GB2207514A - Apparatus for measuring atmospheric humidity in ovens - Google Patents

Abstract

Apparatus for measuring atmospheric humidity in an oven (11), comprising a sampling head (3) within the oven having an external surface adapted to receive condensed moisture thereon, means (5,6) for cooling the external surface without disturbing the temperature conditions in the oven, a temperature sensor (4) for sensing the temperature of the external surface, and an electronic controller (9) outside the oven adapted to control the cooling means and to receive and store time-dependent temperature data from the temperature sensor whereby humidity can be measured by estimating the point of distortion of the rising temperature/time curve after interruption of the cooling means. <IMAGE>

Description

Apparatus for measuring atmospheric humidity in ovens This invention relates to apparatus for measuring atmospheric humidity in ovens, in particular baking ovens.

Many bakers believe that the proportion of water vapour in the atmosphere inside a baking oven is a major factor in setting some of the qualities of the product.

In the case of bread, for example, both the volume and glaze of the loaf are related to humidity and, in I act, steam is often injected into the first section of the oven for this very reason. Steam is an expensive commodity and a ready means of judging the correct level would improve the bakery economy by cutting out excessive consumption. A similar situation applies to some biscuits for which steam injection into the oven is required for the creation of the traditional surface finish. Water sprayed into the oven is not much (if any) cheaper than steam because extra fuel has to be used to supply the latent heat of evaporation.

For certain other products, the level of humidity occuring naturally within the baking chamber is also important to properties of the product, such as the thickness of some semi-sweet biscuits. In this case, the control of humidity via the agency of the extraction dampers perhaps could be valuable for helping to maintain more consistent properties of the product. The commercial value of such a controller is not as easily assessed as that of saving steam, but in terms of enhanced reputation might be considerable.

For these reasons, several attempts to build instruments for the on-line measurement of atmospheric humidity inside baking ovens have been made, some of them quite successfully from a technical point of view.

They have not been taken up by the baking industry on a commercial scale for a number of reasons ranging from unreliability to high prices. These setbacks have their origins within the oven itself, whose elevated temperature, high turbulence and contaminating substances make the use of normal hygrometric methods impossible.

Most of the methods used to overcome the problems have relied on drawing a sample of air from the oven through a pipe and lowering its temperature en route. This is a difficult operation to perform without adversely affecting the performance of the instrument. For example, in the presence of high humidity values it is very difficult to avoid over-cooling the pipe, giving rise to premature precipitation and false readings, or even the complete shut-down of the instrument. Some of the sensors used in these instruments have also been sensitive to small deposits of contaminants from the oven such as fat and sugar. The only instrument which substantially overcame these problems was one which actually chilled the water (plus contaminants) out of the withdrawn sample of atmosphere in order to make measurement.In all cases, however, the use of a suction pump and heavily insulated sampling pipes necessarily leads to a somewhat bulky and expensive device.

The instrument according to the present invention overcomes- these problems to a considerable extent by installing the sensor actually in the oven, preferably in the baking chamber itself, at a point where the atmosphere is assumed to be representative of the general mass of atmosphere in that section-of the oven.

The consequent elimination of sample-withdrawing mechanisms cuts an appreciable amount from the cost, prevents interference with turbulence patterns in the baking chamber and lends itself to greater flexibility in mounting the equipment. It is also reasonable to claim that, because the oven atmosphere is not being drawn through narrow pipes, the accretion of fatty and other deposits will be greatly reduced. It will also become clear later in this description that, since the surface presented to the oven is simply inert metal, its tolerance of deposits will be substantial. Cleaning the surface by robust methods such as abrasion and jets of steam will also be possible.

The principle by which the instrument operates is a variant on the dewpoint method . This well-known method is not in common use at ambient temperatures because other methods such as the wet-dry bulb hygrometer and modifications of the hair hygrometer are so much more convenient. It does however have the merit of being absolute, ie calibration is not necessary, at least in its classical form, eg Regnault's Hygrometer.In essence the instrument possesses a mirrored surface, one side of which is exposed to the unknown atmosphere and the other can be subjected to a controlled regime of heating and cooling. The mirror also possesses a means of measuring its temperature either directly or inferentially.When in operation the mirror is gradually cooled until a mist is seen to form on the outside and the temperature of the mirror at this point gives an estimate of the dewpoint from which the specific humidity of the atmosphere can be calculated.

A more accurate estimate may be made if the rate of heating of the mirror can be controlled to a low rate and a second dewpoint is taken as the temperature at which the mist just disappears. The best estimate is then taken as the mean of the two assessments. This explanation of a well-known principle is given to emphasize the fact that the key to this method is the recognition of the dewpoint when it occurs. For on-line instruments the recognition must be made by an automatic means and there is no shortage of these.

The most obvious is the use of collimated light and a photo-electric cell which loses signal when the reflected light changes from specular to diffuse.

A capacitance method of detecting a film of moisture has also been successfully used and there are semiconductor devices which change their properties in response to the presence of water or high atmospheric humidity. All these methods will work well in favourable conditions but are adversely affected by high temperatures or contamination or both.

The method proposed in this invention relies on the idea of an endothermal distortion to indicate a change of state from dew to vapour. In its simplest form the instrument will not be as accurate as, say, a photo-electric method, particularly at low humidity values, but it will be reproducible and sufficiently accurate for industrial purposes.

The present invention thus provides apparatus for measuring atmospheric humidity in an oven, comprising a sampling head within the oven having an external surface adapted to receive condensed moisture thereon, means for cooling the external surface without disturbing the temperature conditions in the oven, a temperature sensor for sensing the temperature of the external surface, and an electronic controller outside the oven adapted to control the cooling means and to receive and store time-dependent temperature data from the temperature sensor, whereby humidity can be measured by estimating the point of distortion of the rising temperatureltime curve after interruption of the cooling means.

Reference is now made to the accompanying drawings, in which: Figure 1 is a schematic representation of an instrument according to an embodiment of the invention; Figure 2 is a temperature/time curve illustrating the use of such an instrument; and Figure 3 is a temperature/time curve and derivative curves which may be used in computation of the dew point.

In this Patent Application the invention will be described in a simple form. This is the form likely to be of greatest interest because of its low costs and ruggedness; but elaborations are possible which in general would be aimed at extending the range downwards. A schematic representation of the instrument is shown in Figure 1. Item 1 is a box mounted within the oven baking chamber 12, the wall of the chamber represented as 11. The box 1 is completely sealed from the oven atmosphere. Its body 2 is made from a rigid thermally insulating material, whereas its cover-plate 3 is made from a thermally conductive material such as copper, aluminium or even a suitable brass. The means of fixing 3 to 2 is not shown.Attached to the inner side of the cover 3 is a temperature-sensing element 4 such as a thermocouple or a surface-mounting platinum resistance element; it must be in intimate thermal contact with 3.

The electrical connections from 4 are led out of the body of the box 2 via a suitable sealing gland, not shown.

Also connected into the body of the box 2 via sealing glands are air pipes 5 and 6 both of which are led out through the oven wall 11 to the exterior and both of which are thermally insulated within the oven by sheaths 13. Pipe 5 is the air inlet pipe.

Outside the oven it is connected through an electricallyoperated control valve 7 to the compressed air supply of the bakery, here represented as a storage cylinder 10.

The pressure of such a supply may typically be set at about 5 Bar (751b/sq.in) and to match the requirements of the instruments it may have to be connected through a pressure reducing valve, not shown. Similarly, a filter may have to be connected in the line to remove excess water and/or oil; this also is not shown. The pipe 6 is merely an outlet, and is taken to exhaust outside the oven to avoid disturbances to the oven atmosphere, but in some circumstances, exhaust might be permitted within the oven. Outside the oven, the temperature sensor 4 and the electric activator 8 of the valve 7 are connected electrically to a controller 9, which typically will be a small digital computer or similar digital controller.

For fail-safe reasons, the air supply would be shut off from the lead-in pipe 5 when the actuator 8 is de-energised.

The instrument is cyclical in action, the cycle being imposed by the program of the controller 9. At the beginning of a cycle, the temperature of the cover-plate 3 will be well above the dewpoint of the atmosphere in the oven. Of course, if this is the first cycle after the instrument has been switched on, the temperature of 3 will be close to that of the air in the oven. The first act of the cycle is to energise the valve actuator 8 so that the valve opens and compressed air is connected into the box 1 via pipe 5, and a high-speed stream of cold air is forced across the inner face of the cover-plate 3, thus forcing the temperature down. The cooling power of this air stream is, in fact, sufficient to force the temperature of the cover-plate 3 well below the dewpoint of the oven atmosphere.Water therefore begins to condense on the outer surface of cover-plate 3, and given favourable circumstances, will accumula-tethere. Favourable circumstances would include positioning the box 1 so that the outer surface of the cover 3 is horizontal, facing upwards so that the dew has no tendency to run off. The actuator 8 is then de-energised, the valve closes and the supply of cooling air is cut off. The - FrD coverplate 3, being exposed to oven conditions, will take up heat and its temperature will begin to rise.

The rising temperature is recorded by the controller 9 and the temperatures stored in its memory, at the rate of, perhaps, four readings per second. When the water on the cover-plate is totally evaporated, the cycle is at an end and the controller can either start immediately on the next cycle or go into a waiting period if frequent cycling is not programmed. Assuming it does go immediately into the next cycle, the cooling air is switched on and since the cooling process will take several seconds, the controller has time available to go into a data-processing routine in which the array of temperatures from the previous cycle are analysed.

An alternative form of the invention uses an external source of water which can be deposited on the upper surface of the coverplate at the beginning of a new cycle.

To avoid deposits of scale, the water should be free from dissolved solids. In this form of the invention, the temperature of the coverplate can be forced down below the dewpoint by the water itself, provided the water is delivered at a low enough temperature to the coverplate. This obviates the need for a supply of cooling air.

Analysis of the data depends on the fact that if the oven atmosphere is effectively dry, the rising temperature of the coverplate 3 will follow a curve in time described quite closely by the following expression: T(t) = To + (Ti-To) exp(-Kt) where: t is time in seconds measured from the instant the cooling air was switched on T(t)is Temperature of cover-plate at time t To is Temperature of the oven atmosphere Ti is Temperature is cover-plate at t=O K is The heat transfer coefficient from the oven atmophere to the coverplate, divided by the thermal mass of the coverplate; it is taken as a constant for the duration of the temperature curve.

A typical curve of this kind is shown in Figure 2. If the oven atmosphere contains a substantial amount of water, at a level, for example, of 15% specific humidity,dew will begin to precipitate on the cover-plate at temperatures below the dewpoint, about 620C in this instance. The cooling air might take the temperature down to6 say, 400C. After the cooling air is shut off, the rise in temperature will follow a path like the firm line of Figure 2.

Initially the rise will be faster than that of the "dry" curve because the continued precipitation will increase the rate of heat transfer above that of the "dry" level.

As the temperature rises up towards the dewpoint however, the rate of rise diminishes sharply because heat is now being taken up by evaporation of water from the cover-plate.

This is shown as a flattening of the curve, a point of inflexion. In extreme cases, the slope of the curve attains the value of zero for a short while. As soon as the water is evaporated off, the temperature curve takes up a shape parallel to the "dry" curve.

The controller 9 begins its analysis by subjecting the array of temperature points to a numerical smoothing routine, thereby reducing the point-to-point random variation which could lead to error in the computation.

A suitable smoothing method can be effected by a running weighted mean of a suitable number of points, the sum of the weights being exactly equal to unity. Assuming five points, the process might be described mathematically as: Ts(t)=O. lixT(t-2)+0.22xT(t-1)+0. 34xT(t)+O.2xT(t+l)+O1lxT(t+2) where Ts(t) is the smoothed temperature at time t.

Then the controller enters a routine to estimate the dewpoint temperature. This is done, in essence6 by locating the point of inflexion and applying a correction to allow for a departure of the temperature curve from the expected shape, induced by the high heat-flux in the oven.

The whole computation may be performed by a routine here described. After the initial smoothing pass, the timederivative of the curve is computed by any suitable numerical method. Methods of numerical differentiation are described in the book Computing Methods for Scientists and Engineers by L.Fox and D.F Maters, published by Clarendon Press, Oxford, 1968; in particular, the Lagrangian method, which was used in trials, is described in Chapter 8, paragraphs 9 to 11. The derivative curve is then subjected to a smoothing pass, a further differentiation performed, and a final smoothing pass made. Figure 3 shows the results of such a computation carried out on data from an actual trial in a pilot-scale oven, using an experimental probe.

The smoothed temperature-time curve is drawn as continuous line, the smoothed first derivative as a dashed line and the smoothed second derivative as a dotted line. The point of inflexion of the temperature curve is at A, and this corresponds to a minimum of this first derivative and a zero of the second derivative at B. Both points B become less well marked as the atmospheric humidity decreases and are therefore difficult to detect by an automatic computational method. However, the minimum of the second derivative at C is a very strongly marked feature right down to the lowest levels of humidity found in baking ovens, and can therefore always be reliably detected by an automatic computational method.If the value of the second derivative is T"min at C in Figure 3j then the dewpoint temperature is taken to be that temperature at which the modulus of the value of the second derivative falls below T"min divided by 8. The divisor value of 8 was arbitrarily chosen; the computed dewpoint temperature is insensitive to the divisor value because the temperature of the coverplate is changing slowly at the point of inflexion. The point C is not arbitrary, it is related to-the cessation of condensation on the coverplate and the beginning of evaporation; it may be regarded as the threshold of the evaporation period. It is therefore a reliable pointer to the imminence of the dewpoint.

Once the dewpoint temperature has been estimated, the controller 9 can go on to convert it to a humidity value, usually expressed as Specific Humidity in percentage form. This is a straight-forward arithmetical process and can be performed in the controller 9 either by direct calculation (preferably) or by storing a table of dewpoint temperatures against specific humidity values and using a form of tabular interpolation where necessary. The specific humidity can then be displayed directly at the controller and/or converted to an electrical signal which can be used to motivate remote displays and humidity controllers.

The cycle time of the instrument is dictated largely by the ability of the compressed air to cool the cover plate 3. It is expected that the shortest achievable cycle time will be about 30 seconds.

Because of these long cooling periods, the controller 9 is under-used and could therefore control more than one sampling head. Since sampling heads would be substantially cheaper than controllers, a multiple-head system could give considerable cost advantage.

This invention thus provides an improved means of measuring atmospheric humidity within baking ovens or any devices within which'the temperature is high and there may be risk of contaminating deposits. The invention has two parts, an electronic controller outside the oven and a sampling head within the oven. The sampling head has a metallic member with a temperature sensor attached. The temperature of this metallic member can be forced down by a cooling medium, conveniently compressed air. Humidity is measured by estimating the point of distortion of the rising temperature curve after the cooling medium is switched off.

The instrument can be incorporated into a system for regulating the humidity within the oven by controlling the inlet of steam or the opening of extraction dampers.

Claims (5)

CLAIMS:

1. Apparatus for measuring atmospheric humidity in an oven, comprising a sampling head within the oven having an external surface adapted to receive condensed moisture thereon, means for cooling the external surface without disturbing the temperature conditions in the oven, a temperature sensor for sensing the temperature of the external suface, and an electronic controller outside the oven adapted to control the cooling means and to receive and store time-dependent temperature data from the temperature sensor whereby humidity can be measured by estimating the point of distortion of the rising temperature/time curve after interruption of the cooling means.

2. Apparatus according to claim 1, in which the external surface is a metal plate.

3. Apparatus according to claim 1 or 2, in which the cooling means is acompressed air supply directed within the sampling head on the internal face of the external surface.

4. Apparatus according to claim 3, wherein the compressed air supply is controlled by a valve actuated by the electronic controller.

5. Apparatus for measuring atmospheric humidity in an oven, substantially as hereinbefore described with reference to the accompanying drawings.