GB2178539A - Improvements in or relating to a humidity sensor - Google Patents
Improvements in or relating to a humidity sensor Download PDFInfo
- Publication number
- GB2178539A GB2178539A GB08516119A GB8516119A GB2178539A GB 2178539 A GB2178539 A GB 2178539A GB 08516119 A GB08516119 A GB 08516119A GB 8516119 A GB8516119 A GB 8516119A GB 2178539 A GB2178539 A GB 2178539A
- Authority
- GB
- United Kingdom
- Prior art keywords
- container
- humidity
- prongs
- relative humidity
- band
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
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-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
- G01N27/02—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating impedance
- G01N27/04—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating impedance by investigating resistance
- G01N27/12—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating impedance by investigating resistance of a solid body in dependence upon absorption of a fluid; of a solid body in dependence upon reaction with a fluid, for detecting components in the fluid
- G01N27/121—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating impedance by investigating resistance of a solid body in dependence upon absorption of a fluid; of a solid body in dependence upon reaction with a fluid, for detecting components in the fluid for determining moisture content, e.g. humidity, of the fluid
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- Chemical & Material Sciences (AREA)
- Chemical Kinetics & Catalysis (AREA)
- Electrochemistry (AREA)
- Physics & Mathematics (AREA)
- Health & Medical Sciences (AREA)
- Life Sciences & Earth Sciences (AREA)
- Analytical Chemistry (AREA)
- Biochemistry (AREA)
- General Health & Medical Sciences (AREA)
- General Physics & Mathematics (AREA)
- Immunology (AREA)
- Pathology (AREA)
- Investigating Or Analyzing Materials By The Use Of Fluid Adsorption Or Reactions (AREA)
- Investigating Or Analyzing Materials By The Use Of Electric Means (AREA)
Abstract
A sensor for measuring the relative humidity within a closed container electrically comprises a plastics plug 4 screwed into an aperture in the wall 1 of the container and projecting into the container. An electrical connector 10 sealingly fitted in the plug provides socket contacts on the outside of the container, and, on the inside of the container, two metal prongs 12, 14 between which a band 16 of salt- impregnated tape is stretched, The humidity within the container is measured by measuring the electrical resistance between the prongs 12, 14. A water vapour permeable membrane in the form of a cover 18 of silicone rubber is fitted over the inner end of plug 4 to enclose the prongs 12, 14 and band 16. The cover 18 serves to render the sensor relatively insensitive to short term variations in humidity which need not indicate exhaustion or failure of a desiccant used to keep the relative humidity within the container low, but may arise from a temperature charge, for example. <IMAGE>
Description
SPECIFICATION
Improvements in or relating to a humidity sensor
THE PRESENT INVENTION relates to a humidity sensor and more particularly to a humidity sensor adapted to measure the relative humidity within a closed space, such as the interior of a sealed container which contains electronic or electro-mechanical equipment, or a transit or storage container for such equipment.
Such a container is usually provided with an appropriate desiccator to maintain the desired low internal relative humidity within the container. The humidity sensor is provided to indicate when the internal relative humidity reaches a level at which it is necessary to replace the desiccator, or the charge of desiccant within the desiccator, this level being generally taken to be 50% r.h.
One prior proposed type of humidity sensor utilises a humidity sensing element which is made from a sodium chloride impregnated cotton material. A washer of the material is mounted on a supporting plug which is made of nylon or other suitable plastics material.
Two conductive washers are located respectively on either side of the sodium chloride impregnated cotton washer, so that the cotton washer separates the conductive washers. A portion of the plug is provided with threading to enable the plug to be screwed into an appropriate bore provided in the wall of a container containing electronic or other equipment so that the sodium chloride impregnated cotton washer is exposed to the atmosphere within the container. On the external face of the plug, that is to say the face of the plug that is on the exterior of the container, there is a weatherproof two-pin socket, the contacts of which are electrically connected to the two conductive washers.
The sodium chloride impregnated cotton washer acts as the humidity sensing element, since the sodium chloride is hygroscopic, and its electrical resistance depends upon its moisture content which in turn varies with the relative humidity of the surrounding air. Thus the relative humidity within the container can be determined by applying an appropriate voltage to the contacts of the two pin socket and measuring the electrical resistance of the sodium chloride impregnated cotton washer.
One feature of this prior proposed device is that the sodium chloride impregnated washer may have its resistance characteristics permanently changed by exposure to a relative humidity in excess of 75%, so that if the relative humidity then drops, the sensing element will not indicate the true relative humidity.
Thus after the relative humidity within the container has reached a level of 75% or more, the calibration of the cotton washer will have been changed so that the sensor will indicate that relative humidity is high when in fact it is low. Whilst this is a desirable fail-safe characteristic, it would clearly be preferable for the 'fail' condition represented by such a calibration change to be avoidable, particularly in certain fairly common situations as indicated below.
A relative humidity as high at 75% can occur within a container enclosing electronic equipment of the type discussed above as a result of a sudden drop in temperature. Even if the temperature remains at the lower level reached in such a drop for a longer time, the relative humidity within the container may remain high for only a relatively short time, for example for only a few hours, since the desiccant within the container will act to reduce the high relative humidity.This short exposure to high relative humidity may not be damaging to the electronic equipment within the container, but if the sensing element of the type described above undergoes a permanent change of the kind referred to (as it will do as a result of even a very short exposure to a relative humidity of 75% or more) then when a test voltage is applied to the sensor a high current will flow, indicating a high relative humidity (even if the interior of the container then has a low relative humidity) and thus the electronic equipment within the container may well be needlessly inspected, or the desiccant will be needlessly replaced.
The present invention seeks to provide a humidity sensor which can be utilised to sense the humidity within a container containing electronic equipment, or any other container or enclosure, the sensor being adapted so that the sensing element will only respond when internal relative humidity is in excess of predetermined figure for a relatively long period of time, the sensor thus not responding to short duration changes in internal relative humidity due to temperature variations.
According to this invention there is provided a sensor for measuring the relative humidity within a container, comprising an electrically insulating mounting member adapted to be at least partially mounted within the container, said member supporting a sensing element the electrical resistance of which is indicative of the relative humidity to which the sensing element is exposed, the sensor including electrically conductive elements spaced apart from one another and in contact with said sensing element, insulating means electrically insulating said conductive elements and said terminals from one another, said electrically conductive elements being electrically connected with respective electrical terminals by way of which a test voltage may be applied to said conductive elements, the sensor including a cover member formed from a material which is permeable to water vapour and which confines said conductive elements and said sensing element within a closed space to form a permeable cover which separates said element from the interior of the container.
Preferably said mounting member includes a body of insulating material and said conductive elements comprise two resilient metal prongs having respective mounting portions fixed in said body of insulating material and respective leg portions, extending in substantially the same direction from said body of insulating material and terminating in adjoining free ends, and wherein said sensing element is in the form of a band of permeable absorbent material formed as a closed loop which encompasses the free ends of said prongs, the free ends of said prongs being held by said band in a condition in which they are sprung towards one another somewhat from their unstressed state, whereby the band is held in tension between said prongs to hold the band on the prongs and ensure reliable contact between the prongs and the band.
Preferably said cover member is formed of silicone rubber.
Conveniently said cover member is provided with an internally directed bead which is engaged with a groove formed in said member.
This invention also relates to a humidity indicator, comprising a humidity sensor as described above, together with means for applying a test voltage to said electrical leads of the sensor and means for measuring electrical resistance. Preferably said means for measuring electrical resistance determine whether the resistance is such that the sensed relative humidity is, or is not, in excess of predetermined critical relative humidity. The indicator may comprise a plurality of said sensors and means for connecting any selected sensor to said voltage applying means and said current determining means.
In order that the invention may be more readily understood and so that further features thereof may be appreciated an embodiment of the invention will now be described by way of example with reference to the accompanying drawings, in which:
Figure 1 is a cross sectional view of a humidity sensor device embodying the invention mounted within the wall of a container which contains electronic equipment, and
Figure 2 is a block diagram of a monitoring installation utilising sensors as shown in Fig.
1.
Referring now to the drawing, the wall 1 of the container is provided with an internally screw-threaded bore 2 into which is screwed a mounting member 3 which includes a tubular outer body 5 having a generally cylindrical externally threaded intermediate part 5 for engagement in the correspondingly screwthreaded bore 2, an inner end part 6, of reduced diameter, which projects into the container, and an outer end part in the form of a radially outwardly directed flange 7 which abuts the exterior surface of the wall 1. A resilient O-ring 8 may be located between the flange and the wall 1.
A central bore 9 extends through the body 4 and fitted sealingly within this central bore, is a standard electrical connector 10 which is generally cylindrical in form and includes an inner portion which extends part of the way along the bore 9, an outer portion 11 which projects axially outwardly from the body 4 and a flange 12 which engages the outer face of the flange 7. The socket contains two electrical terminal pins which extend outwardly within said socket, from the base of said socket. The socket is intended to receive a complementary plug (not shown) forming part of an electronic or electrical testing facility and including conductors for making electrical contact with said terminal pins. The outer portion 11 may be provided with a dust cap (not shown) which can be removed to allow the appropriate plug to be inserted in the socket.
The two terminal pins in the socket are connected to the respective resilient metal prongs 12, 14 which pass through the axial bore 9 to project slightly from the end part 6, into the interior of the container. The two prongs 12,
14 are made of corrosion resistant metal, for example, of Inconel Alloy 600. Passing in a closed loop around the free inner ends of both prongs 12, 14 is a sensing element 16 in the form of a flexible endless band of highly absorbent material such as cotton tape or paper which is impregnated with sodium chloride. The prongs 12, 14 are displaced resiliently towards one another by the band 16 which is thus held in tension between the prongs. The band 16 is capable of sustaining the light tensile force exerted by the prongs
12, 14 tending to spring apart without undergoing extension or damage, even when the band is damp.
Adjacent the free end of the inner end part 6 is a peripheral groove 17 formed around end part 6. Fitted over the end part 6 and projecting inwardly therefrom is a top hat-like cover 18 which is formed of silicone rubber.
The cover 18 has a wall thickness of approximately 0.75 mm and has an inwardly directed bead 19 around its rim that seats within the groove 17 so that the cover 18 may be snapped into position over the sensing element 16. The cover defines, with the body 4, an internal space within which the sensing element 16 is supported by the prongs 12, 14.
The cover acts as a permeable membrane between the sensing element 16 and the interior of the container. Silicone rubber is the preferred material for the cover, because of its high permeability to water vapour, Also the material is elastic, easy to mould, resistant to long term ageing and is unaffected by a wide range of contaminants. Hence, when the interior of the container defined by the wall 1 is subjected to a particular relative humidity for a lengthy period of time, moisture will penetrate the silicone rubber cover 18 and the relative humidity within the space defined by the cover 18 will become equal to the relative humidity within the container.When an appropriate voltage is applied to the prongs 12, 14 an appropriate current will flow indicative of the resistance of the sensing element 16 and thus of the humidity within the space defined by the cover 18 which is substantially equal to the humidity within the container. The resistance of the sensing element 16 can be determined with a suitable meter.
It will be appreciated that under stable temperature conditions, such as in storage, the relative humidity within the container will only change slowly, and thus the relative humidity indicated by applying a voltage to the prongs 12 and 14 and measuring resistance of the sensing element 16 will be indicative of a relative humidity which is substantially equal to the humidity within the container. However, if the humidity within the container fluctuates rapidly due to changes of temperature over a short period of time, these rapid fluctuations in relative humidity will not be detected by the sensor. As explained previously such rapid fluctuations in relative humidity can occur despite the provision of a desiccator even if the desiccator is not exhausted.It is to be appreciated that such rapid fluctuations in relative humidity, if only of a short duration, are not damaging to the electronic equipment within the container
It will be appreciated that in utilising a sensor in accordance with the invention, if the humidity within the container exceeds 75% for a short period of time, the relative humidity within the cover 18 will not necessarily rise as far as 75%, and thus the salt impregnated band 16 will not be permanently damaged.
However, if the relative humidity within the container is in excess of 75% for a lengthy period of time, then sufficient moisture will migrate through the silicone rubber forming the cover 18 so that the relative humidity within the chamber defined by the cover 18 will also be in excess of 75%. This high relative humidity will damage sensing element 16 so that it will subsequently indicate a high relative humidity within the container, even though no such high relative humidity may exist. Thus, effectively, the sensor will fail in a "safe" condition.
The thickness of the silicone rubber cover 18 will determine the duration of the period of high relative humidity within the container necessary to cause damage to the sensing element. However, in preferred embodiments of the invention the sensor will not significantly respond to short term relative humidity variations within the container but will respond to longer term variations, say having a duration of seven or eight hours or more.
The silicone rubber cover 18 thus acts to 'average out' the relative humidity to be sensed over a relatively long period and incorporates in the sensor a time-lag which is larger than the period of the short-term variations in relative humidity which are liable to occur.
Whilst the silicone rubber cover is primarily provided to ensure a slow response to conditions of high relative humidity within the container, the cover also protects the sensing element, i.e. the band 16, from contamination which can lead to calibration drift resulting in a progressive loss of accuracy. The cover also prevents the escape of sodium chloride from the band 16 into the housing, eliminating the risk of salt corrosion of components within the container. Also the cover protects the sensing element from any short term high relative humidity within the housing which might otherwise damage the sensing element.
Referring now to Fig. 2 of the accompanying drawings, a plurality of containers 20 are illustrated, each container being provided with a sensor 21, each such sensor being as illustrated in Fig. 1. The sensors are connected by appropriate leads 22 to a multi-position switch 23. The multi-position switch is connected to a control and display box 24. The multi-position switch can be actuated to-connect any selected sensor 21 to the control box 24.
When a sensor 21 is connected to the control box 24 an appropriate voltage is applied to the two prongs 12, 14 of the respective sensor 21, and its resistance is measured. From the resistance it is determined whether the measured relative humidity is above or below a predetermined critical level. If the relative humidity is below the predetermined critical level, indicative of the fact that the desiccator is operating satisfactorily, or of the fact that there is only a low relative humidity within the respective container 20, then a green light emitting diode 25 on the control and display box 24 is illuminated. However, if the sensed relative humidity is above the predetermined critical level, then a red light emitting diode 26 is illuminated, this being indicative of the fact that the relative humidity within the container is above the maximum acceptable level. This may be due to the fact that the desiccator is exhausted.
Whilst the apparatus may, as described above, simply operate to indicate whether or not the sensed relative humidity is or is not above a predetermined threshold, the sensor described may be used to provide an indication of the actual value of the relative humidity over a continuous range, in accordance with a calibration of the sensor in terms of resistance vs. relative humidity, (this range, being, of course, below the value of relative humidity at which the sensing element 16 is damaged).
Accordingly the apparatus may be used to obtain a measurement of relative humidity.
Claims (14)
1. A sensor for measuring the relative humidity within a container, comprising an electrically insulating mounting member adapted to be at least partially mounted within the container, said member supporting a sensing element the electrical resistance of which is indicative of the relative humidity to which the sensing element is exposed, the sensor including electrically conductive elements spaced apart from one another and in contact with said sensing element, insulating means electrically insulating said conductive elements and said terminals from one another, said electrically conductive elements being electrically connected with respective electrical terminals by way of which a test voltage may be applied to said conductive elements, the sensor including a cover member formed from a material which is permeable to water vapour and which confines said conductive elements and said sensing element within a closed space to form a permeable cover which separates said element from the interior of the container.
2. A humidity sensor according to claim 1 wherein said mounting member includes a body of insulating material and said conductive elements comprise two resilient metal prongs having respective mounting portions fixed in said body of insulating material and respective leg portions, extending in substantially the same direction from said body of insulating material and terminating in adjoining free ends, and wherein said sensing element is in the form of a band of absorbent material formed as a closed loop which encompasses the free ends of said prongs, the free ends of said prongs being held by said band in a condition in which they are sprung towards one another somewhat from their unstressed state, whereby the band is held in tension between said prongs to hold the band on the prongs and ensure reliable contact between the prongs and the band.
3. A humidity sensor according to claim 2 wherein said band is of woven textile material, non-woven fabric or paper.
4. A humidity sensor according to any of claims 1 to 3 wherein said mounting member is provided with an externally screw threaded portion to enable the member to be screw threadedly mounted in a bore formed in the wall of a container.
5. A humidity sensor according to any preceding claim wherein said mounting member is provided with a two pin socket formed thereon.
6. A humidity sensor according to any one of the preceding claims wherein the sensing element is formed from a cotton material impregnated with sodium chloride.
7. A humidity sensor according to any one of the preceding claims wherein said cover is formed of silicone rubber.
8. A humidity sensor according to any one of the preceding claims wherein said cover is provided with an internally directed rib which is engaged with a groove formed in said member.
9. A humidity indicator, comprising a humidity sensor according to any one of the preceding claims, together with mens for applying a test voltage said electrical terminals of the sensor and means for measuring the electrical resistance.
10. A humidity indicator according to claim 9 wherein said means for measuring resistance indicate a level of relative humidity.
11. A humidity indicator according to claim 9 or 10 comprising a plurality of said sensors and means for connecting any selected sensor to said voltage applying means and said current determining means.
12. A humidity sensor substantially as herein described with reference to and as shown shown in Fig. 1 of the accompanying drawings.
13. A humidity indicator substantially as herein described with reference to and as shown in the accompanying drawings.
14. Any novel feature or combination of features disclosed herein.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB08516119A GB2178539B (en) | 1985-06-26 | 1985-06-26 | Improvements in or relating to a humidity sensor |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB08516119A GB2178539B (en) | 1985-06-26 | 1985-06-26 | Improvements in or relating to a humidity sensor |
Publications (3)
| Publication Number | Publication Date |
|---|---|
| GB8516119D0 GB8516119D0 (en) | 1985-07-31 |
| GB2178539A true GB2178539A (en) | 1987-02-11 |
| GB2178539B GB2178539B (en) | 1989-02-08 |
Family
ID=10581331
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| GB08516119A Expired GB2178539B (en) | 1985-06-26 | 1985-06-26 | Improvements in or relating to a humidity sensor |
Country Status (1)
| Country | Link |
|---|---|
| GB (1) | GB2178539B (en) |
Cited By (6)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US5189902A (en) * | 1990-01-08 | 1993-03-02 | E. G. & G. | Humidity sensor, and a measurement installation including a plurality of such sensors |
| WO2002048702A2 (en) * | 2000-12-15 | 2002-06-20 | Sensirion Ag | Material sensor with protective layer |
| WO2014174015A1 (en) * | 2013-04-24 | 2014-10-30 | Zim Plant Technology Gmbh | Sensor system for the continuous real-time detection of the water content in moisture-carrying layers that are subject to drying out |
| US10161895B2 (en) | 2014-12-23 | 2018-12-25 | 3M Innovative Properties Company | Electronic moisture sensor |
| US11079340B2 (en) | 2014-12-23 | 2021-08-03 | 3M Innovative Properties Company | Methods of monitoring wetness utilizing a resonant circuit |
| WO2022155562A1 (en) * | 2021-01-15 | 2022-07-21 | Dwyer Instruments Llc | Humidity sensor with improved contaminant sealing |
Citations (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| GB2163970A (en) * | 1984-07-20 | 1986-03-12 | Nippon Mining Co | Method of manufacturing a humidity-sensing element |
-
1985
- 1985-06-26 GB GB08516119A patent/GB2178539B/en not_active Expired
Patent Citations (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| GB2163970A (en) * | 1984-07-20 | 1986-03-12 | Nippon Mining Co | Method of manufacturing a humidity-sensing element |
Cited By (7)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US5189902A (en) * | 1990-01-08 | 1993-03-02 | E. G. & G. | Humidity sensor, and a measurement installation including a plurality of such sensors |
| WO2002048702A2 (en) * | 2000-12-15 | 2002-06-20 | Sensirion Ag | Material sensor with protective layer |
| WO2002048702A3 (en) * | 2000-12-15 | 2002-10-17 | Sensirion Ag | Material sensor with protective layer |
| WO2014174015A1 (en) * | 2013-04-24 | 2014-10-30 | Zim Plant Technology Gmbh | Sensor system for the continuous real-time detection of the water content in moisture-carrying layers that are subject to drying out |
| US10161895B2 (en) | 2014-12-23 | 2018-12-25 | 3M Innovative Properties Company | Electronic moisture sensor |
| US11079340B2 (en) | 2014-12-23 | 2021-08-03 | 3M Innovative Properties Company | Methods of monitoring wetness utilizing a resonant circuit |
| WO2022155562A1 (en) * | 2021-01-15 | 2022-07-21 | Dwyer Instruments Llc | Humidity sensor with improved contaminant sealing |
Also Published As
| Publication number | Publication date |
|---|---|
| GB8516119D0 (en) | 1985-07-31 |
| GB2178539B (en) | 1989-02-08 |
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| PCNP | Patent ceased through non-payment of renewal fee |