Lib
er
at
ors
Theory of Operation
VI
Liquid Level Measurement
Liberators are equipped with a unique liquid level measurement
system. This system measures the level of liquid oxygen inside the
unit with a capacitance probe and displays that liquid level on the
level meter’s LEDs.
The liquid level probe consists of two concentric stainless steel
cylinders extending inside the inner vessel. As the liquid oxygen
level rises, the capacitance of this assembly goes up. The level
meter then displays the liquid level in the cylinder based on a
calibration relating capacitance to fill level. The higher the liquid
level in the dewar, the more LEDs are activated, beginning at the
left
most LED.
Electrical connection between the level meter and the probe is
made via a single conductor JST connector. This male plug is at-
tached to its female counterpart extending from the probe, creat-
ing a watertight connection. A single ground wire is connected
from the meter to a male spade terminal on the mounting bracket.
The meter is powered by an internal battery offering battery
life of 5 years or more at 30 cycles per day. The meter has a low
battery (LOW BATT) indicator which signals the need for bat-
tery replacement. The meter battery is covered under a 2-year
limited warranty. If battery failure occurs within 2 years of the
Liberator shipment date, contact CAIRE, Inc. customer service
for a replacement meter. If the meter battery is no longer under
warranty, the CR2032 coin cell battery can be replaced (RP5).
Replacement batteries can be found at most hardware stores or
they can be ordered through CAIRE customer service.
The new level meter improves upon the previous meter by in-
tegrating all components within its casing, simplifying removal
and replacement (RP4). Even more importantly, there is a much
improved calibration procedure that requires no additional tools
and a range of error reporting codes which can be read directly
from the LEDs to report calibration errors. These can be found
in the calibration procedure (RP7).
JST Connector
FIGURE 6:
Liquid Level Meter Circuit
NOTE: Tampering with meter battery housing will void the
battery’s 2-year warranty.
Liberators Technical Service Manual • PN 13350704 Rev
K
13
1
3
