Humidity measurement arrangement and cooking oven provided therewith

A food cooking oven, particularly for use in commercial or collective food service applications, includes a cooking cavity and a heat generating device. A fan is provided for fan-assisted cooking and is adapted to transfer heat to the inside of the cooking cavity. A zirconium-oxide cell is located at a wall of the oven and has one of its sides open towards the outside atmosphere and the other one of its sides open towards the inside of the cooking cavity and is capable of measuring the relative concentration of oxygen within the cavity. The terminals of the zirconium-oxide cell are connected to an indicator, which is arranged so as to be visible from the outside of said oven. The cell terminals are also connected to a controller, which preferably includes at least one connected electronic microprocessor unit and is arranged so as to be able to automatically determine the moisture content of the gas mixture inside the cooking cavity.

BACKGROUND OF THE INVENTION 
The present invention relates to a cooking oven, in particular an oven for 
cooking food, provided with a moisture measurement arrangement so as to 
enable the degree of humidity in the cooking cavity to be automatically 
adjusted to a pre-set level during food cooking processes. 
In the following description of the present invention, reference is made in 
particular to an oven for cooking food in catering or food service 
establishments. It will, however, be appreciated that what is described 
and claimed here equally applies to any other type of food cooking ovens. 
Food cooking ovens are already known that are substantially formed of a 
box-like casing defining an inner cooking cavity in which the food to be 
cooked is properly arranged. A forced-draft hot air circulation is 
generated in said cooking cavity by means of a gas-fired or electric heat 
generator and at least one fan. These food cooking ovens are provided with 
appropriate thermostatic arrangements for automatic adjustment of a 
selected food cooking temperature. These thermostatic control means are 
associated with corresponding temperature selection means which are 
adjusted by the user to the desired temperature setting before each 
cooking process is started, in accordance with the type and the amount of 
food to be cooked. The thermostatic temperature control means of these 
ovens operate in such a manner as to keep the cooking temperature within 
pre-determined limits inside the cooking cavity of the oven, generally by 
cyclically switching on and off the heat generator and/or a steam 
generator if the oven is designed to also cook by steam. The fan is also 
controlled so as to appropriately vary the flow of hot air circulating 
through the cooking cavity. 
These cooking ovens, however, are not provided with any arrangement for 
adjusting the humidity of the air inside the cooking cavity. Humidity of 
the air being in this case a factor that, combined with the temperature, 
decisively affects both the quality of the cooking process as a whole and 
the taste of the food being cooked. Since said humidity is variable 
depending on a number of parameters of the cooking process, such as the 
cooking temperature, the type and the actual amount of food to be cooked, 
the replacement rate of the air in the cooking cavity, and so on, these 
appliances are unable to ensure desirable food cooking results. On the 
other hand improved cooking would be achieved in a most desirable manner, 
under the same performance conditions, if such a humidity adjustment 
function could be carried out in the process. 
Humidity control and measurement devices are known in connection with a 
number of applications of different kinds. They generally include 
semiconductor-type sensors, which are arranged in humid rooms or 
compartments to be controlled and are connected with electric bridge-like 
measurement circuits that are adapted to detect corresponding electric 
signals generated by said sensors so as to automatically convert the 
signals into respective levels of humidity in the room being measured. 
Other types of moisture measurement arrangements include elements for 
measuring concentration of oxygen in gaseous mixtures, such as 
zirconium-oxide cells, operating according to well-known principles. 
However, all such humidity measurement devices and arrangements, while 
ensuring accurate and correct measurement results, are generally poorly 
suited on applications involving the measurement of humidity in a food 
cooking environment, where temperatures prevail that may be anywhere 
between approximately 100.degree. C. and 250.degree. C. according to the 
type and the amount of food to be cooked. These devices can be damaged, 
and therefore unable to correctly perform their duty any longer, and their 
sensing elements are subject to soiling causing alteration of the related 
measurement values. 
SUMMARY OF THE INVENTION 
It is therefore a purpose of the present invention to overcome the 
drawbacks and the limitations of the afore cited humidity measurement 
devices and arrangements by providing a humidity measurement arrangement 
in a food cooking oven according to the appended claims and the following 
description.

DETAILED DESCRIPTION OF THE PREFFERED EMBODIMENTS OF THE INVENTION 
Referring now to FIGS. 1 and 2, an interior of a cooking cavity 2 of a food 
cooking oven, for example, for professional kitchens, includes a 
motor-driven fan 1 arranged on a rear side of said cooking cavity 2. 
Appropriate arrangements (not shown) to generate and propagate microwaves 
inside said cooking cavity can also be provided, for instance of the type 
described in the European patent application no. 90119715 filed by the 
same applicant. 
Food contained in said cooking cavity may be processed in a number of 
different ways, such as for instance by forced-draft circulation of hot 
air produced by suitable gas-fired or electric heat generating means (not 
shown) of some known type, by steaming, by a combination of steaming and 
forced-convection hot air, or by microwaves alone or combined with 
forced-convection hot air and/or steam. 
The oven also comprises a humidity measurement arrangement 3, such as a 
probe, according to the present invention, which is shown housed in the 
cooking cavity 2 and is arranged to operate in a way that is described 
below. Such an arrangement substantially includes probe means adapted to 
measure partial pressure of oxygen inside the cooking cavity with respect 
to the external environment. Said probe means are appropriately arranged 
and insulated to exclude any interference by the other gases present in 
the cooking cavity. It is well known that, based on such a measurement, 
the value of the humidity prevailing in the cooking cavity can then be 
inferred. 
The state of the art in this particular field is briefly recalled here. A 
zirconium-oxide based probe for detecting oxygen concentrations generally 
includes a cell that features a pair of electrodes (usually of platinum) 
applied thereto, and which is kept at a constant temperature T (normally 
350.degree. C.). "Reference air" with a known and constant oxygen 
concentration is delivered on one of the two sides of the cell. By 
measuring with a suitable millivoltmeter the electromotive force of the 
cell, a direct indication of the concentration of oxygen in contact with 
the cell, on the "measurement" side thereof, is obtained. The above 
described method has for instance has found a wide application in the 
measurement of oxygen concentration in flue gases. On the other hand, it 
may be also used advantageously for measuring humidity in hightemperature 
air-steam mixtures. 
Based on the assumption that the oxygen/nitrogen ratio in the air is 
constant, a measurement of the concentration of oxygen enables the 
percentage of a third gas, water vapor in this particular case, to be 
determined directly when the third gas is added to a sample of the air 
being measured. In fact, the presence of a third gas added to a sample of 
air has the effect of reducing in a proportional manner the presence of 
oxygen and nitrogen in the sample, as is shown in FIG. 5. As a result, 
determining the relative concentration of oxygen in such a sample, or in a 
particular room which was originally filled only with air, such as the 
interior of a cooking cavity at the beginning of the cooking process, 
allows the amount of gases, such as water vapor, introduced in and 
differing from the original components of the mixture to be determined as 
well. 
Therefore, by using a normal zirconium-oxide probe along with an 
appropriate converter means it is possible to measure the humidity 
prevailing in air/steam mixtures at temperatures of up to approximately 
600.degree. C. It has been observed that the output signal from the probe 
is not affected to any significant extent by the variations in the 
temperature and humidity of the ambient air used as a reference. 
Returning to FIGS. 1 and 2, the oven may further be provided with a boiler 
4 for steam generation purposes, as well as an appropriate conduit 5 for 
delivering said steam from the boiler 4 to the cooking cavity 2. 
The oven may be additionally provided with a flue riser 6 for exhausting 
flue gases and excess vapors, as well as a venting shutter 7 for shutting 
said flue riser for the purpose of adjusting the internal atmosphere in 
the oven cavity. The oven may be further provided with a conduit 8 for 
letting in water against blades of the fan 1 so as to produce an 
atomization effect when the water comes into contact with the heating 
elements. A corresponding shut-off valve 9 is associated with said conduit 
8. Finally, a drain conduit 26 is connected to the bottom of said cooking 
cavity so as to enable residual liquids, such as fat, water and other 
cooking by-products, to be conveniently removed therefrom. 
The present invention provides a cooking oven with a humidity measurement 
arrangement as described. The arrangement 3 and the cooking oven are 
adapted for a mutually compatible operation so as to optimize the 
performance capabilities of the oven. 
Referring now to FIG. 3, the humidity measurement arrangement 3 includes 
zirconium oxide (ZrO.sub.2) based solid electrolyte 11, a layer of porous 
ceramics 12 protecting a porous platinum electrode 10 against corrosive 
agents, and a resistive element 27 to heat the zirconium oxide to a 
temperature of approximately 350.degree. C. in view of increasing its 
electric conductivity. 
This arrangement is accommodated in a recess 15 provided, preferably by 
press forming, in a wall 16 of the cooking cavity of the oven. The 
arrangement also comprises a surface of porous platinum 17 having a sealed 
tubular form which, like the afore cited electrode 10, acts as a 
catalytic-effect electrode to amplify the electromotive force that is 
generated. due to the difference between the partial pressure of the 
oxygen in the ambient air and the partial pressure in the cooking cavity. 
Fat and other soiling particles from cooking ingredients could settle upon 
the internal electrode of said probe during the use of the oven, thereby 
sealing up the electrode and, as a result, putting the probe out of 
service. Thus, a fat filter or protective baffle 14 (such as a labyrinth 
or metal gauze) with fat and soil retaining properties is removably 
provided at the entrance 13 of the recess 11 that houses the probe. 
In order to be removable, such a protective baffle requires some 
constructional components for assembly and disassembly. In view of 
eliminating such practical drawbacks, another embodiment of the present 
invention is further proposed. This is illustrated in FIGS. 6 to 8 which 
show that the measurement arrangement 3 is housed in the recess 15 
provided in the wall 16 of the cooking cavity 2. However, the recess 15 is 
provided in a re-entering portion 35 (FIG. 7) of the wall 16. Said 
re-entering portion is partially covered by a cover 31 so that hollow 
spaces or double-wall configurations 32 are obtained between said cover 31 
and said re-entering portion 35. 
Slits M1 and M2 through which said hollow spaces open towards the interior 
of the cooking cavity are provided to enable steam to diffuse so as to 
reach the probe 3. The lower slit M2 serves additionally the purpose of 
ensuring draining of washing or condensation liquids that seep through the 
other slit M1. 
In a most advantageous manner, the cover 31 is arranged in such a way that 
said slits M1 and M2 are flush with the wall so as to avoid affecting the 
flow generated by the fan arrangement. Furthermore, the cover is oriented 
in such a manner that the slits are arranged orthogonally with respect to 
air flow R which is generally conveying fat and other soil particles that 
may occlude the probe. 
For obvious reasons, particles tend to follow the trajectory of the air 
flow R without penetrating into the slits, whereas humidity diffuses 
quickly into the hollow space 32 and, from there, into the interior of the 
recess 15. Humidity reaches the probe without being assisted by air 
circulation. 
It is possible that, during operation transients or during phases provided 
to wash the cooking cavity, soiling particles will seep through the hollow 
space 32 and, from there, enter the recess 15. In order to prevent such 
particles from being deposited onto the probe, it is advantageous to 
provide a labyrinth-type filter, which may for instance, include a 
plurality of walls 33, 34. The walls 33, 34 force the air flow to follow a 
path extending partly backwards between the entrance 13 of the recess and 
the probe, so as to stop the flow of particles. 
In a preferred manner, such a filter is made, as shown in FIG. 8, so that a 
preferential air-circulation path S inside the filter is orthogonal with 
respect to the direction of the air flow coming from the hollow space 32 
and making its way into the recess 15, thereby maximizing the stoppage 
effect. 
It will be readily appreciated that a timely and reliable measure in the 
form of an analog electric signal which is representative of the humidity 
prevailing in the cooking cavity of the oven can be used to generate 
indications for display outside the oven and also to automatically control 
some actuating members. For instance, the signal can be used for switching 
the boiler 4 on and off, adjusting the venting shutter 7 of the flue riser 
6 and controlling the shut-off valve 9 of the water conduit 8, so as to 
generate and maintain pre-selectable temperature and humidity conditions 
inside the cooking cavity. 
A flow-chart illustrating the operation of the various members is shown in 
the form of a block diagram in FIG. 4. The measurement arrangement 3, may 
be defined as a lambda probe in accordance with international conventions 
designating the lambda excess air factor as the ratio of the value being 
examined to the stoichiometric value of the combustion air. The probe is 
connected to a voltage meter 21 supplying an analog/digital voltage 
converter 22 which in turn sends the converted signal to an appropriately 
programmed processing and control means 23. The control means is adapted 
to both supply an appropriately provided display 24 on the outside of the 
oven with signals indicative of the detected humidity contents in the oven 
cavity, and receive humidity-degree signals as set through appropriate 
adjusting means 25. 
A particularly advantageous opportunity created by such a function lies in 
the possibility of said processing and control means 23 automatically 
adjusting the humidity inside the cooking cavity of the oven to the 
pre-set value by constantly processing and comparing said pre-set value 
with the value being detected by the measurement arrangement 3. Then, with 
any of a number of well-known state-of-art techniques, the processor 
controls the operation of the boiler 4, the water inlet valve 9 and the 
venting shutter 7 accordingly, so as to automatically attain and maintain 
said pre-set value. 
The oven is of course provided with a number of further elements and 
arrangements which, due to their not being an object of the present 
invention, are omitted. It will be appreciated that, although the present 
invention has been described in an example of a preferred embodiment 
thereof and using a generally known terminology, it nevertheless cannot be 
considered as being limited thereby, since anyone skilled in the art will 
be able to make a number of variations and modifications pertaining to 
both the construction and the shape of the arrangement according to the 
present invention.