Analog liquid level sensor

Details Analog liquid level sensor Analog liquid level sensor

1998-10-01

2001-03-20

Williams, Hezron (Department: 2856)

Measuring and testing

Liquid level or depth gauge

Thermal type

C073S292000

Reexamination Certificate

active

06202486

ABSTRACT:

TECHNICAL FIELD
The present invention relates generally to the field of liquid level sensors, and, more particularly, to an improved sensor for determining, on an analog basis, the location of the surface of liquid in a container.
BACKGROUND ART
It is sometimes desirable to know the specific level of a liquid in a tank or container. Many different techniques have been developed to accomplish this objective. Some are mechanical, as by determining the level of the liquid from a wetted dip stick; some are optical; and still others are electrical.
U.S. Pat. No. 3,898,638 discloses a differential temperature sensor in which a heated electrode
16
is located at different distances from two sensing probes. If fluid flows through the conduit in which the probes are mounted, the two sensors do not sense an appreciable temperature differential therebetween. On the other hand, if the flow through the conduit stops, then the sensing probes will discern a differential change in the electrical resistivity of the sensing probes.
U.S. Pat. No. 4,449,403 discloses a guide tube-inserted liquid level sensor. This device as a chain of series-connected resistance temperature devices (“RTDs”) to measure the temperature gradient within a guide tube. As shown in
FIG. 2
of this reference, there are two RTDs, indicated at
29
and
38
. One RTD is heated by a proximate heater
30
. The sensors are wetted. Boiling and void conditions are sensed by the difference in resistance of the RTDs.
U.S. Pat. No. 4,532,799 discloses a liquid level sensor having a probe that is adapted to be inserted into a container having a liquid therein. A resistance wire is helically wound on a core within a heating element. More heat is carried away by the vapor than by the liquid. Hence, the electrical resistance of the entire length of wire (i.e., the resistance of the portion in the vapor and the resistance of the portion in the liquid) will provide an analog signal indicative of the change in liquid level. However, upon information and belief, it is critically important to know the temperature of the air and the temperature of the liquid in order to determine the particular level of liquid. In other words, the device shown in the '799 patent appears to be useful in showing a change of liquid level, but not indicating the absolute analog value of the liquid level itself.
Finally, U.S. Pat. No. 5,210,769 discloses a liquid level measuring system for a vehicle. Basically, the fluid levels are determined as a function of the electrical resistance of a positive temperature coefficient conductor, part of which is wetted and part of which is not. This patent discloses that such sensors may be used to determine the level of engine oil, coolant, windshield washer fluid, transmission fluid, and fluid in a differential.
Accordingly, it would be generally desirable to provide an improved resistance-type sensor that can be used to determine the absolute level, as opposed to a change in level, of a liquid within a container on an analog basis.
DISCLOSURE OF THE INVENTION
With parenthetical reference to the corresponding parts, portions or surfaces of the disclosed embodiment, merely for purposes of illustration and not be way of limitation, the present invention broadly provides an improved sensor (
20
) for determining, by electrical means, the analog location of the surface of a liquid in a container.
The improved sensor broadly includes: a probe (
21
) inserted into the container, the probe having one portion (
21
a
) arranged above the surface of the liquid and having another portion (
21
w
) submerged in the liquid; a first temperature sensor (TC
1
), operatively arranged within the probe to sense the temperature above the liquid surface; a second temperature sensor (TC
2
) operatively arranged within the probe to sense the temperature of the liquid at a point beneath the surface; a resistance wire (
53
) operatively arranged within the probe; a current source (
60
) operatively arranged to apply a current to the ends of the wire; and detector means (
61
) for measuring the resistance of the wire according to the equation:
R
T
=ƒ[k
a
T
a
Hh+k
w
T
w
H
(1
−h
)]
where R
T
is the resistance of the wire, ƒ is the symbol for “a function of”, k
a
is a constant for the probe first portion, T
a
is the temperature sensed by the first temperature sensor, k
w
is a constant for the probe second portion, T
w
is the temperature sensed by the second temperature sensor, H is the vertical distance between the first and second temperature sensors (i.e., the maximum range of measurement), h is the distance from the first temperature sensor to the liquid surface, V is the voltage sensed by said voltmeter, and i is the current in said wire—, change “whereby” to—such that—, and insert immediately before the period—when the liquid level is between said first and second sensors, but will be indicated by a first constant value if the second temperature sensor is above said liquid surface and will be indicated by a second constant value if the first temperature sensor is arranged beneath said liquid surface; whereby the liquid level may be indicated by the value of h.
In the preferred embodiment, the current source is arranged to apply a constant current to the ends of the wire, and each of the temperature sensors is a thermocouple. The probe may be an elongated tube formed of stainless steel and suitably closed at both ends. This probe may extend upwardly into the container by penetrating the bottom thereof, or may hang or depend into the container from above. The improved sensor may further include a third thermocouple (TC
3
) arranged within the probe above the liquid surface and a fourth thermocouple (TC
4
) arranged within the probe below the liquid surface, and a pump (
63
) associated with a source of liquid, wherein the third and fourth temperature sensors are arranged between the first and second temperature sensors, and wherein the third and fourth thermocouples are arranged to control the operation of the pump or valve to maintain the liquid level between said third and fourth thermocouples.
Accordingly, the general object of the present invention is to provide an improved sensor for determining the location of the surface of a liquid in a container.
Another object is to provide an improved liquid level sensor which determines by electrical means the analog value of the liquid level within a container.
Another object is to provide an improved liquid level sensor that is capable of determining the level of a liquid surface from within a protected environment.
These and other objects and advantages will become apparent from the foregoing and ongoing written specification, the drawings, and the appended claims.


REFERENCES:
patent: 3279252 (1966-10-01), Barlow
patent: 3653262 (1972-04-01), Ehrenfried et al.
patent: 4590797 (1986-05-01), Beaubatie et al.
patent: 4805454 (1989-02-01), Le Vert
patent: 622247 (1927-05-01), None
patent: 271708 (1927-06-01), None
patent: 158522 (1982-09-01), None
patent: 402102419 (1990-04-01), None
patent: 1672228 (1991-08-01), None

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