Heat Loss Vacuum Gauges

Gas pressure is determined by measuring the pressure dependent heat loss

Pirani vacuum gaugeThe particle-number density dependent thermal conduction is another characteristic of a gas. The energy is transferred by molecule collisions. The efficiency of the transfer depends on the mean free path length of the particles. In a certain pressure range the thermal conduction is proportional to the pressure. However, distracting effects such as gas convection, heat radiation, thermal losses at connector pins, etc. have to be considered.

The basic concept of a Pirani vacuum gauge relies on a thin wire heated to a constant temperature, which is significantly higher than the temperature of the surrounding gas in the vacuum. The wire (typical diameter: 5…25 µm; length between 50 and 100 mm) transfers the heat to the gas and is simultaneously part of an electrical circuit, for example of a Wheatstone bridge. The pressure dependent temperature can be derived from the monitored wire resistance. Due to the technical realization in the Pirani devices the electrical power is monitored, which is directly proportional to the wire resistance.

A Wheatstone bridge, which is used for the temperature stabilization, can measure the difference between the heater voltage and a defined reference voltage as well. The difference between the actual and the reference voltage is adjusted to zero at a pressure value, where heat conduction does not play a role anymore. This calibration mainly eliminates the unwanted effects of heat radiation and thermal conduction at the connector pins of the wire.

Advantages of the Pirani vacuum gauge are the wide measurement range from 10-4 mbar up to atmospheric pressure, as well as the accuracy of ±10 % that is sufficient for most applications.

The thermal conductivity of a gas is a function of the average velocity of the molecules as well and thus of the molecular mass of the gas particles. Due to this fact, heat loss vacuum gauges measure the pressure depending on the gas type. Generally the instruments are calibrated for nitrogen and air. Measurements with other gases (e.g. argon; helium) should be interpreted with caution. For example, chamber venting with argon can generate a slight overpressure in the chamber, which cannot be displayed by a Pirani gauge.

In pressure ranges above 10 mbar gas convection increases and the movement is influenced by gravitational forces. Therefore, it is important to consider the mounting position of the gauge and to follow the instructions of the manufacturer. Improvements in the accuracy in the range between 500 up to 1300 mbar are achieved in convection enhanced Pirani gauges, where the housing wall temperature is measured in a similar way as the Pirani-filament and considered in the signal processing step. This compensates not only the variations in the heat loss caused by varying conditions; it also improves the accuracy up to 2 % via the regulation of the convection and the defined mounting position.

Low-priced versions of heat-loss vacuum gauges are thermocouples. Those heat the wire with a constant power and determine its temperature. An clear advantage of this setup is the simple design. However, thermocouples are limited in their measuring measurement range and their accuracy.

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