A definition of active, passive and combination gauges

Vacuum gauges can be categorized into active (including single and combination heads) and passive gauge types. There is a suitable gauge type for all vacuum processes. Leybold offers a complete range of gauges, just as we have a range of pumps for each applicable process. The fundamental properties of the basic gauge types are covered below. As always, please contact us if you have any specific requirements or to request assistance during the gauge selection process. We will be happy to assist you.

Features of active gauges

Active vacuum gauges include both the control unit electronics and the actual sensor. This means that active gauges are a complete, self-contained head and driver electronics in one compact unit. The sensor part generates an electrical magnitude from the measured signal.

These gauges typically require 24 V DC and then provide a 0 - 10 V output related to pressure. The bakeout temperature through electronics is limited to <212°F (<100°C), so as a result, bake-out is only possible to a limited extent with the electronics connected. With the electronics removed they can be baked normally up to 150°C.

Linearized output signal, either analogue or digital, means that there are no transmission errors on the cable. The gauge cables are standardized and there is no influence of the line on the measured pressure value.

The controller is simply a display device, so the power supply comes from the plant. The display of signals is on the central display. Pressure is indicated with an analog measurement or via a digital interface. Digital gauges are (actively) independent of a controller and communicate the measured values serially. 

It is also important to note that active gauges are sensitive to radiation and magnetic fields.

Combined sensor gauges or combination gauges

As might be expected from their name, combination gauges combine various measuring principles in one gauge. This means that individual weaknesses are eliminated, operational safety is increased, and they are also more cost effective. There is only one measurement for two sensors, along with single housing, single connection and single display. Combination gauges also offer better pressure monitoring over larger ranges. An example of combination gauge is the Pirani / Cold cathode gauge – see the page on indirect pressure measurement for more details. 

Features of passive gauges

Unlike active gauges, in passive gauges only the sensor is included, while the electronics are excluded. Passive measurement devices completely separate the measurement part from the driver electronics. The head attached to the system contains only the parts needed for the physical measurement and all other electronics are contained in the control unit. Usually, the control units are also fitted with analog voltage outputs, switch-point relay contacts and digital interfaces.

Passive gauges offer direct measurement of the sensor signal and the cable has an influence on the measuring signal. They require particular cables according to indicator type.

The controller is also the measuring device, so that the power supply only comes from the controller. In a similar fashion, the display is located uniquely on the controller.

Passive gauges are tolerant of radiation and high bakeout temperatures are possible, in contrast to active gauges.

Maintenance

Finally, it is also important to note that, as with any equipment, maintenance of gauges plays a crucial role in their functioning to expected standards. It is essential that gauges of all types are kept clean, in order to ensure reliable pressure readings. 

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