GB2097130A

GB2097130A - Moisture measurement

Info

Publication number: GB2097130A
Application number: GB8209814A
Authority: GB
Original assignee: Moisture Control & Mesurement
Current assignee: Moisture Control & Mesurement
Priority date: 1981-04-04
Filing date: 1982-04-02
Publication date: 1982-10-27
Also published as: GB2097130B

Abstract

In the control of an industrial drying operation by reference to the moisture content of exhaust gas, automatic maintenance of a reliable control signal is provided in spite of derating through contamination of the moisture sensor (hygrometer) by systematically recalibrating the moisture sensor at intervals of time. A predetermined relationship between the moisture sensor output signal and the control signal is stored in a microprocessor, and this relationship is periodically varied in accordance with the condition of the moisture sensor in response to subjecting the moisture sensor to air sources of different known values of moisture content. A moisture content output signal may also be provided which is uninfluenced by the condition of the sensor. If, when using an air source having known moisture content, the moisture output signal varies by more than a predetermined amount in a given period, a warning signal is provided to indicate that the sensor is unsuitable for further use.

Description

SPECIFICATION Method and apparatus for moisture measurement The invention relates to a method of and apparatus for the measurement of the moisture content of the environment at a given location.

The invention is particularly but not exclusively concerned with the control of an industrial drying operation wherein parameters are controlled by reference to the measured moisture content of exhaust gas.

At a time when fuel is expensive it is economically very desirable to ensure that the parameters are accurately controlled so that the operation can be conducted with the maximum efficiency, but it is a matter of common experience that hygrometers and other moisture sensitive devices tend to de-rate with time owing to atmospheric contamination.

Hitherto, attempts have been made to improve the moisture sensitive devices so as to eliminate or minimize the derating thereof, but these endeavours having not met with complete success an alternative approach has been investigated.

According to one aspect of the present invention there is provided a method for controlling a process or operation by means of a control signal in accordance with the moisture content of the environment in the vicinity of a moisture sensitive device capable of producing an output signal according to said moisture content and the condition of the device, said method comprising the steps of producing a control signal in response to said output signal according to a predetermined relationship, and varying the relationship in accordance with the condition of the moisture sensitive device.

According to a further aspect of the present invention there is provided apparatus, including a moisture sensitive device, for controlling a process or operation by means of a control signal in accordance with the moisture content of the environment in the vicinity of said moisture sensitive device, wherein the moisture sensitive device is capable of producing an output signal according to said moisture content and the condition of the device, and said apparatus further comprises output means arranged to yield a control signal in response to said output signal according to a predetermined relationship, and means for varying the relationship in accordance with the condition of the moisture sensitive device.

Preferably the apparatus further comprises recorder means for recording said relationship and the relationship varying means are operative on the relationship as recorded.

Preferably the means for varying the recorded relationship are activated at predetermined intervals of time, and may comprise calibrating means for recording the output signal of the moisture sensitive device at one or more predetermined values of moisture content.

Preferably means are provided to produce a warning signal if the output signal at any of said predetermined values of moisture content varies with time by more than a predetermined extent, or fails to reach a predetermined level.

Embodiments of the invention will now be described by way of example and with reference to the accompanying drawings, of which:- Fig. 1 represents in diagrammatic form an industrial drying oven; Fig. 2 represents in diagrammatic form a control arrangement for the oven of Fig. 1.

Referring firstly to Fig. 1 an oven A containing material B to be dried is supplied through entry port D with air at temperature 50"C and a dew point of 20"C. Passing through the material B the air acquires moisture from the material and is exhausted at exit port E at a reduced temperature but increased dew point. A proportion of the exhaust air is voided to waste at Fwhilstthe remainder is recirculated by pipe P to re-enter the oven A at entry port D having been augmented by hotter dryer air entering pipe P through port G. In order to operate the drying process in a most efficient manner, it is commonly required to control the proportion of air voided at F and the amount of compensating air introduced at G according to the moisture content of the air exhausted at E.

In Fig. 2 block H represents a hygrometer or other moisture sensitive device located within the air exhausting from port E of Fig. 1. The initial sensitivity of the hygrometer is represented by the curve a according to which a moisture contentx produces an output signal of y.

The output of the hygrometer is fed to a microprocessor, one of the functions of which is to retain in a memory a relationship represented in Figure 2 by the curve as a function of output signal y. A further function of the microprocessor is to produce a control signalxx according to the recorded relationship. The control signal is fed to the controller C whereby the proportion of exhaust air voided at F and possibly other parameters are controlled.

In the course of time and according to experience, the sensitivity of the hygrometer H may vary to a condition represented by the curve da according to which the original moisture contentx yields a reduced output signal dy. This reduced signal would in turn by application of relationship curve yield a diminished control signal dxx, the control parameters would be varied to a condition not appropriate to the actual moisture contentx and the efficiency of the system would be reduced.

In order to compensate for the varied sensitivity of the hygrometer, it is arranged at intervals of time determined by the microprocessor to be connected, in turn, to sources of air of different known values of moisture content so that in the same sequence revised output signals are produced corresponding to the said known values which will, in general, be different from those originally yielded by the hygrometer in environments of those values of moisture content.

These revised output signals are fed to the microp rocessor so as to vary the relationship retained in the memorythereof in such a manner that the control signal produced from a hygrometer output signal dy will vary from the value dxx to, or at least in the direction of, the value.

In some embodiments it may be necessary to employ only one source of air of predetermined moisture content so to vary the relationship retained in the memory of the microprocessor, whilst in other arrangements where the sensitivity of the hygrometer is so complex and/or the requirements of the controller so precise that sources of air of three or more different predetermined values of moisture content will be employed.

The relationship retained in the memory of the microprocessor may be in the form of an equation d = fyn + K, wherein the values off, n and K are such that the equation corresponds to the sensitivity curve of the hygrometer. In another embodiment the relationship may be such as to yield a smaller or larger range of values of control signal for a given variation in output signal of the hygrometer.

The arrangement may be that the output signal from the hygrometer, when in an environment consisting of air from one of the predetermined sources, is monitored over a finite period, and if during that period the output signal varies by more than a predetermined amount, awarning signal may be provided to indicate that the hygrometer has derated to such an extent as to be too unstable for further use.

In a refinement of the apparatus, means may be provided for decontaminating the moisture sensitive device when necessary, for example when derating has become so extreme that the warning signal has been given. The decontaminating means may consist of vapour displacement, liquid washing, or heating means depending on the type of moisture sensitive device and the nature of the contaminant.

It will be appreciated that during the process of varying the relationship recorded by the microprocessor, the controller C may have to be disconnected from the microprocessor and in many cases the process can be carried out during periods in the drying cycle at which loss of control will cause least difficulty. In other arrangements it may be necessary to duplicate the combination of hygrometer, microprocessor and controller and arrange for one of these combinations to be put in operation whilst the other is being subjected to the process of varying the relationship recorded by the microprocessor.

The microprocessor may be provided with graphical, numerical or other display means whereby the extent of derating or any parameter or condition determined by the apparatus can be indicated.

Moreover, the microprocessor may be programmed to predict the final equilibrium condition of the moisture sensitive device, or the time to reach such condition, following a change in environmental condition, by reference to recorded response characteristics.

Whilst the apparatus described relates to the determination of moisture content of air, the invention may with appropriate variation be employed in circumstances in which the moisture content is measured in other gases, in liquids or solids or in mixtures of two or more of these materials.

It will be understood that although the control signal in the above described embodiments are applied to the control of a process or operation, it may in other circumstances be used merely for giv ing an accurate indication of the moisture content of the environment relatively uninfluenced by the con dition of the moisture sensitive device, and accord ing to yet further aspects of the present invention there are provided apparatus, including a moisture sensitive device, for producing a signal indicative of the moisture content of the environment in the vicin ity of said moisture sensitive device, wherein the moisture sensitive device is capable of producing an output signal according to said moisture content and the condition of the device, and said apparatus further comprises output means arranged to yield a moisture-content-indicative signal in response to said output signal according to a predetermined relationship, and means for varying the relationship in accordance with the condition of the moisture sensitive device, and a method for producing a signal indicative of the mositure content of the environment in the vicinity of a moisture sensitive device capable of producing an output signal according to said moisture content and the condition of the device, said method comprising the steps of producing a moisture-content-indicative signal in response to said output signal according to a predetermined relationship, and varying the relationship in accordance with the condition of the moisture sensitive device.

Claims

1. A method for controlling a process or operation by means of a control signal in accordance with the moisture content of the environment in the vicin ityofa moisture sensitive device capable of producing an output signal according to said moisture content and the condition of the device, said method comprising the steps of producing a control signal in response to said output signal according to a predetermined relationship, and varying the relationship in accordance with the condition of the moisture sensitive device.

2. A method according to Claim 1 wherein the relationship is varied at predetermined intervals of time.

3. A method according to Claim 1 or Claim 2 wherein a warning signal is given if the output signal at any predetermined value of moisture content varies with time by more than a pre-determined extent, or fails to reach a predetermined level.

4. A method according to any one of the preceding Claims further comprising the step of decontaminating the moisture sensitive device.

5. Apparatus, including a moisture sensitive device, for controlling a process or operation by means of a control signal in accordance with the moisture content of the environment in the vicinity of said moisture sensitive device, wherein the moisture sensitive device is capable of producing an output signal according to said moisture content and the condition of the device, and said apparatus further comprises output means arranged to yield a control signal in response to said output signal according to a predetermined relationship, and means for varying the relationship in accordance with the condition of the moisture sensitive device.

6. Apparatus according to Claim 5 further comprising recorder means for recording said relationship and the relationship varying means are operative on the relationship as recorded.

7. Apparatus according to Claim 5 or Claim 6 wherein the means for varying the relationship comprise calibrating means for recording the output signal of the moisture sensitive device at one or more predetermined values of moisture content.

8. Apparatus, including a moisture sensitive device, for producing a signal indicative of the moisture content of the environment in the vicinity of said moisture sensitive device, wherein the moisture sensitive device is capable of producing an output signal according to said moisture content and the condition of the device, and said apparatus further comprises output means arranged to yield a moisture-content-indicative signal in response to said output signal according to a predetermined relationship.

9. A method for producing a signal indicative of the moisture content of the environment in the vicinity of a moisture sensitive device capable of producing an output signal according to said moisture content and the condition of the device, said method comprising the steps of producing a moisturecontent-indicative signal in response to said output signal according to a predetermined relationship, and varying the relationship in accordance with the condition of the moisture sensitive device.

10. A method for controlling a process or operation by mean of a control signal in accordance with the moisture content of the environment in the vicinity of a moisture sensitive device capable of producing an output signal according to said moisture content and the condition of the device substantially as described with reference to the drawings.

11. Apparatus, including a moisture sensitive device, for controlling a process or operation by means of a control signal in accordance with the moisture content of the environment in the vicinity of said moisture sensitive device substantially as described with reference to the drawings.