1. Introduction

A helium level meter measures and displays the liquid helium level of helium vessels. The model developed at the Helmholtz-Zentrum Berlin offers a wide range of additional functions including the following ones:

1. Utilize customized superconducting helium sensors
2. Install a pressure sensor to measure the pressure inside the vessel
3. Perform automatic measurements during the filling procedure and the storage time
4. Use the device for automatic filling procedures
5. Integrate the device into the institute-wide helium monitoring system developed at the Helmholtz-Zentrum Berlin in order to supervise the usage of helium and detect leakage

If used as part of the helium monitoring system, each helium level meter should be assigned one vessel and stay attached to it until the helium is depleted. During that period it periodically transmits measured values via a wireless network to a database.

The helium level of a vessel can be determined via the electrical resistance of a superconducting wire inside the vessel, which is called helium sensor. Only the warmer part of the wire in the gas phase is non-superconducting and has an electrical resistance so that the resistance corresponds to the filling level. Because the wire just above the surface of the liquid helium is also superconducting due to its thermal conductivity it has to be heated by electrical current for some seconds to ensure that the whole part in the gas phase is non-superconducting. In case that the whole wire is superconducting, a small resistor is installed at the upper end of the wire inside the vessel which works as a starting point for the heat propagation.