GB2161277A - Humidity sensor - Google Patents

Abstract

A humidity sensor comprises an elongate wooden sensing element 16 with the grain arranged transverse to the longitudinal axis. The element is surrounded by an air permeable reinforcing member 17 and is mounted in a housing 10, which is exposed via openings to the external atmosphere, for longitudinal expansion and contraction. A switch 18 is operable in response to longitudinal expansion of the sensing element to activate a warning device (eg for protecting electronic equipment enclosures). An adjuster 26, with an associated indicator 27, 28 movable along a scale (30, fig 2) is provided to set the sensor, so as to operate the switch at a preselected humidity level. The material of the sensing element may be yellow or white pine, eg Quebec Yellow Pine. <IMAGE>

Description

SPECIFICATION Humidity sensor This invention relates to a humidity sensor.

There is a need for a humidity sensor which has no driven parts and which can be used for example to detect an increase in humidity resulting say from the failure of a seal of a sealed enclosure such as of the type containing electronic equip ment and which can be used to initialise a warning device.

According to the invention, there is provided a humidity sensor comprising an elongate wooden sensing element in which the grain in the wood extends transverse to the longitudinal extent of the sensing element, an air permeable reinforcing member closely surrounding the sensing element, an elongate housing in which the sensing element is mounted for longitudinal expansion and contraction and which is provided with openings so as to expose the sensing element to the atmosphere external to the sensor, switch means mounted at one end of the housing and operable in response to longitudinal expansion or contraction of the sensing element, adjustment means at the other end of the housing for adjusting the position of that end of the sensing element remote from the switch means to set the sensor so as to operate the switch at or substantially at a pre-selected humidity level, and indicator means movable in response to movement of the adjustment means for providing an indication of the humitidy level at which the switch will operate.

Preferred and/or optional features of the invention are set forth in claims 2-8.

The invention will now be more particularly described with reference to the accompanying drawings, in which: Figure 1 is a vertical section through one em bodiment of a humidity sensor according to the present invention, and Figure 2 is a view of the housing of the sensor of Figure 1.

Referring to the drawings, the humidity sensor shown therein has a three part plastics housing 10 consisting of an open ended tubular body 11 provided with a plurality of openings 12, an adjuster housing 13, with cover, snap-fittably mounted in one end of the body 11, and a switch housing 14, having a hinged cover 15, snap-fittably engaged with the other end of the body 11.

The sensor also has an elongate sensing element 16 and is conveniently of non-circular and typically square cross-section. The sensing element is formed of soft wood and its grain is arranged to extend cross-wise or transverse to the longitudinal extend of the sensing element. With its grain orientated as aforesaid the sensing element 16 will expand considerably more, and as has been found, as much as five times more than a sensing element in which the grain extends in a direction parallel to the longitudinal extent of the sensing element. Moreover, yellow or white pine (preferably grade 1 yellow or white pine) and especially Quebec Yellow Pine (Pinus Strobos) has been found to be particularly suitable as the material of the sensing element because of its lack of any significant warping and its ability to respond rapidly to changes in humidity over a large humidity range.Hardwoods are slow to respond and while very soft woods respond well they lack reliability when subjected to end pressure. Quebec Yellow Pine has been found to behave better than the coarser grained pines, and being selected primarily for pattern making is less prone to distortion.

The sensing element is closely surrounded by a perforated reinforcing sleeve 17 of non-ferrous metal to support the sensing element 16 against bending and breaking. However, the fit between the element 16 and the sleeve 17 is such as to allow the element 16 to be slid into the sleeve 17.

A sealed snap-action pressure operated on/off electrical micro-switch 18 is contained within the switch housing 14 for operation by a switch activator 19 secured to the lower end (as viewed in Figure 1) of the sensing element 16 and guided for longitudinal movement by a bush 20 fitted in an aperture in a part of the switch housing 14 which snap-fittably engages with the tubular body 11.

The lower end of the sensing element 16 is supported against rotation relative to the housing 10 by a boss 21 which projects from the switch housing 14, has a cross-sectional shape corresponding to that of the sensing element 16, and surrounds the lowermost end of the sensing element 16 and perforated sleeve 17.

The adjuster housing 13 is in the form of a hollow plug and the upper end (as viewed in Figure 1) of the perforated sleeve 17 is rigidly attached to an externally threaded spigot 22 which projects through an aperture in lower wall of the housing 13. A stainless steel compression spring 23 surrounds the spigot 22 and is interposed between a flange 24 of the spigot and the underside of the lower wall of the housing 13. An internally splined externally threaded sleeve 25 is contained with the housing 13 and interposed between its upper and lower ends. An externally splined adjuster 26 is mounted within the sleeve 25 but projects through the upper wall of the housing 13 for engagement by a screw-driver or the like. The adjuster 26 has an internally threaded blind bore in its lower end, which engages with the threaded spigot 22.Hence rotation of the adjuster will cause longitudinal movement of the sensing element 16 towards or away from the micro-switch 18.

An indicator has a tubular portion 27, which is internally threaded and is mounted on the sleeve 25, and a pointer 28, which extends radially outwards from the portion 25 through aligned elongate slots 29 provided in the peripheral wall of the housing 13 and the upper end of the tubular body 11. A graduated scale 30 (see Figure 2) is marked on the tubular body 11 alongside the slots 29 and may for example have a range of between 0 and 100- relative humidity. As the adjuster 26 is rotated to move the sensing element 16 longitudinally, the sleeve 25 also rotates, but as the indicator is prevented from rotating it will move along the sleeve 25 due to interengagement of their respective threads and the pointer will move along the scale 30.

In operation, the humidity sensor is installed in an enclosure in which humidity control is important so as to protect for example electronic equipment and the micro-switch 18 is connected to a warning device (not shown), which may be either optical or audible, or even both. As the humitidy in the enclosure increases the sensing element 16 will expand and if this expansion extends the length of the actuator beyond a predetermined length previously set by the adjuster 26 the actuator 19 will operate the micro-switch 18 to close the circuit of the warning device.

The spring 23 will compress to take up excessive expansion of the sensing element 16 which could otherwise damage the micro-switch 18 or the sensing element 16.

In the above described arrangement it is to be noted that the sensing element is only under stress when the humidity exceeds an acceptible level set by the adjuster and read off by the user against the scale 30.

The humidity sensor could also be arranged to give an indication of a reducing humidity level.

This could be done by using a switch which acts to open a circuit when pressure is applied to it and by setting the sensing element so that at acceptible humidity levels the activator presses on the switch contact.

Claims (9)

1. A humidity sensor comprising an elongate wooden sensing element in which the grain in the wood extends transverse to the longitudinal extent of the sensing element, an air permeable reinforcing member closely surrounding the sensing element, an elongate housing in which the sensing element is mounted for longitudinal expansion and contraction and which is provided with openings so as to expose the sensing element to the atmosphere external to the sensor, switch means mounted at one end of the housing and operable in response to longitudinal expansion or contraction of the sensing element, adjustment means at the other end of the housing for adjusting the position of that end of the sensing element remote from the switch means to set the sensor so as to operate the switch at or substantially at a pre-selected humidity level, and indicator means movable in response to movement of the adjustment means for providing an indication of the humidity level at which the switch will operate.

2. The humidity sensor of Claim 1, wherein the sensing element is supported against rotation relative to the housing and the adjustment means comprises a rotatable adjuster in screw-threaded engagement with a part fixed to the sensing element such that rotation of the adjuster causes longitudinal movement of the sensing element towards or away from the switch means, the indicating means comprising a non-rotatably mounted indicator movable lengthwise across a scale on the housing, the indicator being in screw-threaded engagement with the adjuster or a part rotatable therewith for movement across the scale as the adjuster is rotated and the sensing element is moved.

3. The humidity sensor of Claim 1 or Claim 2, wherein the switch is a snap action switch.

4. The humidity sensor of any one of the preceding claims, wherein the sensing element is of soft wood.

5. The humidity sensor of Claim 4, wherein the sensing element is of yellow or white pine.

6. The humidity sensor of Claim 5, wherein the sensing element is of Quebec Yellow Pine (Pinus Strobos).

7. The humidity sensor of any one of the preceding claims, wherein the reinforcing member is a perforated non-ferrous metal sleeve.

8. The humidity sensor of any one of the preceding claims, in combination with a warning device operable by the switch means of the sensor.

9. A humidity sensor substantially as hereinbefore described with reference to, and as shown in, the accompanying drawing.