GB2278203A - Device and method for calibration of sensors - Google Patents

Abstract

A device for calibrating a gas sensor comprises a block (7) of material having at least one channel or passage (8) to increase the surface area thereof, the material of the block having a thermal conductivity greater than 10Wm<-1>K<-1>, a circuit (1, 8, 3) for passing a gas over the surface of said block and temperature sensing means (3) which are disposed in the block in thermal contact with the point in the circuit where the gas leaves the block. The device can be used in a method of calibrating humidity sensors which involves saturating the surface of the block with water and then passing the gas over the saturated surface to establish thermal equilibrium at the temperature of the block and so produce a gas of defined dewpoint. A heater (9) and cooling coil (12) allow the temperature of the block (7) to be controlled. <IMAGE>

Description

DEVICE AND METHOD FOR CALIBRATION OF SENSORS The invention relates to a device for calibrating gas sensors, in particular humidity sensors and to a method of calibration of such sensors in situ. In another aspect the invention relates to a device for producing a gas mixture comprising a carrier gas and a condensable gas in which the condensable gas is at a defined vapour pressure in the carrier gas. The device is particularly useful for producing a gas of defined humidity such as would be required for the calibration of a humidity sensor.

In many industrial processes it is important to keep the humidity of process gases at the lowest achievable level. Water vapour is generally removed by the application of quite simply constructed systems employing molecular sieve or other humidity absorbers. In order to measure the residual humidity of gases for the purpose of monitoring and control, various types of humidity sensors (hygrometers) are used. These may operate by making use of a variety of physical or chemical principles but generally the accuracy of commercially available humidity sensors, especially at low humidity levels, is not adequate for process control. The humidity of a gas is directly calculatable from its dewpoint, i.e. the temperature at which water vapour in the gas starts to form droplets of liquid.It can be shown that, for example, if several supposedly calibrated and identical sensors are located at the same process monitoring point in a gas stream of low humidity, dewpoint values as divergent as 20 0C apart may be obtained with different instruments. Thus it is important to find a reliable method of calibrating and recalibrating humidity sensors and to be able to do this while the humidity sensors remain in situ in the process stream. Removal of a sensor to a calibration facility results in loss of process information and also affects the sensitivity of the sensor by exposing it to a high humidity environment i.e. ambient air, while in transfer.

Whether or not calibration of the humidity sensor is carried out in situ, calibration requires monitoring the sensor readings in relation to a known standard. One known calibration method employs a condensing mirror hygrometer as a secondary standard to define the dewpoint of a gas. The condensing mirror device employs the principle of detecting by suitable optical means the formation of condensation on a mirror during cooldown of the mirror at constant cooling rate. The temperature at which condensation forms, either as liquid droplets or frost, is the dewpoint of the gas being measured. If the condensing mirror hygrometer is connected into the process stream at the same point as the humidity sensor to be calibrated the readings of both instruments can be compared for calibration of the sensor.

As an alternative to the condensing mirror system other sources of gas of known dewpoint can be used. One commonly used method to generate gas streams of varying moisture levels involves passing a gas through a saturated solution of lithium chloride. The solution has a low vapour pressure which can be predictably calculated from the temperature of the solution. Since the gas picks up the equilibrium vapour pressure of the saturated solution at the temperature of the solution, that temperature will represent the resulting dewpoint and hence humidity level of the gas so produced.

Another method of producing gas of known humidity is to mix a very dry gas with a gas which has been saturated with moisture, in different amounts and then to calculate the final moisture concentration of the mixture from the mixing proportions.

A problem with all three calibration methods discussed above is that they cannot be used to calibrate a sensor for providing readings at very low humidity levels such as is required in some industrial processes. The condensing mirror device only works reliably down to dewpoints in the range of 0 -60 C. Similarly the lithium chloride solution cannot generate standard gases with very low humidities. With the mixed gas method the problem arises that in order to achieve low dewpoint mixtures, dilution ratios in excess of 10,000:1 are required which leads to large inaccuracies arising from the residual humidity content of the "dry" component (this is certified but not measurable), the mixing ratio and the mixing process itself.

A particular example where the above calibration standards would be unsuitable is in relation to humidity sensors for air detritiation processes used on gases produced by a nuclear fusion reactor. In such processes removal of substantially all residual humidity is required.

The present inventors have thus developed a calibration device suitable for use with humidity sensors which is free from the drawbacks of known calibration systems in that it can be used in situ in specially adapted process gas lines and in that it can produce gases of known dewpoint over an infinitely wide range, including dewpoints low enough to calibrate a humidity sensor in an air detritiation system.

Further, apart from the general applicability of the device to humidity sensors it may be adapted for calibration of other gas sensors because the device has a general capability to produce gas mixtures which contain one condensable gas of defined vapour pressure in one or more carrier gases.

A device in accordance with the invention for calibration of gas sensors comprises a block of material having at least one channel or passage and preferably a plurality of channels or passages, therein to increase the surface area thereof, said material having a thermal conductivity greater than l0Wm1K1, a circuit for passing a gas over the surface area of said block, couplings within said circuit for attachment to the sensor to be calibrated and temperature sensing means being disposed in said block in thermal contact with the point in the said circuit where the gas leaves the block. The device may include the means for heating or cooling the block to a predetermined temperature.

It is to be understood that as used herein the term "block" is intended to include a body of any variety of geometric shape or of an irregular shape.

The block may have any number of surface depressions. In particular the shape of the block may be determined by the type of heating or cooling means used.

In the device of the invention the channelled block provides a large isothermal surface area which when the device is in use for calibration of a humidity sensor, is saturated with water. When the block is heated or cooled to a pre-determined temperature and a suitable calibration gas passed across the isothermal surface, the gas becomes saturated with water in thermal equilibrium so that the dewpoint of the gas is identical to the temperature of the surface. Thus accurate temperature measurement of the surface at the point where the gas leaves the block gives the dewpoint of the gas. This is a substantial advantage of the invention.Since the dewpoint of the circulating gas is directly defined by a single temperature measurement, the device can use any suitable thermometer (e.g. platinum resistor) calibrated and traceable to national (or international standards in order to achieve the desired accuracy. The gas of known dewpoint can then be circulated through the humidity sensor to be calibrated. This may be achieved in situ by attaching the couplings in the circuit of the device to the inlet and outlet of the humidity sensor which is especially arranged in the process gas stream to be easily disconnected therefrom while still remaining in the housing and avoiding exposure to ambient air.

Preferably the block and at least part of the circuit for the gas are within a housing to provide a thermally insulated environment. The housing may be packed with an insulating powder such as perlite and evacuated. Alternatively, the housing itself may include or comprise radiation shields and be evacuated.

In order to reduce temperature fluctuations still further, the device may include within the thermally insulated environment a counterflow heat exchanger through which the circulating calibration gas flows on its way to and from the block.

The heating means for the block may be for example an electric heating element disposed around or below the block. The cooling means may be a cooling coil wrapped around the block through which is passed a suitable coolant such as liquid nitrogen. An alternative means for cooling the block is to bring it into contact with for example, dry ice and methanol either by submersing the block or by introducing the dry ice and methanol into a cooling well within the block. A further alternative for cooling the block is to introduce a suitable heat transfer medium into a well or depression within the block and then apply to the medium, conventional refrigeration for example, using a refrigerator with a "cold finger".

Ideally the block is made of a material of high thermal conductivity such as metal, a particularly preferred metal being copper. It is desirable to increase the surface area as much as possible by filling the passages and channels in the block with spheres, granules, wire mesh or knitted metal wool which is made of the same material as the block.

Preferably, the internal surfaces of the passages and channels will be gold plated to reduce surface effects and improve response time.

As mentioned above, calibration of humidity sensors is only a preferred use of the device in accordance with one aspect of the invention. It could be used to calibrate a sensor for gases other than water vapour, the only difference in operation being that the gas which the sensor measures (sensor gas) is first condensed on the isothermal surface of the block. A flow of carrier gas is then passed across the surface at a pre-heated or pre-cooled temperature to establish saturation of the carrier gas by the sensor gas in thermal equilibrium. As with the dewpoint, the vapour pressure of the sensor gas in the carrier gas is a direct function of the temperature of the block. The defined calibration gas mixture is then passed to the appropriate sensor as described above.

As an example of alternative uses the surface of the block may be saturated with tritiated water to produce a gas of defined tritium content which can be used to calibrate a Tritium Monitor for a variety of carrier gases. Another example is the calibration of a halogen detector where a carrier gas such as helium having freon therein at a defined partial pressure is produced by the calibration device.

In a second aspect of the invention there is provided a method of calibrating a humidity sensor which comprises the steps of: (a) providing a block of material having at least one channel or passage and preferably a plurality of channels and passages therein, said material having a thermal conductivity greater than 1OWm 1K 1 (b) saturating the entire surface area of the block with water, (c) heating or cooling the block to a pre-determined temperature, (d) passing a gas across the saturated surface of the block until said gas is saturated with water and in thermal equilibrium with the block so that the dewpoint of the gas is identical to the temperature of the block, (e) passing the equilibrated gas of defined dewpoint to the humidity sensor to be calibrated and (f) repeating steps (c) to (e) above at one or more other pre-determined temperatures to calibrate the humidity sensor.

The above method can be carried out using the calibration device of the invention. A convenient way of saturating the surface area of the block for producing very low dewpoint gases i.e. with dewpoints less than 0 0C is to pass ambient air across the surface. About 500 litres of air provides adequate saturation if passed through the device at -200C to -500C.

In a particularly preferred embodiment of the invention the outputs from the humidity sensor and temperature sensor of the calibration device are fed to an integrated microprocessor to facilitate autocalibration.

In a third aspect of the invention there is provided a device and method for generating a mixture of a carrier gas and a condensable gas in which the condensable gas is at a defined vapour pressure in the carrier gas. For example the gas mixture may be air containing water vapour which is of known humidity.Such a device comprises a block of material having at least one and preferably a plurality of channels and passages therein to increase the surface area thereof, said material having a thermal conductivity greater than 10Wm 1K , means for passing a gas over the surface area of said block and temperature sensing means being disposed in said block in thermal contact with the point where the gas leaves the block; such a device may be used to generate a gas mixture of defined composition for a number of purposes and of course may form part of a device for calibrating sensors as described above.

The invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is a vertical cross-section through a calibration device in accordance with a first embodiment of the invention for calibrating humidity sensors; Figure 2 is a vertical cross-section through a calibration device in accordance with a second embodiment of the invention for calibrating humidity sensors.

Figure 3 is a schematic representation of a humidity sensor as normally fitted in a process gas stream; Figure 4 is a schematic representation of a humidity sensor in a process gas stream with a fitting adapted to be coupled to a calibration device in accordance with the invention and Figure 5 is a schematic representation of a humidity sensor in the specially adapted fitting as shown in Figure 4 and coupled to a calibration device in accordance with the invention.

Referring to Figure 1 one embodiment of the calibration device comprises a copper block (7) having a spiral or otherwise convoluted channel (8) which is filled with copper wool to achieve a surface area sufficiently large for humidity equilibration.

The calibration gas passes from the inlet (1) through the convoluted channel (8) and returns to the outlet (2). In both directions it passes through a counterflow heat exchanger (6) which keeps temperature variations of the block to a minimum.

This gas circuit may be used at the start of the calibration operation to introduce sufficient ambient air to saturate the surface of the block. A thermometer (3), typically an accurately calibrated Pt 100 RTD sensor, is mounted in close proximity to and in intimate thermal contact with the point at which the gas leaves the block. The temperature measured at this location is the dewpoint of the gas leaving the device.

The block (7) is provided with a cooling coil (12) having a coolant supply line (4) and return line (5). Preferred coolants are liquid nitrogen, air or argon. A heater (9) is disposed below the block (7). Both heater (9) and cooling coil (12) allow accurate adjustment of the temperature of the block (7) and hence the dewpoint of the calibration gas leaving the device. The complete assembly is surrounded by a housing (10) which is either filled with an insulating powder or the housing itself includes radiation shields. Either way the housing is evacuated via valve (11) to provide a thermally insulated environment. The block (7), heat exchanger (6) and other components are carefully arranged in the housing (10) so that the temperature of the isothermal block is the lowest temperature in the gas loop.The mass of the copper block (7) in relation to the efficiency of the counterflow heat exchanger (6) is preferably selected such that for design flow rates (typically 10 1 mint) the temperature of the block without additional heating by means of heater (8) rises sufficiently slowly for accurate calibrations e.g. less than 3 0C per hour at a temperature of -1000C. Raising the temperature at a constant rate is particularly suitable for calibration of a wide range of humidity. Continuous recording of the block temperature and sensor gives a full, continuous and complete calibration record.

Figure 2 shows an alternative calibration device in which the block 30 has a central well 32.

A coolant such as dry ice and methanol may be introduced into the well 32 as an alternative to using the cooling coil shown in Figure 1. As another alternative the well may be filled with a suitable heat transfer fluid such as methanol and a "cold finger" introduced into the fluid through the top of the housing 34. The block 30 has a series of sunken holes or channels 36 arranged in an annular formation and filled with copper wool 38. The channels 36 are connected to the gas inlet 40 and outlet 42 by a manifold plate 44. Temperature sensing means 48 is positioned near the gas outlet 26 so as to be in thermal contact with the gas as it leaves the block.

The device is also provided with a heat exchanger 46 within the housing 34 through which the calibration gas is passed on its way in and out of the housing.

As with the device of Figure 1 the heat exchanger serves to minimize temperature variations within the vicinity of the block 30.

The device of Figure 1 or Figure 2 may be used to calibrate a humidity sensor in situ in a gas process stream as shown in Figures 4 and 5.

Figure 3 shows a typical installation of a humidity sensor (16) which is fitted directly in the process line in a conventional manner. Such a sensor is only accessible for calibration after removal from the process line.

Figure 4 shows a humidity sensor (15) installed in such a way that it may be calibrated in situ by the calibration device of the invention. The sensor (15) is connected to the process stream in a separate housing via suitable lines. A flow inducing device (18) such as a pitot tube uses the kinetic energy of the process flow to drive process gas through the open valve (20), the sensor housing and back to process line (13). Self-closing couplings (23) and (24) isolate the process from the environment.

Both Figures 4 and 5 show a calibration device in accordance with the invention.

In Figure 4 the device (27a) is shown with the calibration gas circulating in a closed loop. The block is first cooled to a temperature lower than the dewpoint range to be measured. During cooldown the surface of the block is saturated with water by passing a quantity of ambient air containing about 2 grams of water, typically 500 litres, through the device at about -200C to -500C. Thereafter, calibration gas is circulated through the loop. With the loop closed an engineer may transport the device to the installation where the sensors are to be calibrated.

In Figure 5 the calibration device is shown in use, the sensor 14 being calibrated. Self-closing couplings (21) and (22) are connected to self-closing couplings (23) and (24) on the sensor housing with valve (19) closed so that the sensor is cut off from the process gas. The calibration gas of defined dewpoint is then pumped by means of pump (28) through the sensor (14), the flow rate being adjusted by the pump bypass valve (25) and measured by a flow meter (26). It is important that auxilliary equipment such as pumps, valves, and flow meters be arranged preferably on the side of the gas inlet (1,40) to the isothermal block (7,30) in order to avoid errors caused by humidity retained in and potentially released from such equipment. Only the gas leaving the block is of defined humidity and has to be directly connected to the humidity sensor inlet.

Once readings have been taken from a particular sensor in an installation for the purposes of calibration, the device can be used to check other sensors in the system. It is preferable to start with the sensor at the lowest dewpoint in the system and proceed to connect the calibration device to other sensors in sequence of rising dewpoint, thus performing individual calibrations only in a narrow range below and above the actual operating point of each sensor.

Once the calibration of all sensors is completed, humidity retained on the block is removed by passing ambient air or dry nitrogen through the block and by heating it to a temperature of about +500C.

Claims (24)

CLAIMS:

1. A device for calibration of gas sensors comprising a block of material having at least one channel or passage therein to increase the surface area thereof, said material having a thermal conductivity greater than 10Wm 1K 1, a circuit for passing a gas over the surface of said block, couplings within said circuit for attachment to the sensor to be calibrated and temperature sensing means, said means being disposed in said block in thermal contact with the point in the said circuit where the gas leaves the block.

2. A device as claimed in claim 1 which is provided with means for heating the block.

3. A device as claimed in claim 1 or claim 2 which is provided with means for cooling the block.

4. A device as claimed in any preceding claim wherein the block and at least part of the circuit is surrounded by a housing.

5. A device as claimed in claim 4 wherein the housing includes radiation shields.

6. A device as claimed in claim 4 wherein the housing is packed with an insulating powder and/or evacuated.

7. A device as claimed in claim 4, 5 or 6 wherein the part of the gas circuit housed within the thermally insulated environment includes a counterflow heat exchanger.

8. A device as claimed in any preceding claim wherein the block is made of metal.

9. A device as claimed in claim 8 wherein the block is made of copper.

10. A device as claimed in claim 8 or claim 9 wherein the at least one passage or channel in the block is filled with spheres, granules, wire mesh or knitted metal wool made of the same material as the block.

11. A device as claimed in any one of claims 8 to 10 wherein the internal surface of said channels and passages are plated with gold.

12. A device as claimed in any preceding claim wherein the circuit for the gas is adapted to pass air laden with water through the block in order to saturate the surface with said water.

13. A device as claimed in any preceding claim wherein the said circuit is adapted to recirculate calibration gas through the block after saturation of the surface with water.

14. A method of calibrating a humidity sensor comprising the steps of: (a) providing a block of material having at least one channel or passage therein, said material having a thermal conductivity greater than -l -l l0Wm K (b) saturating the entire surface area of the block with water, (c) heating or cooling the block to a predetermined temperature, (d) passing a gas across the saturated surface of the block until said gas is saturated with water and in thermal equilibrium with the block so that the dewpoint of the gas is identical to the temperature of the surface of the block, (e) passing the equilibrated gas of defined dewpoint to the humidity sensor to be calibrated, and (f) repeating steps (c) to (e) above at one or more other pre-determined temperatures to calibrate the humidity sensor.

15. A method as claimed in claim 14 wherein the block is a metal, preferably copper.

16. A method as claimed in claim 15 wherein the at least one channel or passage in the block is filled with spheres, granules, wire mesh or knitted metal wool made of the same material as the block.

17. A method as claimed in any of claims 14 to 16 wherein the surface area of the block is saturated with water by passing about 500 litres of ambient air across the surface at a temperature of between -200C and -500C.

18. A method as claimed in any of claims 14 to 17 wherein to avoid temperature fluctuations in the block, the feed for the incoming gas and the feed for the outgoing thermally equilibrated gas are disposed in a thermally insulated environment.

19. A method as claimed in any one of claims 14 to 18 wherein the temperature of the block is measured by a temperature sensor whose output is fed to an integrated microprocessor which also receives the output from the humidity sensor being calibrated.

20. A device as claimed in claim 1 and substantially as herein described with reference to Figures 1, 2, 4, and 5 of the accompanying drawings.

21. A method as claimed in claim 14 and substantially as described herein with reference to Figures 1, 2, 4 and 5 of the accompanying drawings.

22. A device for producing a gas mixture comprising at least one carrier gas and a condensable gas in which the vapour pressure of the condensable gas is defined comprising a block of material having at least one channel or passage therein to increase the surface area thereof, said material having a thermal conductivity greater than 10Wm 1K 1, means for passing a gas over the surface of said block and temperature sensing means being disposed in said block in thermal contact with the point where the gas leaves the block.

23. A device as claimed in claim 22 wherein the carrier gas is air and the condensable gas is water vapour.

24. A method of producing a gas mixture comprising at least one carrier gas and a condensable gas in which the vapour pressure of the condensable gas is defined comprising the steps of: (a) providing a block of material having at least one channel or passage therein and a thermal conductivity greater than, 1OWm 1K 1 (b) condensing the condensable gas over the entire surface area of the said block, (c) heating or cooling the block to a predetermined temperature and (d) passing at least one carrier gas across said entire surface area such that the carrier gas becomes saturated with the condensable gas in thermal equilibrium with the block and the vapour pressure of the condensable gas in the said mixture being a function of the temperature of the block.