Theo is based at Essen/Mühlheim Airport and is regularly used for guest flights and as an advertising space. Theo was built in 1972 by WDL Luftschiffgesellschaft, which rebuilt it in 2015 and is currently constructing a new hangar for it. Theo is a bouncing airship with a length of 59.5 m and a diameter of 15 m at its most voluminous point. This type (unlike the Zeppelin, which is better known by name) does not have a covered framework - instead, its shape is determined solely by the gas with which the hull is filled. To ensure that the pilot always has all the information about the pressure inside the airship, digital displays from the measurement technology provider Althen are installed in the cockpit; they have been ensuring that Theo flies safely for more than 30 years.

An impact airship does not need any scaffolding at all. However, this design places special demands on the stability of the pressure conditions for flight operations. When inflated, the airship reacts sensitively to rudder movements and can therefore be controlled very precisely. Any loss of pressure, on the other hand, makes the airship more cumbersome and difficult to maneuver - and therefore potentially less safe.

When Theo is illuminated at night, the three air chambers (the ballonettes) are clearly visible.

© WDL Airship Company

Therefore, the top priority for the safe operation of any airship is to keep the pressure in the interior constant. For this to work, there are three ballonets in the interior - smaller balloons that are filled with air in this case and whose contents are variable in order to compensate for the pressure fluctuations of the large interior. The pressure inside the ballonets is always higher than the external air pressure. This ensures that the entire body is always filled to capacity. This overpressure is only a few mbar. The principle of ballonets dates back to the 18th century - but today's pilots find it much easier to steer an airship safely thanks to digital displays.

Digital pressure display as a redundant addition

Every airship must have a pressure monitoring system. However, how this is implemented is not specified in detail. Theo and the other WDL airships initially used a simple mechanical pointer instrument (manometer).

In 1988, Althen was commissioned by WDL to develop the 'Triple Press Indicator' for Theo and the other airships. TPI is a three-channel system consisting of differential pressure sensors and a display unit with limit value monitoring. A sensor is fitted in the outer hull and in each of the three ballonets, which is connected to the display via a cable and can thus show the measured value. The pilot uses these displays to operate the valves of the individual pressure chambers and thus control the airship.
Each air chamber is monitored by a measuring channel. All displays are housed together in a compact display so that the pilot can see the pressure conditions in the entire airship at a glance and react quickly to changes. There is also another measuring channel as a single-channel system. With these two devices, all pressures in the chambers are permanently monitored.

If minimal pressure differences influence the flight attitude of the airship, the decimal places of the pressure display, which is installed directly in the pilot's field of vision, are important.

© WDL Airship Company

The differential pressure sensors from Althen have an extremely small measuring range of just 125 mm water column, which corresponds to around 12.3 mbar. The static system pressure for Theo is 14 bar. The fact that the measuring range is very small compared to the system pressure places special demands on the pressure transducers used, which are based on the LVDT principle (LVDT = Linear Variable Differential Transformer). This is precise and robust and is still used in various applications today. A wear-free LVDT displacement transducer measures the deformation of a pressure diaphragm. The transducer signal is then electronically converted into an industrial standard signal.

The small pressure differences that influence the flight characteristics of Theo are all the more difficult to measure because the surrounding system pressure is so high. In conjunction with the high-resolution digital display, Althen's pressure measurement and display enables much more precise manual control of the airship's valves compared to the additional mechanical display. The measuring principle has proven itself and is still used today. Holger Piscator, who works in product development at Althen and was involved in the Triple Press Indicator, explains: "Today, you would equip such a system with a touch display and make it programmable via a controller, but the functional principle would remain the same."

Areas of application for differential pressure measurements

Differential pressure measurement is often used in industry for leakage measurements. The principle is also valuable in the areas of air conditioning and energy management, for example to measure the degree of contamination of filters. The accuracy requirements here are somewhat lower than for the airship, but the principle is comparable. Differential pressure measurement is also used in clean rooms to monitor whether there is constant overpressure.