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Method for measuring liquid level of cryogenic tank onboard aircrafts Disclosure Number: IPCOM000237720D
Publication Date: 2014-Jul-07
Document File: 2 page(s) / 721K

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The present invention proposes to measure the liquid level in the tank by measuring the weight of the tank and by extrapolating a "weight level" Measuring a weight is not sensitive to product state and thefore to the duty cycle of the tank. This is the only method to measure residual product inside the tank with a good reliability

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The present document aims at proposing a new method for measuring liquid level of cryogenic tank onboard aircrafts.

Like most of the cryogenic vessel design, a LH2 tank consists in an evacuated cryogenic tank containing the hydrogen. For insulation purposes, the inner tank containing the LH2 is housed inside an outer jacket, the void between the two vessels is evacuated and filled with multilayers insulation. The vacuum space is permanently sealed and doesn't require specific maintenance. The inner tank is supported inside the outer jacket by means of supporting devices designed to withstand the weight of the filled tank and to resist to some specified accelerations and loadings as per its specific use (for example onboard an aircraft). Tank operation of the tank is done by means of pipings, wirings, etc.. for filling, withdrawing; measuring, etc... that are installed inside the vacuum and that links the inner tank with the outside.


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Hydrogen is stored in the container in its liquid state at ambient and low pressure between 20K to 28K. The tank is fitted with an efficient multi-layers insulation in order to minimize the heat leaks. Supporting elements and pipings are causing additional heat leaks of the tank. For small tank which total tank occupancy is a major constraint, pipings and instrumentation amounts the biggest share of the heat leaks.

These heat leaks will cause the inner tank pressure to increase and ultimately a loss of hydrogen will occur when reaching the set pressure of the tank safety valve.

Typically, with an initial 90% filling rate, the pressure in the tank will rise at a rate which depends on the amount of tank heat leaks, up to an inflexion point which corresponds to the ultimate state of expansion of the liquid phase in the tank when the liquid level in the tank reaches 100%.

With such features of cryogenic fluids, measuring accurately a liquid level is very problematic for sealed cryogenic tanks during the duty cycle. Measuring a liquid level with acceptable accuracy is possible upon filling when tank is degassing at the filling station but when in operation, liquid level may vary significantly depending on tank pressurization method and duty cycle, storage time, tank heat losses and therefore level measurement is almost impossible and inaccurate.

Most of the existing technologies compatible with cryogenic temperature for measuring liquid level require for their installation and maintenance, pipings or wirings (housed in pipings) bringing additional heatleaks. Among them, pressure differential gauge, conductance gauge, ultrasonic wave gauge, ect.... Accuracy of most of these technologies is sensitive to product state and this may impact level measurement during duty cycle.

The present invention proposes to measure the liquid level in the tank by measuring the weight of the tank and by extrapolating a "weight level". Measuring a weight is...