GB2216667A - Relative humidity measuring device - Google Patents

Abstract

A device (20) for providing an indication of the relative humidity of an atmosphere comprises a "dry" thermometer, having first temperature sensing means (22), a "wet" thermometer, having second temperature sensing means (21), which in use has water evaporating from it and means (9-11) for determining the relative humidity of the atmosphere from said measured "wet" and "dry" temperatures, each of the first and second temperature sensing means (22, 21) comprising heat activatable liquid crystal compositions (22a-k, 21 a-k). The "wet" thermometer is in the form of an arcuate row of different liquid crystal regions 21 a-k, preferably identifiable against a temperature scale, mounted on a rectangular card 2. The "dry" thermometer is in the form of an arcuate row of different thermochromic liquid crystal regions 22a-k mounted on a disc 3 rotatably mounted on card 2. By rotating the disc 3 so that the "dry" temperature is aligned with the "wet" temperature on index 10 carried on disc 3 indicates the relative humidity on a scale on card 2 through an aperture in the disc. <IMAGE>

Description

RELATIVE HUMIDITY MEASURING DEVICE Background of the Invention This invention relates to a device for providing an indication of the relative humidity of an atmosphere.

One known instrument for measuring relative humidity is a wet-and-dry bulb hygrometer. Such a known instrument comprises two thermometers, the bulb ("wet bulb") of one of which is kept dry and measures the temperature of the ambient air and the bulb ("dry bulb") of the other of which is kept moist, e.g. by means of a thin piece of clean muslin cloth wetted uniformly with distilled water and contacting the "wet bulb". The temperatures of both the "wet bulb" and the air contacting the "wet bulb" are lowered by the evaporation which takes place when unsaturated air moves past the "wet bulb".An equilibrium temperature, termed the "wet-bulb temperature", is eventually reached which temperature closely approaches the lowest temperature to which air can be cooled by the evaporation of water into that air. The water-vapour content or relative humidity of the air is derived from a known conversion table using the temperatures measured by the "wet bulb" and the "dry bulb". The main disadvantages of this known instrument are that it is relatively expensive, is cumbersome and bulky in use, and is a delicate piece of equipment to use.

Another known device for measuring relative humidity comprises a card carrying two circular regions of absorbent material, e.g. blotting paper, one of which represents a "dry bulb" and the other of which represents a "wet bulb".

The card comprises a fixed, arcuately arranged first temperature scale representing the "dry bulb" temperature and, disposed radially inside the first temperature scale, a rotatable disc carrying an arcuate second temperature scale representing the "wet bulb" temperature. In use the "wet bulb" region is wetted with a few drops of distilled water and the card is then waved to-and-fro in the air for a few seconds causing water to evaporate from the wetted circular ("wet bulb") region into the surrounding air.

-Immediately afterwards an electronic infrared temperature scanner is used to read the temperatures of the "wet bulb" and the "dry bulb" regions. The rotatable disc is turned so that the measured "wet bulb" temperature is positioned opposite the measured "dry bulb" temperature and the relative humidity reading is then read off from another scale on the card. Although the card is relatively cheap to produce and easy to use, it can only be used with a relatively expensive electronic infrared temperature scanner. Thus the total cost of the card and scanner is relatively high.

Summarv of the Invention The present invention seeks to provide a device for providing an indication of the relative humidity of an ambient atmosphere which device is relatively cheap to produce, is reliable, is easy to use and is reasonably accurate.

According to the present invention a device for providing an indicaion of the relative humidity of an atmosphere comprising a first thermometer, having first temperature sensing means, for measuring a first temperature of the first temperature sensing means when the latter is dry and is located in the said atmosphere, a second thermometer, having second temperature sensing means, for measuring a second temperature of the second temperature sensing means when the latter has water evaporating from it and is located in the said atmosphere, and means for determining the relative humidity of the atmosphere from said measured first and second temperatures, is characterised in that each of said first and second temperature sensing means comprises heat activatable liquid crystal compositions for providing a visual indication of temperature.

Preferably the said liquid crystal compositions are selected to be activated at temperatures within the normal ambient temperature ranges at which relative humidity is to be measured. Typically such a temperature range is from 10 C to SOoC, although for most practical applications a temperature range of from 150C to 320C is satisfactory.

Conveniently the liquid crystal compositions of each temperature sensing means are selected to be activated at different temperatures within, or over different relatively narrow temperature ranges within, a wide temperature range (i.e. the expected temperature range at which relative humidity is to be measured). The liquid crystal compositions may be of the type which "clear" at a given clearing temperatures - e.g. so that for each temperature sensing means the liquid crystal compositions "clear" at different temperatures within the wide temperature range.

Alternatively the liquid crystal compositions may comprise a number of thermochromic liquid crystal compositions which display "colour play" (i.e. different colours) over different narrow temperature ranges. By way of example, in this latter case, each thermochromic liquid crystal composition of a temperature sensing means may typically exhibit colour play from "start of red" to "start of blue" over a narrow temperature range of 2.20C.Thus for the lowest temperature range, the "start of red" temperature might be 10.80C and the "start of blue" temperature might be 130C with a "mid green" temperature of 12cm. The next temperature range might slightly overlap the lowest temperature range, with the "start of red" temperature being 12.80C, the "start of blue" temperature being 150C and the "mid green" temperature being 140C. The "mid green" temperature range of each succeeding temperature range would be increased by 20C. Typical thermochromic liquid crystal compositions are nematic, chiral-ester liquid crystal compositions.

Preferably the heat activatable liquid crystal compositions are encapsulated and printed as an ink in one or more layers. Typically the or each layer is relatively thin, i.e. less than 25 micrometers, and the measuring device is constructed in sheet material form.

Preferably the liquid crystal compositions of the first and second temperature sensing means are arranged in first and second single rows, respectively, the first and second rows being movable relative to each other to enable them to be brought into registry with each other, the said relative humidity determining means providing said indication of relative humidity in dependence on the set relative position of the first and second rows. The first and second single rows may be linear rows which are slidable relative to each other. It is preferred, however, that both said single rows are arcuate, one arcuate row being carried on a fixed carrier and the other arcuate row being carried on a rotatable or turnable carrier. By way of example only, each arcuate row may subtend an angle of 1800 +450, e.g. 2100. In order to match up relatively movable rows, it is not, of course, necessary for a numerical indication of the first and second temperatures to be given. For example, if the liquid crystal compositions are thermochromic, all that is necessary is to move the two carriers relative to each other so that the colour indications of the two rows are moved into matching positions, the relative humidity then being read from the relative humidity determining means.

Brief DescriDtion of the Drawings An embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which Figure 1 is a view from the front of a known device for providing an indication of the relative humidity of an atmosphere; Figure 2 is a view from the front of a device according to the invention for providing an indication of the relative humidity of an atmosphere; Figure 3 is a view from the rear of the device shown in Figure 2; and Figure 4 is an enlarged schematic sectional view taken on the line IV-IV of Figure 2 but not drawn to scale.

DescriDtion of the Preferred Embodiments Figure 1 shows a known device 1 for measuring relative humidity comprising a rectangular card 2 carrying a rotatable disc 3 turnable about an axis 4. The card 2 has two circular apertures 5, 6 in which are removably located pieces of absorbent material 5a, 6a, respectively designated "wet" and "dry", e.g. of blotting paper. The disc 3 carries around part of its perimeter a first temperature scale 7 which, on turning of the disc 3 relative to the card 2, is moved against a similar, second temperature scale 9 on the card 2. The disc 3 is provided with an aperture 9 and an indicator arrow 10 against an edge of the aperture 9, the aperture 9 revealing a relative humidity scale 11 against which the arrow 10 points when the disc is positioned between two limit positions.

In use of the. device 1, the "wet" piece of absorbent material 6a is wetted with distilled water and the device is then waved in the air for a few seconds so that some of the water applied to the "wet" piece of absorbent material 6a evaporates therefrom into the surrounding atmosphere.

An electronic infrared scanner (not shown) is then immediately applied against each of the pieces of absorbent material 5a and 6a and the temperatures of each is measured. The disc 3 is then turned so that the measured "wet" and "dry" temperatures are matched on the scales 7 and 8, respectively. The arrow 10 indicates against scale 11 the relative humidity of the ambient atmosphere.

Figures 2 to 4 show a device , generally designated 20, according to the invention for measuring relative humidity. Where appropriate the same reference numerals have been used in Figures 2 to 4 as used in Figure 1 to indicate similar parts of the two devices.

The device 20 comprises a rectangular card 2 and a rotatable disc 3 turnable about an axis 4. The card 2 is provided with aperture 9 and an indicator arrow 10 on the disc 3 for indicating the value of relative humidity from a scale 11 on the card 2. However instead of the two temperature scales 7 and 8, the apertures 5 and 6, the absorbent material 5a and 6a, and the electronic infrared scanner (not shown) required for operation of the device 1, the device 20 is provided with two arcuate temperature sensing strips 21 and 22 of heat activatable liquid crystal compositions. As can be seen in Figures 2 to 4, strip 21 (designed to measure the "wet" temperature of strip 21) is mounted on the card 2, strip 22 (designed to measure the "dry" temperature of strip 22) is mounted on disc 3 and the scale 11 runs in the opposite direction to the scale 11 of Figure 1.It will of course be appreciated that the scale 11 can be positioned above the axis 4 with the aperture 9 in the upper half of the disc 3. In this manner the scale 9 would then have its numbers reading from left to right in ascending order.

The card conveniently comprises a transparent outer layer 24 (see Figure 4)of gloss or matt surfaced PVC which has been surface modified to eliminate static and to enable specially developed ink systems to key directly on to the modified surface during printing using, for example, silk screen printing or a web offset lithographic process. Such surface modified PVC film is known in the printing art.

Typically layer 24 has a thickness of about 480 micrometers but this thickness is not critical.

Printed matter, e.g. a temperature scale (not shown), instructions (not shown) and scale 11, is printed on to the inner surface of the layer 24 by a conventional printing process, e.g. a web offset lithographic process. The strip 21 is also printed on to the inner surface of the layer 24.

This strip comprises a plurality of regions 21a - 21k each of a different heat activatable liquid crystal composition and arranged in an arcuate row. The regions 21a - 21k may be applied directly to the layer 24 or on to a previously printed "tie coat" (not shown) to improve adhesion of the "inks" to the layer 24.

The liquid crystal compositions of the regions 21a21k suitably comprise microencapsulated droplets of thermochromic liquid crystal materials, e.g. of nematic, chiral-ester liquid crystal compositions, the microcapsules preferably having a diameter of less than 20 micrometers, typically less than 10 - 15 micrometers. The liquid crystal compositions are selected so that different regions 21a - 21k are activated over different temperature ranges.

For example the strip 21 may be designed to be activated when subjected to a wide temperature range, e.g. from 10500C. However in a typical application, the strip may be designed to respond to temperatures of from 10.8 - 33or, with the different regions 21a - 21k displaying colour change over different narrow temperature ranges within the wide temperature range. Typically the regions 21a - 21k can be designated as follows, where the "colour play" (i.e.

the colour change temperature range from "start of blue" to "start of red") temperatures and the mid-green temperatures of the regions are given in degrees centigrade: Region Start of Red Mid-Green Start of Blue 21a 10.8 12 13 21b 12.8 14 15 21c 14.8 16 17 21d 16.8 18 19 21e 18.8 20 21 Region Start of Red Mid-Green Start of Blue 21f 20.8 22 23 219 22.8 24 25 21h 24.8 26 27 21i 26.8 28 29 21j 28.8 30 31 21k 30.8 32 33 From the above table it can be seen that the "colour play" ranges overlap, each such range being 2.20C and being spaced 2.0C apart. A thin backing layer 26, e.g. of black "ink", may be printed over the strip 21. As shown the various regions 21a - 21k are positioned immediately adjacent each other and the strip 21 thus appears to be continuous.However, the regions 21a - 21k may be spaced apart from each other so that the strip 21 consists of spaced apart regions. Indeed the regions 21a - 21k may be varying distances apart in order to better represent the "non-linear" arcuate scales 7 and 8 shown in Figure 1.

Preferably, however, even if the regions 21a - 21k are gradually spaced closer together along the arcuate row of the strip with increasing temperature, each region suitably extends through a similar angle of arc. As mentioned above temperatures on a scale (not shown) may be provided on the card 2 to identify the temperatures of the different regions 21a - 21k. Although in Figure 2 the regions 21a21k and regions 22a - 22k are arranged to display ascending temperatures in the clockwise direction, the regions could be arranged the other way round - i.e. so as to display descending ranges in the clockwise direction. With such an arrangement of the regions of the strips 21 and 22, the numbers of the scale 11 (as shown positioned in Figure 2) would of course increase from left to right (i.e. as the scale 11 shown in Figure 1).

The layer 24 with strip 21 and backing layer 26 (if provided) are adhered or laminated to a plastics backing sheet 27 with an arcuate aperture 28 formed therein. The aperture 28 is of substantially the same size as, or of slightly smaller size than, the strip 21.

The disc 3 is formed in substantially the same manner as the card 2 and comprises a transparent outer PVC layer 40 on which is printed the strip 22 consisting of a plurality of regions 22a - 22k. Preferably the two arcuate strips 21 and 22 subtend the same angle, e.g. 1800 +450, and the regions 22a - 22k cover the same temperature ranges as the regions 21a - 21k, respectively. A backing layer 41, e.g. of black ink, is printed over the strip 22. The layer 40 with strip 22 and backing layer 41 are adhered to a plastics backing sheet 42 and the entire disc 3 is rotatably mounted on the card 2 by means of a stud 43.

In use of the device 20, the part of the strip which is covered by the layer 26 and which is revealed through the aperture 28, is moistened through the aperture 28 from the reverse side of the card 2. Ideally this moistening is performed with distilled or ionized water, although normal tap water will give acceptable results. The device 20 agitated or waved in the air like a hand fan to cause some of the applied water to evaporate into the surrounding atmosphere. After a short period of time, e.g. 10s, equilibrium in the displayed "temperature" or colour of the strips 21 and 22 is achieved. The disc 3 is then immediately rotated until the region(s) 22a - 22k displaying colour are matched against the region(s) 21a - 21k displaying colour, and the relative humidity reading is read from the scale 11.It will be appreciated that the regions 22a - 22k measure the "dry bulb" temperature, i.e.

the temperature of the strip 22 when dry in the atmosphere, and the regions 21a - 21k measure the "wet bulb" temperature, i.e. the temperature of the strip 21 when the distilled water applied thereto evaporates into the atmosphere. The liquid crystal materials of the strips 21 and 22 should be chosen to exhibit a degree of memory for a period of time, e.g. 30s, and the matching of the regions should, of course, occur in this period of time.

Although not shown, the device 20 may be modified to provide a reading of the "dew point". The dew point can be measured if the "dry bulb" temperature and the relative humidity are known. Thus the reverse side of the device could carry a rotatable disc with printed thereon an arrow and a dew point temperature scale in OC. The disc would then be rotated to position the arrow against a fixed "dry bulb" temperature scale in OC and the dew point temperature would be read opposite a fixed scale giving the relative humidity. The "dry bulb" temperature scale and the relative humidity scale would conveniently be positioned just radially outside the rotatable disc to facilitate positioning of the disc.

In other types of device according to the invention, the strips 21 and 22 could be arranged in straight rows and be slidable linearly relative to each other, e.g in the manner of a slide rule. Alternatively, although less ideally, the strips 21 and 22 may be fixed relative to each other, merely being arranged to provide "wet bulb" and "dry bulb" temperatures for calibrating the relative humidity from known charts or tables.

Other types of liquid crystal compositions could be employed, for example compositions which "clear" at and above specific temperatures (e.g. to reveal specific colours for matching purposes or merely to reveal numerical temperatures in OC).

Although the card 2 of the device 20 is shown as being rectangular in shape, it will be appreciated that it can be of any other suitable shape, e.g. circular, "racket" shaped or the like.

Claims (12)

1. A device for providing an indication of the relative humidity of an atmosphere comprising a first thermometer, having first temperature sensing means, for measuring a first temperature of the first temperature sensing means when the latter is dry and is located in the said atmosphere, a second thermometer, having second temperature sensing means, for measuring a second temperature of the second temperature sensing means when the latter has water evaporating from it and is located in the said atmosphere, and means for determining the relative humidity of the atmosphere from said measured first and second temperatures, characterised in that each of said first and second temperature sensing means comprises heat activatable liquid crystal compositions for providing a visual indication of temperature.

2. A device according to claim 1, in which the said liquid crystal compositions are selected to be activated at temperatures within the normal ambient temperature range at which relative humidity is to be measured.

3. A device according to claim 2, in which the said temperature range is from 10 C to 500 C.

4. A device according to any one of the preceding claims, in which the liquid crystal compositions of each temperature sensing means are selected to be activated at different temperatures within, or over different relatively narrow temperature ranges within, a wide temperature range.

5. A device according to claim 4, in which the liquid crystal compositions are of the type which "clear" at given clearing temperatures so that for each temperature sensing means the liquid crystal compositions "clear" at different temperatures within the wide temperature range.

6. A device according to claim 4, in which the liquid crystal compositions comprise a number of thermochromic liquid crystal compositions which display "colour play" over different narrow temperature ranges.

7. A device according to any one of the preceding claims, in which the heat activatable liquid crystal compositions are encapsulated and printed as an ink in one or more layers.

8. A device according to any one of the preceding claims, in which the liquid crystal compositions of the first and second temperature sensing means are arranged in first and second single rows, respectively, the first and second rows being movable relative to each other to enable them to be brought into registry with each other, the said relative humidity determining means providing said indication of relative humidity in dependence on the set relative position of the first and second rows.

9. A device according to claim 8, in which the first and second single rows are linear rows which are slidable relative to each other.

10. A device according to claim 8, in which both said single rows are arcuate, one arcuate row being carried on a fixed carrier and the other arcuate row being carried on a rotatable or turnable carrier.

11. A device according to claim 10, in which each arcuate row subtends an angle of 1800 + 450.

12. A device for providing an indication of the relative humidity of an atmosphere constructed and arranged substantially as herein described with reference to,- and as illustrated in, Figures 2-4 of the accompanying drawings.