Vacuum measurement technology – reconsidered
Fig. 2: Areas of application of the various sensors. Source: Busch Vacuum Solutions.
At a higher vacuum level, indirect measuring principles based on special properties of the process gas, such as thermal conductivity or ionization probability, are used. Unlike the direct measurement method, these principles depend on the type of process gas. For this reason, a specific correction factor must be applied for each process gas except air or nitrogen.
A number of factors must be taken into account when selecting the correct sensor. Knowledge of the pressure range, the required accuracy, the gas composition as well as the process and ambient conditions are of great importance for optimum measurement results. The VACTEST vacuum metrology comprises the following sensor technologies for direct pressure measurement independent of the gas type:
Piezoresistive sensorThe robust and cost-effective piezoresistive sensor technology measures the change in resistance of a strain gauge resulting from the diaphragm deflection. The robustness and versatility of the sensor means that it is the best choice for absolute pressure measurement in the rough vacuum range. The measuring range is between 1,600 and 1 millibars.
Capacitive sensorCapacitive sensors exhibit a high degree of accuracy and measure the changes in the electrical capacitance of a plate condenser caused by the diaphragm displacement. These high resolution sensors are specially designed for industrial applications requiring high accuracy and chemical compatibility. Capacitive sensors can be used in the range from 200 to 0.1 millibars.
The following sensor technologies are available for gas type-dependent indirect pressure measurement:
PiraniThe robust and reliable Pirani technology measures the thermal conductivity of gases. The heat transfer between a heated filament and its environment changes proportionally to the pressure. Pirani sensors are the best solution for medium vacuum applications.
Pirani sensors can be used reliably in a wide measuring range from 1,000 to 1 · 10-4 millibars.
Cold cathodeThe cost-effective and robust cold cathodes (inverted magnetron) measure the electrical conductivity of a plasma. Neutral gas molecules are ionized by collision with electrons and captured by the cathode. The current generated is an indication of the pressure in high and ultra-high vacuum processes. Cold cathodes can be used in the range from 2 · 10-3 to 5 · 10-9.
In combination with Pirani sensors, they cover measuring ranges from 1,000 to 5 · 10-9 millibars.
Hot cathodeHot cathodes based on the Bayard-Alpert principle measure gas ionization stably and reproducibly via thermal electron emission. The ion collector current is linear to the gas pressure. This technology forms one of the most accurate measuring principles for high and ultra-high vacuum. Together with a Pirani sensor, the measuring range is extended from 1,000 to 5 · 10-10 millibars.
With the various VACTEST products, Busch meets all requirements for the measurement of pressures of a vacuum system or process in terms of accuracy, measurement range and functionality.
VACTEST mobile gauges are the perfect hand-held solution for quality assurance of vacuum process maintenance. These battery-powered gauges offer many functions, such as a USB interface for exporting and displaying data as well as a data logger and can even be used under vacuum.
Busch Vacuum Solutions offers the VACTEST explorer as a software tool to provide fast and efficient visualization, analysis and comparison of process data on a computer or tablet.
VACTEST explorer is available in a Lite and a Pro version and includes numerous functions, for example calculation of leakage rates, Active Sensor Controller remote control, pump-down curve comparison, configuration of all digital transmitter parameters and many more.
The VACTEST viewer is a free Bluetooth monitoring application for mobile devices with an Android operating system, and can display the pressure values of up to 16 connected sensors. The tool was designed for Bluetooth data exchange between software and sensors and is ideally suited for maintenance or calibration purposes. Visual and audible alarms can be preset for maximum and minimum pressure values.