Pirani gauges
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
Mode of operation
Block diagram of Pirani gauge
Curves to convert air readings to other gases
A heated metal wire (also called a filament) suspended in a
gas will lose heat to the gas as its molecules collide with the wire and
remove heat. If the gas pressure is reduced the number of molecules
present will fall proportionately and the wire will lose heat more
slowly. Measuring the heat loss is an indirect indication of pressure.
The electrical resistance of a wire varies with its temperature, so
the resistance indicates the temperature of wire. In many systems, the
wire is maintained at a constant resistance R by controlling the current I through the wire. The resistance can be set using a bridge circuit. The power delivered to the wire is I2R, and the same power is transferred to the gas.[3] The current required to achieve this balance is therefore a measure of the vacuum.
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.
It must be admitted that is not possible gauge manufacturing that can show different pressure ranges with sufficient accuracy. Extensive application of vacuum, eventually, led to the construction of various pressure gauges, which can eliminate easily and completely the needs of users. Thus, each one are designed and manufactured for specific environmental conditions and for specific extent of the amount of vacuum. The most important of them can be noted the Pirani, Penning, McLeod and thermocouple gauges.
In Figure 1, it is shown a number of this gauges, based on measurable pressure range. The most common of these gauges can be noted the thermal conductivity gauges. These gauges are formed based on this fact that heat transferring by one gas is associated with gas pressure.
In this type of gauges current passing through the filament (the sensor) heats it.
One part of the filament heating is transferred to the surrounding gas molecules, which calculates the loss of produced heat of heat transferring to the gas molecules lead to measuring the pressure inside the chamber.
Pirani gauge is one of the most common thermal conductivity gauges, which is used at it of platinum, nickel or tungsten filament. Pirani gauge is designed and built based on the dependence of the heated wire resistance to its temperature. These gauges are applied to measuring pressure about 10-2 MPa.
Figure 1: Measuring range for different gauges
The physics of pressure measurement in these gauges is founded based on filament resistance measurement, by Wheateston Bridge method. As shown in Figure 2, two filaments are used as two arms of the Wheateston Bridge. The reference filament is placed in constant gas pressure. The main filament gauges (sensor) is placed inside a vacuum system, exposed by gases inside the chamber. Both filament are heated due to passing current.
Figure 2: Pirani gauges, 1-Reference resistance, 2-Gauges resistance, 3-filament
The generated heat is transferred in the form of radiation or convection to the surrounding gas of filament. The resistance of gauges filament is achieved by measuring the voltage across the wire and the current inside the wire, and using the Wheateston Bridge formula for resistors, and considering that the resistance of wire is a function of temperature, and temperature changes also is a function of the pressure inside the chamber, eventually, it's calculable the chambers gas pressure .
Of course, the measured resistance of the filament is changed non-linearly with pressure. The gauges are built by Marcelo Pirani, and was registered in his name.
Thermocouple and thermistor gauges work like Pirani gauges, except that, in these gauges the pressure is measured directly and according to the changes in wire temperature. Detectable range, by these gauges, is about 10-1 -10-2 Pascal. Figure 3 shows a schematic view of a thermocouple gauge.
Figure 3: Schematic view of a thermocouple gauge, 1-thermocouple, 2-heated filament, 3&4- mV meter, 5-filament, 6-Thermocouple connection.
Refrence
Book: Coating fundamental and nanostructure analysis, jahanbakhsh mashaiekhy, iup,2015
2-https://en.wikipedia.org/wiki
3Ignoring the power lost by the electrical connections and the wire supports.