The feasibility of a capacitive sensor for measuring the level of liquid nitrogen in a container has been demonstrated. The basic sensor design could also readily be adapted to measurement of the levels of cryogenic liquids other than nitrogen.
The general concept of capacitive sensors is not new. The novel aspect of this particular capacitive sensor lies in the use of the liquid nitrogen as part of the dielectric of a capacitor. Two vertically parallel, electrically conductive plates constitute the electrodes of the capacitor and are partly immersed in the liquid from the top, so that the top surface of the liquid lies somewhere along the plates. As the level of the liquid varies, the amount of dielectric material between the plates, and thus the capacitance, also varies. It is possible to compute the level of the liquid from (1) a measurement of the capacitance or a quantity related to capacitance in a known way and (2) the known relationship between the liquid level and the capacitance.
Several other types of sensors for measuring the levels of cryogenic liquids are available. The transducers in those sensors include, variously, cryodiodes, carbon resistors, hot-wire sensors, and simple floats. Each type is disadvantageous in one or more ways:
- Cryodiodes are expensive (costing about $500 apiece), and the electrical currents through them must be conditioned in order to enable detection of liquid/vapor interfaces for proper measurement of liquid levels.
- Because carbon resistors are insensitive to liquid/vapor interfaces, they give only rough indications of the levels of cryogenic liquids.
- Hot-wire sensors can give inaccurate readings when liquids boil next to the wires.
- Simple floats provide good visual indications of liquid levels, but their outputs cannot be fed directly to computer data-acquisition systems.
The first step in the development of the present capacitive sensor was to establish the relative permittivity of liquid nitrogen (≈1.454), gaseous nitrogen (1.0005480), and air (1.0005364) at the boiling temperature of liquid nitrogen at standard atmospheric pressure. The electrodes of a prototype of this sensor were made from two copper-clad circuit boards with a thickness of 0.010 in. (0.254 mm), a length of 10 in. (25.4 cm), and a width of 1 in. (2.54 cm). The boards were secured with five nylon screws, nuts, and washers spaced 0.070 in. (1.78 mm) apart. A wire was soldered to the top of each plate, and the wires were attached in parallel to another capacitor that constituted part of the time base of a free-running oscillator. The plate structure was placed inside a Dewar flask, into which liquid nitrogen was poured. As the liquid level increased, the capacitance of the plate structure likewise increased. In subsequent demonstrations, this prototype sensor performed consistently, and thus its capability was proved conclusively.
This work was done by Tim E. Roth of AlliedSignal, Inc., for Johnson Space Center. For further information, contact the Johnson Space Center Commercial Technology Office at (281) 483-0474.