Fluid level sensor having capacitive sensor

• US 5,182,545 A
• Filed: 04/24/1991
• Issued: 01/26/1993
• Est. Priority Date: 01/06/1989
• Status: Expired due to Fees

• Current Assignee: Standex Electronics, Inc. (Standex International Corporation)
• Original Assignee: Standex International Corporation
• Inventors: Goekler, Lewis E., Chamberlin, Edward R.
• Primary Examiner(s): Ng, Jin F.
• Assistant Examiner(s): Oda, Christine K.
• Application Number:
  US07/690,502
• Time in Patent Office: 643 Days
• Field of Search: 340/620, 340/450, 340/618, 73/304 C, 324/690, 324/669
• US Class Current: 340/620
• CPC Class Codes: G01F 23/265 for discrete levels
• CPC Class Codes: G01F 23/266 measuring circuits therefor
• CPC Class Codes: G01F 23/268 mounting arrangements of probes

Abstract

A capacitive liquid level sensor utilizes capacitive principles to sense the level of a liquid contained within a receptacle. As a liquid rises and falls in the container, the dielectric effect of the liquid changes the effective capacitance of a sensing capacitor which is detected by electronic circuitry coupled to the sensor. One plate of the sensor capacitor is a probe disposed within a receptacle while a grounded conductive portion of the receptacle is a second plate of the capacitor. The liquid within the receptacle acts as a dielectric between the plates.

Claims

1. A fluid level detector for detecting the level of a liquid contained within a receptacle having an electrically conductive portion, comprising:

a probe mounted in said receptacle and electrically insulated from said receptacle, said probe acting as a first plate of a capacitor and being cooperative with said electrically conductive portion of said receptacle acting as a second plate of said capacitor, the liquid in the receptacle acting as a dielectric between said first plate and said second plate of said capacitor to produce a sensor capacitor having a capacitance which varies in magnitude in accordance with the level of liquid contained within the receptacle;

detecting circuitry coupled to said probe and responsive to a difference between a voltage decay signal derived from said sensor capacitor and a reference voltage decay signal derived from a reference capacitor to provide an output signal when a predetermined voltage difference is detected therebetween; and

indicating means coupled to said detection circuitry and responsive to said predetermined voltage difference as detected by said detection circuitry to provide an indication of the level of liquid contained within the receptacle.

2. The fluid level detector of claim 1, wherein said indicating means includes an output whereat an output voltage is produced which is substantially linearly proportional to the amount of fluid in said receptacle.

3. The fluid level detector of claim 1, wherein said indicating means includes an output functioning as an ON/OFF switch that is ON when the level of fluid in said receptacle exceeds a redetermined level.

4. The fluid level detector of claim 3, wherein said predetermined level is selectable.

5. The fluid level detector of claim 1, wherein said indicating means includes a first output whereat an output voltage is produced, which is substantially linearly proportional to the amount of fluid in said receptacle, an a second output functioning as an ON/OFF switch that is ON when the level of fluid is said receptacle exceeds a predetermined level.