 8 cm apart to which wire is spot welded at either end of the helix in a manner which allows bridge current to flow through the wire.

The present invention is concerned with the provision of an improved gauge head which allows for enhanced sensitivity without the need to increase the size of the head.

In accordance with the invention, there is provided a head for a vacuum gauge of the Pirani type which comprises an envelope within which is supported an electrically conductive coiled filament and having means to allow current to be passed through the filament, wherein the coiled filament is formed in a non-linear configuration.

The non-linear configuration preferably comprises forming the filament with a simple, substantially central fold so that the filament approximates to a "hairpin" shape. Alternatively, the filament may be folded to form a substantially 'V' shape or other similar shape.

For a better understanding of the invention, reference is now made, by way of exemplification only, to the accompanying drawings, in which: Figure 1 shows a typical Pirani type vacuum gauge control circuit in which the gauge head is shown schematically.

Figure 2 shows a gauge head of standard design with a coiled filament in a linear configuration.

Figure 3 shows a side view of a gauge head in accordance with the invention.

Figure 4 shows a top view of the gauge head shown in Figure 3.

With reference to the drawings, the circuit shown in Figure 1 comprises a Wheatstone bridge circuit having the usual four resistances R1, R2, R3 and R4; R2 is the filament of the gauge head H and the other three resistances are nearly equal to that of the gauge head. RM is the resistance associated with the current meter M. In this example, R1 and R3 are of fixed value and R2 and R4 are variable. The balanced condition of the bridge is R2 R1 R4tR3.

One way of measuring gauge head pressure is to vary R4 to balance the bridge and calculate the head resistance R2. Calibration of the gauge will allow conversion of the resistance R4 to pressure.

Alternatively, R1, R3 and R4 are kept constant and the out-of-balance current read from the meter M. Again, conversion of the meter reading to pressure is possible by prior calibration.

The gauge head shown in Figure 2 for use in the circuit of Figure 1 comprises a glass envelope 1 secured to a boss portion 2 made from an insulating material. A metal rod 3 is embedded in the portion 2, one end 3 of which serves as a connecting pin to the Wheatstone bridge circuit and the other end 4 of which is 'L'-shaped.

A shorter metal rod 5 is also embedded in the portion 2 and provides a complementary connecting pin to the Wheatstone bridge circuit. A helically coiled filament wire of tungsten in linear configuration is stretched between the rod 5 and the 'L' shaped end 4 of the rod 3. The filament provides resistance R2 of the Wheatstone bridge circuit of Figure 1.

Turning to Figure 3 and 4, there is shown a modified gauge head according to the invention. It comprises a boss portion 20 made from an electrically insulating material such as PTFE or glass/ceramic in which are embedded the larger arm of each of two 'L' shaped metal pieces 21, 22 which are held relative to each other as shown.

Also embedded in the boss portion 20 is an end of an electrically insulated rod 23, the other end of the rod 23 having a short 'L' shaped portion relative to the main portion to which is attached an electrically insulating bobbin 24; the bobbin has a centrally positioned circumferential grove 25.

The shorter arms 26, 27 of each 'L' shaped piece 21, 22 are used for connection to the bridge circuit and the longer arms 28, 29 have spot welded thereto the end of a filament 30.

The filament 30 is made from a single length of wire made of gold-coated tungsten of 0.025 mm diameter and, except for a very short portion at its centre and at either end in the vicinity of the spot welds, is coiled in the form of a helix of 2 mm diameter. The central uncoiled portion passes over the bobbin 24, in particular within the groove 25.

The filament is slightly tensioned in that position and its non-linear configuration approximates to that of a hair pin.

Claims (3)

1. A vacuum gauge of the Pirani type having a head which comprises an envelope within which is supported an electrically conductive coiled filament and having means to allow current to be passed through the filament, wherein the coiled filament is formed in a non-linear configuration.

2. A vacuum gauge according to Claim 1 in which the configuration comprises forming the filament with a simple, substantially central fold so that the filament approximates to a "hairpin" shape.

3. A vacuum gauge according to Claim 1 in which the configuration comprises forming the filament into a substantially 'V' shape.