GB2126350A

GB2126350A - Dew-point measuring device

Info

Publication number: GB2126350A
Application number: GB08322687A
Authority: GB
Inventors: Horst Waldschmidt
Original assignee: Endress and Hauser SE and Co KG
Current assignee: Endress and Hauser SE and Co KG
Priority date: 1982-08-25
Filing date: 1983-08-24
Publication date: 1984-03-21
Also published as: GB8322687D0; JPS5965246A; FR2532428A1; DE3231534A1

Abstract

The dew-point measuring device comprises a capacitive dew detector having electrodes (5,6) formed on a substrate (2) by the coating method, and comprises a resistance thermometer having a measurement precision resistor applied to the substrate for measuring the temperature of the substrate (2). In order to obtain high accuracy in a cheap and simple manner, the measurement precision resistor of the resistance thermometer is formed by at least one of the electrodes (5,6), e.g. of platinum or nickel. <IMAGE>

Description

SPECIFICATION Dew-point measuring device This invention relates to a dew-point measuring device comprising a capacitive dew detector, the electrodes of which are formed on a substrate by the coating method, and comprising a resistance thermometer, the measurement precision resistor of which is applied to the substrate for measuring the substrate temperature.

In the case of dew-point measuring devices of this type it is known that the temperature of the substrate is reduced by a suitable cooling device, until a condensation coating is formed on the substrate, and it is then controlled by a temperature control device in such a way that the condensation coating retains a predetermined constant thickness. The temperature of the substrate measured by the resistance thermometer is then the dew-point temperature. The condensation coating is ascertained capacitively on account of the fact that as a result of the large dielectric constant of water the condensation coating causes a considerable increase in the capacity measured between the electrodes of the dew detector. Capacitive dew detectors have a number of advantages over dew-point measuring devices which operate by determining the condensation coating optically.

They are considerably simpler, smaller, lighter and cheaper, mechanically more robust and less susceptible to contamination.

Dew-point measuring devices with a capacitive dew detector are known for example from the German Auslegeschrift (Published Specification) 22 35 212, the Swedish Laid-Open Specification 333820 and the paper "A new method of measuring and controlling humidity by electric cooling elements and capacitive sensors for dew" by D. Schreiber in "Measurement and Instrumentation, Acta Imeko 1973", Vol. II, Hungarian Academy of Sciences Publishing House. These known dew-point measuring devices have a number of problems which affect the accuracy of measurement. Usually the measurement precision resistance of the resistancethermo- meter, for example a Pt 100, is applied to the substrate beside the surface occupied by the electrodes of the dew detector.In this way errors in the temperature measurement can occur, since the substrate temperature is not always the same at all points on account of temperature gradients. The temperature must be measured, however, with a high degree of accuracy, since measurement errors of fractions of a kelvin result in displacements of the humidity measurement value by a number of percentage points.

Further disadvantages of the known capacitive dew detectors are that the entire surface of the substrate is not available for the dew detection and that special manufacturing steps are necessary in order to apply the measurement precision resistor to the substrate in addition to the electrodes of the dew detector.

The problems and disadvantages described are intensified when other temperature sensors are used instead of a measurement precision resistor for measuring the substrate temperature.

An object of the invention is to provide a dewpoint measuring device of the type described above, the dew detector of which may be produced in a simple, cheap and compact manner and which permits a high degree of measurement accuracy.

In accordance with the invention in that the measurement precision resistor of the resistance thermometer is formed by at least one of the electrodes of the capacitive dew detector.

By using at least one of the electrodes of the capacitive dew detector as the measurement precision resistor of the resistance thermometer the demands which are made upon the temperature sensor are met in an outstandiing manner. Measurement is carried out directly at the point at which the dew formation is ascertained, and the surface area of temperature measurement coincides with the surface area of dew detection. Any temperature gradients which may occur do not therefore affect the accuracy of measurement, since they have the same effect on the temperature measurement and the dew detection. In addition, the entire substrate surface may be utilized both for dew detection and for temperature measurement.

The simple manufacture of the dew detector is particularly advantageous, since the measurement precision resistor of the resistance thermometer is obtained in the same process step during the production of the electrodes of the dew detector without additional expense. On account of the smaller number of connexion electrodes the dew detector may also be exchanged more easily.

An advantageous embodiment of the invention consists in at least the electrode of the dew detector which forms the measurement precision resistor being made of platinum. The advantageous characteristics of platinum resistance thermometers, in particular their high degree of accuracy, high temperature range and long-term stability, are then obtained. It is also perfectly possible for the measurement precision resistor to be given the standardized value of a Pt 100 to 100 fl. Nickel may also be used instead of platinum.

If the surface of the substrate supporting the electrodes is covered, as in the case of capacitive dew detectors, by a protective layer in an hermetically sealed manner, the measurement precision resistor is also protected from corrosion, mechanical damage and contamination in the same way as the dew detector.

Further features and advantages of the invention are evident from the following description of examples of embodiment, which are illustrated in the drawing, in which Figure lisa plan view of a first embodiment of the dew detector of a dew-point measuring device according to the invention; Figure 2 is a sectional view of the dew detector according to Figure 1; Figure 3 is a block circuit diagram of a dew-point measuring device with the dew detector of Figure 1; Figure 4 is a plan view of a second embodiment of the dew detector; Figure 5 is a dew detector constructed in practice according to the first embodiment, and Figure 6 is the electrical equivalent-circuit diagram of the dew detector of Figure 5.

The capacitive dew detector 1 illustrated in plan view in Figure 1 and in section in Figure 2 comprises an insulating substrate 2 of ceramic material, on which conductor paths 3 and 4 of platinum are applied in accordance with one of the known processes of the thin-film method. The substrate 2 may for example consist of aluminium oxide, or even silicon oxide, which is particularly advantageous since it has approximately the same heat expansion coefficient as platinum. The conductor paths may be produced in conventional manner by a layer of platinum with the desired thickness of the conductor paths first being deposited on the entire surface of the substrate 2 by evaporation and the conductor paths subsequently being formed by the photographic etching technique.

The conductor paths 3 and 4form two combshaped electrodes 5, 6 disposed in the same plane and comprising intermeshing prongs and contact surfaces 7, 8, 9. The comb-shaped electrode 5 formed by the conductor path 3 has the usual shape of a comb-shaped electrode with a crossbar 5a which is joined to prongs 5b formed at a right angle thereto. The crossbar 5a is joined to the contact surface 7. In contrast, the comb-shaped electrode 6 formed by the conductor path 4 is made meandershaped. Each prong 6a, with the exception of the two outermost prongs, comprises two conductor path portions 6b which are arranged beside and parallel to one another and at one end are joined to one another by a short crossbar 6c and at the other end are joined to the conductor path portions of adjacent prongs by short cross-bars 6d.The free ends of the conductor path portions of the outermost prongs are joined to the contact surfaces 8 and 9 respectively.

The surface of the substrate 2 supporting the conductor paths 3,4 is provided with a protective layer 10, which covers at least the conductor paths completely hermetically. The protective layer 10 acts as a protection against corrosion and also allows the dew detector to be cleaned in a simple manner. It may consist of glass, or silicon oxide or silicon nitride which is applied by sputtering.

As known and conventional in the case of dewpoint measuring devices a cooling block 11 is provided, with which the dew detector 1 may be cooled to the dew-point. The cooling device may be in the form, for example, of a Peltier element (not shown) disposed in the cooling block. The cooling block 11 may, as is likewise known, also contain a heating device in addition to the cooling device, so that the temperature of the dew detector 1 may be kept very precisely at the dew-point by simultaneous control of the heating and cooling. If a Peltier eiement is used as a cooling device, it may also be operated as a heating device by reversing the poles.

Figure 3 shows the electrical block circuit diagram of a dew-point measuring device with the dew detector illustrated in Figures 1 and 2. The electrical equivalent-circuit diagram of the dew detector 1 is illustrated in the box shown in broken lines. The capacitor CM represents the capacity between the conductor paths 3 and 4, which occurs between the contact surfaces 7 and 8. The resistance RM is the resistance of the conductor path 4 between the contact surfaces 8 and 9.

In the manner customary in the case of dew-point measuring devices a capacity-measurement circuit 12, which supplies an output signal dependent upon the capacity CM to a temperature regulator 13, is connected to the contact surfaces 7 and 8. The temperature regulator 13 controls the cooling and heating devices contained in the cooling block 11.

The capacity-measurement circuit 12 may be formed in any way known per se. It may for example contain an oscillator, the oscillation frequency of which is affected by the capacity CM.

The characteristic of the dew-point measuring device illustrated in Figure 3 is that the conductor path 4 is used as the measurement precision resistor of a resistance thermometer. For this purpose a resistance-measurement circuit 14, which generates an output signal which is dependent upon the resistance RM and is indicated by a display device 15, is connected to the contact surfaces 8 and 9. The resistance-measurement circuit 14 may for example comprise a bridge circuit which contains the resistance RM in a bridge branch, or it may simply measure the voltage drop at the resistance RM, which is caused by a constant direct current passed over the resistance RM. Since the resistance RM depends upon the temperature of the dew detector 1, the display device 15 may be calibrated directly in kelvins.

The dew-point measuring device of Figure 3 operates in the following manner: When the apparatus is switched on the dew detector 1 is normally at the ambient temperature, which is usually above the dew-point. The capacity CM measured by the capacity-measurement circuit 12 is relatively small. The capacity-measurement circuit 12 supplies the temperature regulator 13 with a signal which causes the temperature regulator to control the cooling block 11 in such a way as to reduce the temperature of the dew detector 1.

When the temperature of the dew detector 1 reaches the dew-point a condensation coating is formed on the surface of the dew detector. On account of the high dielectric constant of water the capacity CM iS thereby changed. The control loop formed by the capacity-measurement circuit 12, the temperature regulator 13 and the cooling block 11 now keeps the dew detector 1 at a temperature which corresponds to a pre-determined value of the capacity CM. This capacity value corresponds in turn to a specific thickness of the condensation coating on the dew detector.

The temperature indicated by the display device 15 in this regulated state is the dew-point.

By using the conductor path 4 as the measurement precision resistor of the resistance thermometer the requirements made upon the temperature sensor for measuring the temperature of a dew detector are met in an excellent manner. Since the conductor path consists of platinum it permits the accuracy of measurement of a platinum resistance thermometer.

It is also perfectly possible for the conductor path to be given the standardized resistance of a Pt 100 of 100fl.

Measurement is thus performed with low resistance, so as to exclude failure on account of contact resistances. Furthermore, measurement takes place directly at the place at which dew formation is determined, there being a very good heat transfer between the condensation coating and the temperature sensor. Finally, the temperature measurement is performed over the entire surface on which dew formation is capacitively determined.

The simple manufacture of the temperature sensor is particularly advantageous, as it is carried out without additional expenditure during the manufacture of the dew detector. Since there are only three connexion contacts, the dew detector is more easily exchangeable, so that maintenance is simpiified. In the same way as the dew detector the temperature sensor is very well protected from damage, corrosion and contamination and it is easy to clean. On account of the materials used (ceramic materials, platinum) the dew detector may be used over a wide temperature range.

As already known in the case of dew-point mirror apparatus, the condensation surface may be cleaned of contamination, in particular by hygroscopic particles, by being periodically heated so that the dirt evaporates. In the case of the dew detector described, the conductor path 4 may be used for this purpose as a heating resistor.

Figure 4 illustrates another embodiment of the dew detector which allows the two conductor paths to be used as a measurement precision resistor for temperature measurement. In the case of the dew detector 21 of Figure 4 the two conductor paths 23 and 24 formed on the substrate 22 are made meander-shaped, so that each of them forms a comb-shaped electrode 25 and 26 respectively, which is in the shape of the comb-shaped electrode 6 in Figure 1. A contact surface 27 and 28 respectively is formed on one end of each conductor path 23, 24.

The other ends of the two conductor paths 23 and 24 are connected to one another with direct current by an inductive resistor 30, but are separate from one another with respect to high frequency. For the purpose of measuring the resistance with direct current the conductor paths 23 and 24 are connected in series between the contact surfaces 27 and 28 by way of the inductor 30. For measuring the capacity with high frequency, on the other hand, the inductor 30 is connected in parallel to the capacity present between the conductor paths 23 and 24. Where dimensions permit, the inductor 30 with this capacity forms a parallel-resonant circuit, the resonance frequency of which depends upon the capacity. This resonance frequency may likewise be measured between the contact surfaces 27 and 28.In this embodiment, therefore, the resistancemeasurement circuit and the capacity-measurement circuit are connected to the same contact surfaces 27 and 28, so that a connexion point is dispensed with.

The inductor 30 may be wound from wire and mounted directly on the substrate.

The design of the conductor paths according to Figure 4, but without the inductor 30, makes it possible for a conductor path, for example the conductor path 23, to be used as a measurement precision resistor and the other conductor path 24 to be used as a heating resistor. This permits the temperature to be monitored during heating and the heating to be switched off when a pre-determined temperature is reached. The heating resistor may also be used for temperature control in dew-point measurement if this is carried out by simultaneous heating and cooling.

Figure 5 illustrates a dew detector of the type shown in Figure 1 as constructed in practice. The same reference numerals as in Figure 1 have been used for designating corresponding parts. The substrate 2 consists of a silicon oxide wafer with a thickness of 0.3 mm and dimensions of 10 x 20 mm.

The conductor paths 3 and 4 of platinum have a thickness of approximately 1 Fm and a width of 0.2 mm. A glass layer of 25 m with the relative dielectric constant Er = 5.84 is applied as a protective layer.

The dew detector of Figure 5 has the following special features as compared with the simplified embodiment of Figure 1: A further contact surface 8a and 8b respectively is formed parallel to each of the two contact surfaces 8 and 9. This corresponds to a step which is customary in the case of measurement precision resistors of the type Pt 100 for the purpose of eliminating the effect of contact resistances during the resistance measurement. As indicated in the equivalent-circuit diagram of Figure 6, the measurement direct current I is passed over the contact surfaces 8, 9 and the voltage drop is measured at the contact surfaces 8a, 9a.

In position A two parallel conductor path portions of the conductor path 4 are joined to one another by conductor bridges 4a arranged at uniform intervals.

These conductor bridges shorten the length of the conductor path 4 acting as the measurement precision resistor, while they do not affect the capacity measurement. By removing or separating one or more of the conductor bridges 4a the effective length of the conductor path 4 may be increased in stages.

This permits a rough adjustment of the measurement precision resistance. The conductor bridges 4a may be removed or separated with the aid of a laser beam.

In position B two parallel conductor path portions of the conductor path 4 are joined by a continuous conducting layer 4b, which is shown cross-hatched for the sake of ciarity. The length of the conductor path 4 acting as the measurement precision resistor is likewise reduced by this conductive layer 4b. By removing part of the conductive layer 4b from the end, as indicated at 4c, the effective length of the conductor path 4 may be continuously varied. This permits a fine adjustment of the measurement precision resistance. The conductive layer 4b may likewise be removed with the aid of a laser beam.

In the case of the dimensions of the dew detector given above the conductor path 4 of the embodiment illustrated in Figure 5 has an effective length of 157.6 mm if all the conductor bridges 4a and the entire conductive layer are present. Since platinum has the resistivity p = 10-5 Q cm, this results in a resistance of 78.8 Q. By removing the conductor bridges 4a the effective length of the conductor path 4 may be increased by a maximum of 29.2 mm, as a result of which the measurement precision resistance is increased by 14.8 Q. Altogether, therefore, the resistance of the conductor path 4 may be set between 78.8 Q and 108.2 Q by the rough and the fine equalization. In this way it is possible to give the measurement precision resistance of the resistance thermometer the standardized value of 100 flat 0HC.

The capacity between the conductor paths 3 and 4 of the dew detector illustrated in Figure 5 amounts to 16.4 pF in the dry state and approximately 48 pF in the moist state, if the dew layer has been formed.

The dew detector may of course be varied in a number of ways. Thus production is possible with the thick-film method instead of the thin-film method. Instead of ceramic material another suitable material may be used for the substrate. The conductor paths and the protective layer may likewise be made from materials other than those mentioned above. Further variations are obvious to the person skilled in the art.

Claims

1. A dew-point measuring device comprising a capacitive dew detector having electrodes formed on a substrate by the coating method, and comprising a resistance thermometer having a measurement precision resistor applied to the substrate for measuring the substrate temperature, the measurement precision resistor of the resistance thermometer being formed by at least one of the electrodes of the capacitive dew detector.

2. A dew-point measuring device as claimed in Claim 1, wherein at least the electrode of the dew detector forming the measurement precision resistor is formed from platinum.

3. A dew-point measuring device as claimed in Claim 1, wherein at least the electrode of the dew detector forming the measurement precision resistor is formed from nickel.

4. A dew-point measuring device as claimed in any one of Claims 1 to 3, wherein at least the electrode of the dew detector forming the measurement precision resistor is produced by the thin-film method.

5. A dew-point measuring device as claimed in any one of Claims 1 to 4, wherein the or each electrode of the dew detector forming the measurement precision resistor is meander-shaped.

6. A dew-point measuring device as claimed in any one of Claims 1 to 5, wherein the electrodes of the dew detector are formed as comb-shaped electrodes arranged in one plane.

7. A dew-point measuring device as claimed in any one of the preceding Claims, wherein the two electrodes of the dew detector are connected in series with direct current for the measurement of resistance by an inductor.

8. A dew-point measuring device as claimed in any one of the preceding Claims, wherein the surface of the dew detector supporting the electrodes is covered in an hermetically-sealed manner by a protective layer.

9. A dew-point measuring device as claimed in Claim 8, wherein the protective layer is formed from glass.

10. A dew-point measuring device as claimed in Claim 8, wherein the protective layer is formed from silicon oxide or silicon nitride.

11. A dew-point measuring device as claimed in any one of the preceding Claims, wherein the conductor path portions of the electrode forming the measurement precision resistor are joined by conductor bridges which are disposed at intervals and which may be removed or separated in orderto adjust the measurement precision resistor.

12. A dew-point measuring device as claimed in any one of the preceding Claims, wherein the conductor path portions of the electrode forming the measurement precision resistor are joined by a continuous conductive layer which may be completely or partially removed in order to adjust the measurement precision resistor.

13. A dew-point measuring device as claimed in any one of the preceding Claims, wherein the electrode forming the measurement precision resistor is used as a heating resistor.

14. A dew-point measuring device as claimed in any one of Claims 1 to 12, wherein one electrode of the dew detector is formed as a measurement precision resistor of the resistance thermometer and a further electrode of the dew detector is formed as a heating resistor.

15. Adew-pointmeasuring device substantially as herein described with reference to any one of the embodiments shown in the accompanying drawings.