GB2300266A

GB2300266A - High temperature atmosphere humidity measurement

Info

Publication number: GB2300266A
Application number: GB9508634A
Authority: GB
Inventors: Ian Andrew Boyle; Paul Kenneth Dyche
Original assignee: Royal Doulton UK Ltd
Current assignee: Royal Doulton UK Ltd
Priority date: 1995-04-28
Filing date: 1995-04-28
Publication date: 1996-10-30
Anticipated expiration: 2015-04-28
Also published as: GB9508634D0; GB2300266B

Abstract

The humidity of a high temperature atmosphere e.g. in a kiln 5 is measured by continuously withdrawing a sample of the atmosphere by means of a probe 6, cooling the sample while maintaining its temperature above the dew point by passing it through a pipe 7 and delivering the cooled sample to a sensing unit 8 in which the humidity of the sample is measured either capacitively (figure 2) or by psychrometric means (figure 3).

Description

METHOD AND APPARATUS FOR THE MEASUREMENT OF HUMIDITY This invention relates to a method and apparatus for measuring humidity.

The production of ceramic tableware and giftware incorporating coloured decoration may involve the use of colouring materials and glazes incorporating metallic oxides which can give rise to release of metals such as lead and cadmium in service. The release of lead and cadmium is regulated in the U.K. by statute which specifies a test method and permissible limits. Similar regulations exist in most other countries. The extent to which such metal release takes place can be affected by the conditions under which the articles are fired during production and in particular reductions in metal release have been brought about by controlling the moisture content of the atmosphere in the kiln or furnace during firing of the product.

Maintenance of uniform moisture content during the firing process requires regular monitoring of the kiln atmosphere and hitherto this has been done by withdrawing known volumes of gas from the kiln over a known weight of dried silica gel or other desiccant which is then re-weighed and the moisture content assessed based on the increase in weight. While this process operates satisfactorily it cannot be carried out continuously and there is therefore a need for a means of continuously monitoring and measuring the humidity within ceramic kilns. This presents problems since hydrocarbon materials, incorporated in the decorating mediums employed to decorate ceramic products, burn off during firing and condense on conventional sensing or measuring elements interfering with their operation and producing false readings.

The invention provides a method of continuously measuring the humidity of a high temperature atmosphere comprising continuously withdrawing a sample of the atmosphere, cooling the sample while maintaining its temperature above the dew point of the atmosphere, delivering the cooled sample to a sensing unit, maintaining the temperature of the sample in the sensing unit within a predetermined range above the dew point of the atmosphere, and measuring the humidity of the sample in the sensing unit.

Preferably the temperature of the sample in the sensing unit is controlled by altering the rate of flow to the sensing unit when the sample temperature reaches predetermined upper and lower limits.

Advantageously the rate of flow is reduced when the sample temperature exceeds a predetermined upper limit and is increased when the sample temperature reaches a predetermined lower limit.

The invention also provides apparatus for measuring humidity of a high temperature atmosphere comprising means for continuously withdrawing a sample volume of said atmosphere, means for cooling said sample while maintaining its temperature above the dew point of the atmosphere, means for delivering the cooled sample to a sensing unit, means for maintaining the temperature of the atmosphere in said sensing unit between predetermined limits, and means for measuring the humidity of the temperature-controlled sample in the sensing unit.

Preferably said means for cooling the sample comprises pipe means through which the sample passes before delivery to said sensing unit.

Preferably the apparatus includes pump means connected downstream of said sensing unit and operable to draw said sample through said sensing unit.

Preferably said pump means is adapted to be connected both to said sensing unit and direct to atmosphere, connection of said pump means to atmosphere serving to reduce the rate of flow of said sample through said sensing unit and thereby cause the temperature of the atmosphere in the sensing unit to decrease.

Connection of said pump means to atmosphere is preferably controlled by valve means operative in response to changes in temperature of the sample within said sensing unit.

Measurement of humidity of the sample within said sensing unit is preferably effected by a capacitive probe but may alternatively be affected by use of wet and dry thermometer bulbs or other means. Preferably means is provided for presenting a visual read-out of humidity in response to signals derived from said probe.

The invention also provides a method of controlling the atmosphere in furnace or kiln comprising withdrawing a sample of the kiln atmosphere into a sampling unit, measuring the humidity of the sample, generating a signal dependant on the humidity measurement reading, and utilising the signal to alter the atmosphere within the kiln.

Embodiments of the invention will now be described, by way of example only, with reference to accompanying diagrammatic drawings, in which: Fig. 1 is a schematic diagram showing the principal components of one form of apparatus according to the invention; Fig. 2 is a diagrammatic illustration of one form of sampling apparatus according to the invention; Fig. 3 is an illustration similar to Fig.2 showing a modified form of apparatus; and Fig. 4 is a vertical cross-section through a sensing unit incorporated in the apparatus of Fig. 3.

Referring to Fig. 1, the apparatus is used for measuring humidity of the atmosphere in a continuous kiln used to fire articles of ceramic ware such as tableware or giftware. The articles of ware are decorated using transfers or other means of decoration incorporating ceramic pigments and fluxes and are fired in the kiln at temperatures in the region of 750-10000C.

During the firing process a predetermined level of humidity is maintained within the kiln by injecting water into the kiln atmosphere. A level of 6 to S, Ó moisture content is generally maintained in a modern fast fire kiln and around 4 to 5 moisture content in a traditional kiln.

In order to monitor the moisture content of the kiln atmosphere, kiln gas for sampling purposes is continuously withdrawn from the kiln 5 through a tubular ceramic probe 6 and a coiled copper pipe 7 into a sampling apparatus 8. The length, diameter and composition of the pipe 7 are selected to produce the desired drop in temperature of the extracted sample from the kiln temperature of around 750-10000C to a temperature of 55-600C used for testing purposes.

Typically a coil of soft copper tube nominally 1Omm in internal diameter and approximately 1200mm in length may be employed. The sampling apparatus 8 may incorporate an integral control and display unit and/or may be connected to a separate unit shown at 9 as described hereafter or to an existing kiln instrument panel.

Fig. 2 shows a first form of sampling apparatus according to the invention. Gases leaving the pipe 7, pass through a flexible heat-resistant hose 7A which is connected to a sensing unit 10 to which a thermostatic control device 11 is connected. This incorporates a capillary type thermostat 12, having a measurement bulb communicating with a sensing chamber 10A in the unit 10 and an electrical cartridge heater 13 also communicating with the sensing chamber.

The thermostatic control unit serves to raise the temperature in the sensing chamber to a level'above the anticipated dew point temperature of the gases being sampled, so preventing the formation of condensate which would otherwise cause erroneous measurement and could lead to other problems. The control unit serves primarily to pre-heat the sensing chamber to a working temperature of around 45-500C when starting from cold as will be described hereafter.

In order to measure the humidity of the gases in the sensing unit 10, a capacitive probe 15 extends into the sensing chamber. The probe varies in electrical capacitance according to the temperature and humidity of the gas surrounding it and is connected through an electrical circuit 16 to a humidity analyser 17 incorporating a screen 18 which displays a reading of relative humidity. The analyser also produces a temperature output signal which is transmitted via a circuit 19 to a temperature controller 20, the function of which will be described hereafter, and an alarm signal which may be arranged to provide an audible and/or visual warning in the event the humidity reading is outside the acceptable range.

The sample to be tested is drawn into the sensing unit 10 by a pump 22 connected by a pipe 23 to the sensing chamber 10A of the unit 10. A branch pipe 24 is connected to the pipe 23 between the sensing unit 10 and the pump and is connected to atmosphere through a bleed valve 25 operated by a solenoid 26. The pump 22 discharges to atmosphere through an outlet pipe 27.

The temperature controller 20 incorporates a first relay (not shown) which is operable to control operation of the solenoid 26 to open or close the bleed valve 25.

The controller 20 also incorporates a second relay 30 which controls the power supply to the pump 22.

In operation of the apparatus, the pump 22 is first switched on to draw gases from the kiln 5 through the pipe 7 and hose 7A to the sensing unit 10. During their passage through the pipe 7 the gases are cooled to the chamber operating temperature of around 55-600C.

During initial start-up conditions the temperature in the kiln will be relatively low and the thermostatic control unit 11 will energise the heater 13 to maintain the temperature in the sensing chamber within the appropriate range. As the temperature of the gases drawn from the kiln increases, the heater 13 is switched off. Contact of the gases with the probe 15 produces electrical signals which are transmitted through the circuit 16 to the unit 17 to produce a humidity reading in the display window 18.

As the kiln increases in temperature, the temperature of the test sample continuously withdrawn into the sensing unit 10 will increase and would ultimately result in overheating. In order to avoid this, the temperature signal transmitted from the probe 15 through circuit 16, analyser 17 and circuit 19 to the temperature controller 20 serves to operate the solenoid 26 and open the bleed valve 25 when the temperature within the sensing chamber exceeds 600C. This results in the pump 22 drawing air from atmosphere through the valve 25 and pipe 24 thus reducing the flow through the sensing chamber via pipe 23. This allows more heat loss to take place in the pipe 7, causing the temperature in the sensing chamber to fall.Once the temperature falls to the 55-600C operating temperature of the sensing chamber, the valve 25 is closed and the volume of gases drawn by the pump 22 through the sensing chamber from the kiln is increased. This sequence of operations continues throughout operation to maintain the temperature of gases within the sensing unit 10 at the desired level to ensure proper operation of the probe 15 and accurate readings of humidity on the display 18.

The relay 30 serves to cut off the power supply to the pump 22 in the event the operating temperature in the sensing unit 10 varies from the desired operating temperature by more than a predetermined amount, generally about 120C above or below the operating temperature. This prevents the pump from drawing humid air into the sensing unit before it has reached a workable temperature. It also protects the probe 15 from overheating in the event of failure of the solenoid-operated bleed valve 25.

Figs. 3 and 4 show a second form of apparatus in which the sensing unit 10 of the embodiment shown in Fig. 2 is replaced by a modified unit 35 incorporating a psychrometer incorporating wet and dry bulbs. The general arrangement of the sampling unit is similar to that of Fig. 2 and corresponding parts are identified by the same reference numerals. Thus, gases to be sampled enter the sensing unit 35 from hose 7A and are drawn through the unit via pipe 23 by a pump 22 exhausting to atmosphere at 27. A bleed valve 25 operated by a solenoid 26 is connected in the line 23 and serves to connect the pump to atmosphere when operated by a temperature controller 20. A relay 30 serves to cut-off power supply to the pump 22 in response to excessive variations of temperature in the sensing unit in the same manner as the Fig. 2 embodiment.

The sensing unit 35 is shown in greater detail in Fig. 3 and incorporates a sensing chamber 36 into which project a "dry bulb" platinum resistance thermometer assembly 37 and a "wet bulb" platinum resistance thermometer assembly 38. The "wet bulb" sensor is maintained moist by a close fitting cotton lamp wick 39 which draws a supply of water by capillary action from a water reservoir 40 machined into the sensor housing.

These devices operate in the manner of a psychrometer or wet and dry bulb hygrometer, the first sensor measuring the actual gas temperature and the second being cooled by latent heat of evaporation of the water in which it is soaked. The cooling effect varies according to the temperature and humidity of the surrounding gas and the humidity can therefore be determined from the temperature readings of the two sensors.

In order to prevent water being sucked up by the wick 39, the measuring chamber 36 and reservoir 40 are interconnected by a port 41 to allow pressure equalisation. The level of water in the reservoir is sensed by a liquid level sensor 42 connected in an electrical circuit through a control unit 43 with a peristaltic pump 44 operable to draw water from a larger reservoir 45 mounted at a lower level in the apparatus.

the reservoir 45 is provided with a further liquid level sensor 46 connected to the control unit 43 and operable to initiate an alarm when the water supply in the reservoir 45 requires replenishment. Actuation of the level sensor 46 is also arranged to disconnect the power supply to the pump 22 to prevent the wick 39 drying out and becoming contaminated.

The signals from the wet and dry sensors are converted to a linear electrical signal of one millivolt per centrigrade degree in the control unit 43 and are transmitted to a control panel 9 shown in Fig. 1 of the drawings. The control panel incorporates an amplifer 50, a computer 51 operable to convert the amplified signals from analog to digital form and a combined chart recorder and digital display unit 52 on which the humidity readings are graphically and visually displayed. In other respects the arrangement shown in Figs. 3 and 4 operates in the same manner as the arrangement shown in Fig. 2.

By virtue of the arrangements described it is possible to continuously withdraw kiln atmosphere for sampling despite the high temperature and corrosive nature of the kiln atmosphere. Maintenance of the temperature of the test sample within the desired operating limits is ensured by virtue of the facility to connect the pump to atmosphere to control the flow of high temperature gases into the sensing unit when the kiln is in full operation, supplemented in the case of the Fig. 2 embodiment by the thermostatic control associated with the sensing unit.

Various modifications may be made without departing from the invention. For example, alternative means of sensing humidity within the sensing unit may be employed and the sample gases may be cooled by other means. If desired the temperature within the sensing chamber could be monitored and displayed and the auxiliary bleed valve manually operated to connect the pump to atmosphere by an operator in response to inspection of the temperature readings.

In a further modification the signals generated by the apparatus could be employed to automatically alter the humidity within the kiln by increasing or reducing the quantity of water introduced, thereby producing automatic control of the kiln atmosphere on a continuous basis.

Whilst endeavouring in the foregoing specification to draw attention to those features of the invention believed to be of particular importance it should be understood that the Applicant claims protection in respect of any patentable feature or combination of features hereinbefore referred to and/or shown in the drawings whether or not particular emphasis has been placed thereon.

Claims

aaims:-

1. A method of continuously measuring the humidity of a high temperature atmosphere comprising continuously withdrawing a sample of the atmosphere, cooling the sample while maintaining its temperature above the dew point of the atmosphere, delivering the cooled sample to a sensing unit, maintaining the temperature of the sample in the sensing unit within a predetermined range above the dew point of the atmosphere, and measuring the humidity of the sample in the sensing unit.

2. A method according to claim 1 wherein the temperature of the sample in the sensing unit is controlled by altering the rate of flow to the sensing unit when the sample temperature reaches predetermined upper and lower limits.

3. A method according to claim 2 wherein the rate of flow is reduced when the sample temperature exceeds a predetermined upper limit and is increased when the sample temperature reaches a predetermined lower limit.

4. A method of continuously measuring the humidity of a high temperature atmosphere substantially as hereinbefore described.

5. Apparatus for measuring humidity of a high temperature atmosphere comprising means for continuously withdrawing a sample volume of said atmosphere, means for cooling said sample while maintaining its temperature above the dew point of the atmosphere, means for delivering the cooled sample to a sensing unit, means for maintaining the temperature of the atmosphere in said sensing unit between predetermined limits, and means for measuring the humidity of the temperature-controlled sample in the sensing unit.

6. Apparatus according to claim 5 wherein said means for cooling the sample comprises pipe means through which the sample passes before delivery to said sensing unit.

7. Apparatus according to claim 6 wherein the length, diameter and composition of said pipe means are selected to produce a drop in temperature of said sample from 750-1000"C to 55-60"C.

8. Apparatus according to claim 6 or 7 wherein said pipe means is of coiled form.

9. Apparatus according to claim 6, 7 or 8 wherein said pipe means is made from copper.

10. Apparatus according to claim 9 wherein said pipe comprises a tube of lOmm. nominal internal diameter and around 1200mm. in length.

11. Apparatus according to any of claims 5 to 10 including pump means connected downstream of said sensing unit and operable to draw said sample through said sensing unit.

12. Apparatus according to claim 11 wherein said pump means is adapted to be connected both to said sensing unit and direct to atmosphere, connection of said pump means to atmosphere serving to reduce the rate of flow of said sample through said sensing unit and thereby cause the temperature of the atmopshere in the sensing unit to decrease.

13. Apparatus according to claim 12 wherein connection of said pump means to atmosphere is controlled by valve means operative in response to changes in temperature of the sample within said sensing unit.

14. Apparatus according to claim 13 wherein changes of temperature in said sensing unit are sensed by electrical signals derived from said means for measuring humidity of the sample in the sensing unit.

15. Apparatus according to claim 13 including means operable to cut off the power supply to said pump in the event the operating temperature in said sensing unit varies from the desired operating temperature by more than a predetermined amount.

16. Apparatus according to claim 15 wherein said variation is +12"C.

17. Apparatus according to any of claims 5 to 16 wherein said means for measuring humidity comprises a capacitive probe.

18. Apparatus according to any of claims 5 to 16 wherein said means for measuring humidity comprises wet and dry thermometer bulbs.

19. Apparatus according to any of claims 5 to 18 wherein means is provided for presenting a visual read-out of humidity in response to signals derived from said means for measuring humidity.

20. Apparatus according to any of claims 5 to 19 including means for producing an audible and/or visual warning in the event the humidity reading is outside the acceptable range.

21. Apparatus according to any of claims 5 to 20 including means for preheating said sensing unit to a predetermined temperature when starting from cold.

22. Apparatus according to claim 21 wherein said pre-heating means comprises a thermostatic control unit.

23. Apparatus according to any of claims 5 to 22 wherein said atmosphere is the atmosphere in a furnace or kiln.

24. Apparatus according to claim 23 including means for generating a signal dependent on the humidity of said sample and means operable in response to said signal to alter the humidity of the atmosphere in the furnace or kiln.

25. Apparatus for measuring humidity of a high temperature atmosphere substantially as hereinbefore described with reference to Fig. 2 of the accompanying drawings.

26. Apparatus for measuring humidity of a high temperature atmosphere substantially as hereinbefore described with reference to Figs. 3 and 4 of the accompanying drawings.

27. A furnace or kiln provided with apparatus for measuring humidity according to any of claims 5 to 26.

28. Any novel subject matter or combination including novel subject matter disclosed, whether or not within the scope of or relating to the same invention as any of the preceding claims.