Humidity sensor alarm unit

• US 4,092,635 A
• Filed: 09/20/1976
• Issued: 05/30/1978
• Est. Priority Date: 09/20/1976
• Status: Expired due to Term

• Current Assignee: Hygrodynamics Incorporated
• Original Assignee: Baxter Travenol Laboratories Incorporated (Baxter International Inc.)
• Inventors: Warner, Roger M.
• Primary Examiner(s): Habecker, Thomas B.
• Application Number:
  US05/724,811
• Time in Patent Office: 617 Days
• Field of Search: 340/235, 340/409
• US Class Current: 340/507
• CPC Class Codes: G01N 27/121 for determining moisture content, e.g. humidity, of the fluid moisture content of the tested material G01N27/048
• CPC Class Codes: G05D 22/02 characterised by the use of electric means
• CPC Class Codes: G08B 21/20 responsive to moisture

Abstract

An alarm unit for a humidity sensor activates an alarm circuit when the relative humidity rises above a certain value or under certain malfunction conditions such as a missing sensor or a cut cable, includes a first circuit for establishing a reference signal, and a second circuit adapted to be connected to a humidity sensor for responding thereto and for generating a humidity out-of-limits signal. A third circuit responds to the reference signal and to the humidity out-of-limits signal to generate an alarm signal. The third circuit includes a level detector, and the first circuit comprises an impedance device which cooperates with the second circuit which in turn connects the sensor to the input of the level detector, the sensor being a variable impedance device.

Claims

1. A fail-safe humidity sensing alarm apparatus, which comprises:

an electric humidity sensor for detecting changes in humidity and having a resistance which varies in response to humidity variations;

a level detector for detecting a substantially null condition and for providing a first signal when said substantially null condition is detected and a second signal when other than a substantially null condition is detected;

a bi-stable device having a first state in response to said first signal and a second state in response to said second signal;

alarm means for providing an alarm when said bi-stable device is in said second state; and

circuit means for providing to said level detector (a) a substantially null condition when said humidity sensor detects a relative humidity which is below a predetermined threshold, (b) other than a substantially null condition when said humidity sensor detects a relative humidity which exceeds the predetermined threshold, (c) other than a substantially null condition when said humidity sensor is short-circuited, and (d) other than a substantially null condition when said humidity sensor is open-circuited.

2. A humidity sensing alarm apparatus as described in claim 1, said circuit means including first and second secondary windings of a transformer connected with the humidity sensor and an impedance device to form a bridge circuit with the level detector.

3. A humidity sensing alarm apparatus as described in claim 2, said circuit means further including a shunting resistance connected across said humidity sensor.

4. A humidity sensing alarm apparatus as described in claim 1, said bi-stable device comprising an electromagnetic relay having a plurality of relay contacts, said relay contacts being operative to close an alarm circuit when said relay is in said second state and to open said alarm circuit when said relay is in said first state.