New reference chamber system of the 87x9 gas density monitor generation

In addition to the advantages just mentioned, the 87x9 generation reference chamber measuring unit can now also monitor the low pressure range down to vacuum in addition to the operating range and cover a larger range between the lowest and highest switching points with up to five switches.

SF₆-free gas density monitor

The new measuring system of the 87x9 generation is based on the same reference chamber principle as the models of the proven 87x6 generation (see illustration below). Both the manufacturing processes and the central, critical elements of the design have been preserved. This means that the measuring system of new generation guarantees the same unsurpassed reliability and robustness as the systems that have been tried and tested for over 20 years. The reference chamber is no longer filled with the same insulating gas as the switchgear, but with a mixture of nitrogen and CO₂, which reflects the isochoric (constant volume) behavior of the system gas. When calibrating the measuring system, the switching points and the scaling of the display are precisely matched to the values of the effective insulating gas in the system.

With regard to the upcoming alternative insulating gases, this has the advantage that the exact gas mixture no longer has to be provided, but that the reference chamber filling can be determined purely mathematically based on the physical data (isochore values) of the specific insulating gas mixture in the system and produced on this basis.

In the case of systems with sulphur hexafluoride as an insulating gas, the SF₆-free measuring systems of the new generation offer advantages above all in terms of disposal at the end of their service life: despite the very small quantity of less than 1g SF₆ per gas density monitor, conventional gas density monitors with SF₆-filled reference chambers must be disposed of specially depending on national legislation. And it is to be expected that the disposal regulations for SF₆ will become even stricter. The new 87x9 generation reference chamber measuring system is filled with a gas that can be safely disposed of without further measures, regardless of the insulating gas in the system. Particularly in systems with sulphur hexafluoride, which are to be converted to alternative insulating gases in the medium term, the disposal of gas density monitors designed for SF₆ should already be considered today.

Reference chamber-based full-range display

A major advantage of Trafag's reference chamber-based gas density monitors is the zoomed display. In contrast to manometer-based gas density monitors, the scaling is not linear over the entire range, but the operating range (slightly below the lowest alarm switching point to above the overpressure alarm switching point) is displayed much larger because it is decisive for the system safety. The integrated low-pressure range below the operating range, extends from vacuum to approximately 4 bar and is used for monitoring during storage, transportation and commissioning.

If you compare the display of the new 87x9 generation with that of the previous 87x6-based generation, two significant differences stand out: on the one hand, the display of the new generation is fundamentally larger and it is visually based on traditional pressure gauges. On the other hand, the low pressure range is no longer on a separate scale with its own pointer, but is integrated directly into the full-range display. The full-range display consists of the operating range, the low-pressure range and the intermediate range in between.

The difference between 87x6 and 87x9 in the display of the low-pressure range is not only purely visual; the measuring principles also differ: the separate low-pressure display of the 87x6 generation is based on a separate Bourdon tube pressure gauge measuring mechanism; the new, integrated low-pressure display of the 87x9 generation is based directly on the reference chamber. Its measuring range has been extended so that it also covers the low pressure range down to vacuum.

Indication in the low pressure range

The device types 87x9 and 87x6 differ in the measuring principle in the low pressure range. This leads to a display difference when the gas density monitor indication is read against the ambient pressure: The Bourdon tube pressure gauge measures the pressure relative to the environment; the reference chamber on the other hand — an absolute measuring system due to its principle — does not measure the pressure, but the gas density, displayed as the pressure of the system gas normalised to 20°C.

If the display of both gas density monitor generations is compared with a manometer at 20°C, all three devices display almost the same values within the measurement uncertainty. The relative measuring systems of the pressure gauge and low pressure indicator of the 87x6 generation display 0bar relative in the unconnected, unpressurised state, while the reference chamber system displays the current barometric pressure (absolute). This is usually between 0.96 and 1.02bar, for example 0.98bar absolute, depending on the altitude above sea level and the weather. An absolute pressure gauge would also display the same.

Under identical conditions, but with an ambient temperature of only 5°C, the displayed values differ significantly: the relative measuring systems still display 0bar relative, and the absolute pressure gauge also still displays 0.98bar absolute. But the pointer of the reference chamber-based, absolute measuring mechanism is significantly higher, at approximately 1.4bar absolute. However, the display is not scaled to absolute or relative pressure in bar, but to the (isochoric) gas density at 20°C, e.g. of SF₆. The density of SF₆, which generates an absolute pressure of 0.98bar in a sealed volume at 5°C (the barometric pressure applied to the gas density monitor), would generate an absolute pressure of 1.4bar if the sealed volume were heated to 20°C.

If pressure measuring systems such as pressure gauges (absolute or relative pressure) are used for maintenance work or filling, the low pressure display of the 87x9 gas density monitor must be converted for comparison with the pressure gauge. This conversion is not necessary for the low pressure display of the 87x6 gas density monitor, as it is already based on a relative pressure gauge. If the density, which is shown as the pressure of the reference chamber gas at 20°C on the display of the gas density monitor, is converted to pressure, the isochoric temperature change must be taken into account accordingly during the conversion.

Extended operating range with up to five switches

The operating range between the lowest and highest switching point is significantly larger in the new 87x9 generation: for variants with partial range display it is 250kPa@20°C, for variants with full range display it is 180kPa@20°C. The switching point accuracy of 10kPa@20°C is still unsurpassed on the market. The number of possible switches has also been increased: five instead of four microswitches can now be integrated as normally closed and normally open contacts. This means, for example, that both the first alarm point and the lock-out can be operated with redundant switches for even greater safety, and the fifth switch can also be used as a high-pressure alarm to prevent overfilling.

Measuring system based on the reference chamber principle

If you are not sure which Trafag product is best suited for your application, please contact us. Our specialists will be happy to provide you with competent advice.

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If you are not sure which gas density monitor is best suited for your application, please contact us. Our specialists will be happy to provide you with competent advice.