Controlling the indoor climate is in itself a difficult task since the desired result is inherently difficult to define. This not only applies to the method by which the desired result is obtained but also to the desired goal which is equally difficult to define as it depends on the parameters chosen and determined with the purpose of obtaining an optimal climate for the individual person.

Examples of controllable variation parameters may be temperature, humidity, air circulation speeds in indoor rooms and radiation asymmetry generated by e. g. windows or floors.

Apart from that, an adjustment of some of the above- mentioned parameters may influence the other parameters in an undesirable direction or at least towards a non- predictable end condition.

Similarly, it must be kept in mind that there is often only a limited number of options available with which a desired goal may be obtained. Examples of such options may be heating panels, radiators etc. for temperature regulation, actual air-conditioning, ventilation etc.

Yet another and quite significant problem is that most methods of regulation result in a significant increase in the consumption of energy which is the reason why all

methods of regulation must often be weighed against a financial element. An intervention and the result of such an intervention in the existing indoor climate must also be weighed against the size of the costs it may cause. An example of such a deliberation is ventilation of an indoor climate with air from the outside since the achieved result-which is cleaner and better air- should be weighed against the temperature variation and consequently the need for reheating or cooling that may arise.

A problem in connection with many indoor environments is that high humidity provides improved living. conditions for e. g. dust mites. Since a large part of the population suffers more or less from allergy towards dust mites, it is necessary to prevent the existence of the dust mites to the extent that it is possible. This could typically be done by ventilating air with the purpose of reducing relative humidity below a given limit value.

Meanwhile, it is often difficult to determine whether ventilation is advisable or not since a reading of the outdoor relative humidity does not provide an estimate of whether ventilation is advisable or not. An outdoor relative humidity level above the indoor relative humidity level may very well cause a reduction of indoor humidity since relative humidity contents in the air depend on temperature. Even when using a hygrometer showing both indoor and outdoor relative humidity, it is extremely difficult to get an impression of whether ventilation may cause a reduction of indoor relative humidity.

Background of the invention When, as stated in claim 1, the hygrometer comprises an input for a first temperature representation signal and an input for a first humidity representation signal, said hygrometer comprising means for computing of relative humidity as a function of a first temperature representation signal and a first humidity representation signal, and the hygrometer further comprises a least one further input for an further humidity representation signal, and said hygrometer comprising means for computing and indicating of relative humidity as a function of the first temperature representation signal (T1) and the further humidity representation signal (H2), it is possible for the hygrometer to produce a clear and easily interpretable display of whether the outdoor relative humidity may cause a reduction of the indoor humidity if a ventilation is carried out.

Thus, the invention takes into account that outdoor humidity often differs from indoor humidity since it in according to the invention is assumed that air ventilation from the outside to the inside causes a change in relative humidity as the air temperature will usually adapt to the prevailing first indoor temperature if the indoor and outdoor temperatures differ significantly.

The invention is particularly advantageous in connection with ventilation of a room with a given relative humidity since knowledge of the outdoor relative humidity does not provide any indication of whether ventilation will be useful or not. High outdoor relative humidity may thus easily cause a lower relative indoor humidity if the

outdoor temperature is below the temperature inside.

Thus, the invention provides the opportunity to decide whether ventilation is advisable or not with great accuracy.

In this connection, it must be emphasised that the invention may basically be used to obtain an increase or a reduction of humidity since the well-being of a person in an indoor environment would in reality depend on an interval of relative humidity and an underlying desire of a given room temperature, all things being equal.

If relative humidity of the room in question is shown in relation to relative humidity computed on the basis of outdoor humidity and the indoor temperature, it is possible to obtain a very useful indication of whether ventilation is desirable or not.

The invention is particularly useful in connection with allergy afflicted homes in which dust mites must be kept at a minimum. Dust mites reproduce and live poorly below a certain limit value of relative humidity. Information indicating whether relative humidity may be reduced or not thus constitutes a valuable parameter if the number of dust mites is to be kept at a minimum.

Basically, the invention is based on the assumption, or rather approximation, that a room being ventilated will subsequently be capable of heating or cooling the ventilated air to the room (typically indoor) temperature (the first temperature representation signal) that forms the basis for computing of the relative humidity.

This approximation will not always reflect the exact prevailing ventilation but it has turned out that the assumption may in fact form the basis of a very simple ventilation estimate. According to the invention it is also possible to carry out various adjustments that take into account the actual prevailing temperature fluctuations arising from ventilation of a room. Other similar adjustments may be carried out automatically or manually with knowledge of the present application.

By using the above-mentioned estimate-the air ventilated from the outside to the inside will assume the indoor temperature-it also becomes possible to provide the user of the hygrometer with a knowledge of whether relative humidity may be changed by ventilation, just as it will in reality provide the user with a surprisingly accurate estimate of whether the room will have a relative humidity below or above the limit value determining the existence or not of the dust mites after ventilation.

Thus, in its generic form, the invention offers a particularly effective ventilation estimate through surprisingly simple technical measures. However, it should be noted that the invention may also be modified to carry out more or less complicated adjustments when computing the ventilation indication.

These adjustments may e. g. be carried out solely by software, just as they may be made by adding additional input signals.

It is implicit that the temperature representation and humidity signals may be fed to the hygrometer wireless.

When, as stated in claim 2, the hygrometer comprises a test switch, which creates an acoustic or visual indication of whether ventilation is advisable or not when manually activated, it is possible to achieve a particularly simple technical solution according to the invention.

When, as stated in claim 3, the hygrometer comprises a display unit for displaying of a relative humidity as a function of the first temperature representation signal and the first humidity representation signal and said hygrometer further comprises means for displaying of a relative humidity as a function of the first temperature representation signal (T1) and the further humidity representation signal (H2), a further advantageous embodiment of the invention has been achieved with the advantages mentioned above.

In this respect, a displaying will mainly be a numeric estimate.

In addition, it is also easy for the user to form an overall view of not only whether ventilation may in fact influence humidity in direction of the desired humidity interval but also whether the result obtainable will equal the effort necessary to obtain a given change. An example of this is when humidity is only reduced by a few per cents through ventilation and results in a temperature reduction or increase of several degrees for which it may subsequently be necessary to compensate.

When, as stated in claim 4, the display unit also comprises means to display the average level of an indoor relative humidity over a given period of time, a particularly advantageous embodiment of the invention is created since an average displaying of the relative humidity over a given period of time will produce an improved estimate of ventilation in relation to e. g. dust mites.

According to the invention, the ideal period of time is e. g. a week or a couple of days as this will provide the user with an impression of how more permanent humidity conditions over a longer period of time will contribute to reducing the survival potential of the dust mites.

This embodiment of the invention thus provides the user with a more solid basis for determining whether ventilation should be initiated or not since a momentary display of the relative humidity, depending on the purpose of the potential ventilation, does not always offer a good indication of the condition of the room over a long period of time.

When, as stated in claim 5, the hygrometer further comprises at least one input for a further temperature representation signal, it is possible to obtain the advantage of combining the display with both indoor and outdoor temperatures.

When, as stated in claim 6, the hygrometer comprising a display unit and said hygrometer comprising means for computing and displaying a relative humidity as a

function of an indoor temperature representation signal and an indoor humidity representation signal on the display unit, and the hygrometer comprises means for computation and displaying a relative humidity between an indoor temperature representation signal and an outdoor humidity representation signal, a particularly advantageous embodiment of the invention has been achieved.

When, as stated in claim 7, the outdoor humidity representation signal and possible temperature representation signals are fed to the hygrometer from an externally connected sensor, it is possible to obtain a particularly advantageous and practical embodiment of the invention through a simple hardware set-up.

Brief Description of the Drawings An example of an embodiment of the invention is described below referring to the accompanying drawings, in which fig. 1 shows an example of the application of a hygrometer according to the invention, and fig. 2 shows the basic set-up according to an embodiment of the invention.

Description of the Preferred Embodiment Figure 1 shows a typical indoor environment.

According to the invention, a hygrometer 1 is installed in an indoor room. The room may for instance be ventilated by a window 3 being opened or closed or by a passive or active air duct 2.

According to the invention, the hygrometer 1 will be able to produce an indication of how efficient a potential ventilation in fact will be.

Figure 2 shows a principle view of a preferred embodiment of the invention.

The hygrometer 1 features a display unit-a display 11- that at least comprises means to show the indoor relative humidity and"outdoor"relative humidity as a function of the indoor temperature.

The hygrometer 1 is connected to four external sensors through connecting wires 12,14,16,18.

The sensors comprise an outdoor humidity sensor 13, an indoor humidity sensor 17, an outdoor temperature sensor 15 and an indoor temperature sensor 19 that are individually connected to the hygrometer through connecting wires 12,16,14 and 18.

Internally, the hygrometer consists of display drivers, power supply, a microprocessor arrangement etc. as generally known within the art.

The microprocessor arrangement comprises at least software routines for computation of the indoor relative humidity and the relative humidity between outdoor humidity in relation to the indoor measured temperature.

According to the illustrated embodiment of the invention, typical ceramic ohm humidity sensors 13,17 have been

used just as ohm platinum resistors have been used for temperature sensors 15,19. It is implicit that it is as an example also is possible to use capacitive humidity sensors.

In connection with the hygrometer described above, it must be emphasised that the applied sensors measuring the indoor temperature and humidity will in reality often be incorporated in the hygrometer housing itself.

Apart from that, it will in reality also be advantageous to provide the display unit with a layout that also creates outdoor and indoor temperature indications since the applied sensors create a temperature representation signal anyway.

Also, within the scope of the invention, it will be advantageous to create an indication that is the average result of a given longer period of time, e. g. a day or a week, so that the user may get a reasonable idea of the general indoor environment.

Additionally, the hygrometer may feature various other options within the scope of the invention. An example of this is that the numeric display of relative humidity may alternatively be replaced by a simple visual LED indicator or an acoustic statement of whether ventilation is advisable or not.