Measuring and testing – Moisture content or absorption characteristic of material
Reexamination Certificate
2002-10-24
2004-07-06
Williams, Hezron (Department: 2856)
Measuring and testing
Moisture content or absorption characteristic of material
C073S077000, C340S604000, C324S664000
Reexamination Certificate
active
06758084
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention is directed to the detection of dry/wet states of a thermistor bead, in general, and more particularly, to a method and apparatus for detecting a dry/wet state of a thermistor bead based on a difference in voltage across the thermistor bead in response to a difference in current conducted therethrough.
Sensing liquid level in a container, like fuel in an aircraft fuel tank, for example, has been performed using resistive bead type thermistors which have an inverse temperature coefficient. When current is conducted through a thermistor bead in air, i.e. a dry state, the bead increases in temperature and exhibits a low resistance to the current. In contrast, when the thermistor bead is submersed in a liquid, like jet fuel, for example, the bead is cooled and its resistance to current conducted therethrough is increased. Current vs. Voltage (I/V) characteristics of a typical thermistor bead at various temperatures is shown in the graph of FIG.
1
. It is readily observable from the graph of
FIG. 1
that the voltage across the thermistor becomes a viable measurement for detecting a wet vs. dry (wet/dry) state of the thermistor bead as the current conducted therethrough becomes greater than 45 milliamps. For example, by passing a constant current through a thermistor bead and lowering the bead from the top of the container, it may be determined at what level in the container the thermistor bead becomes submersed into the liquid by detecting the wet/dry state thereof based on the voltage across the thermistor bead.
Known interface circuitry which uses the above described method of detecting the dry/wet state of a thermistor bead is shown in the block diagram schematic of FIG.
2
. Referring to
FIG. 2
, a thermistor bead
10
is coupled between a constant current source
12
and a common or ground return. The constant current source
12
is powered by a power supply and is operative to conduct current through the thermistor bead
10
. The voltage across the thermistor bead
10
is sensed by one input of a comparator circuit
16
which is also powered by the supply
14
and common return. A reference voltage is generated by a circuit
18
which is powered by the supply
14
. The comparator circuit
16
compares the reference voltage which is coupled to another input thereof with the thermistor bead voltage. When the thermistor bead voltage exceeds the reference voltage, a wet bead state is effected at the output of the comparator
16
and when the thermistor bead voltage is less than the reference voltage, a dry bead state is effected at the output of the comparator
16
.
As noted above, to insure proper performance of the thermistor bead and detection circuitry using the above described method, the bead
10
should be biased with a constant current equal to or greater than 45 milliamps where voltage levels across the bead are dry/wet distinct for all practical temperature environments ( see the graphs of
FIG. 1
, for example). For level sensing of combustible liquids in a container with a thermistor bead, the bias current level of 45 milliamps may not be considered safe, and thus unacceptable. For example, the FAA has deemed this bias current level unacceptable in terms of the maximum allowable current that may enter an aircraft fuel tank. Only currents less than 30 milliamps with justification have been deemed acceptable for aircraft fuel tanks. Accordingly, a thermistor bead may not be an acceptable level measurement sensor for combustible liquids in all cases using interface circuitry implementing the above described traditional method.
SUMMARY OF THE INVENTION
In accordance with one aspect of the present invention, a method of detecting a dry/wet state of a thermistor bead comprises the steps of: conducting current through the thermistor bead selectively between first and second current levels; measuring a difference in voltage across the thermistor bead in response to the first and second current levels; and detecting the dry/wet state of the thermistor bead based on the measured difference in voltage. In one embodiment, the first and second current levels conducted through the thermistor bead are both less than thirty milliamps.
In accordance with another aspect of the present invention, apparatus for detecting a dry/wet state of a thermistor bead comprises: a first circuit coupled to the thermistor bead for conducting current through the thermistor bead selectively between first and second current levels; a second circuit coupled to the thermistor bead for measuring a difference in voltage across the thermistor bead in response to the first and second current levels; and a third circuit coupled to the second circuit for detecting the dry/wet state of the thermistor bead based on the measured difference in voltage.
REFERENCES:
patent: 4872340 (1989-10-01), de Yong
patent: 4901061 (1990-02-01), Twerdochlib
patent: 5534708 (1996-07-01), Ellinger et al.
patent: 5880480 (1999-03-01), Ellinger et al.
patent: 6208254 (2001-03-01), McQueen et al.
Calfee Halter & Griswold LLP
Frank Rodney
Percio David R.
Simmonds Precision Products Inc.
Williams Hezron
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