Indicating apparatus for indicating a vacuum

An indicating apparatus for indicating a vacuum, particularly for indicating the vacuum on the filtered-air side of an air filter for an internal-combustion engine, including a housing and a piston which is movably disposed in the housing and which separates a vacuum area from an anbient-pressure area. A snap element is provided which is activated by the piston and, at a predetermined vacuum, triggers an electric contact.

BACKGROUND OF THE INVENTION 
This invention relates to an indicating apparatus for indicating a vacuum 
comprising a housing, a piston movably arranged in the housing, a spring 
which exerts a force on the piston, and a membrane which seals off a 
vacuum area on one side of the membrane from an ambient-pressure area on 
the other side of the membrane. 
An indicating apparatus of this type is known, for example, from U.S. Pat. 
No. 4,369,728. This indicating apparatus is used for indicating the degree 
of dirt accumulation on an air filter of an internal-combustion engine. 
Normally, such indicator devices operate with the vacuum which exists on 
the filtered-air side of the air filter. This vacuum causes the position 
of the membrane to shift. When the degree of dirt accumulation on the 
filter increases, the vacuum will naturally rise. This is sensed by the 
indicating apparatus and converted to an indicating position. 
It is a disadvantage of the known system that, because of the design of the 
membrane and of the piston, a low accuracy is achieved. Also, there is the 
risk that the detent element of the piston will tilt the piston and cause 
the piston to jam. 
Another indicating apparatus is known from Published German Patent 
Application No. DE-OS 2,847,278, which operates by means of an electric 
contact element. A piston or a membrane exerts a force on an electric 
switch against the pressure of a spring. As soon as the force exceeds a 
specific amount, this electric switch will open or close and generate a 
signal. It is a disadvantage of this apparatus that the indication is very 
dependent on the characteristic spring curve and the elasticity of the 
piston or of the electric contact. 
SUMMARY OF THE INVENTION 
It is an object of this invention to provide an indicating apparatus which 
has a high precision. 
A further object of the invention is to provide an indicating apparatus 
which operates in a reliable manner. 
These and other objects of the invention are achieved by providing an 
indicating apparatus for indicating a vacuum comprising a housing, a 
membrane which seals off a vacuum area on one side thereof with respect to 
an ambient pressure area on the other side thereof, a piston associated 
with the membrane and movably arranged in the housing, and a snap element 
acted upon by the piston in response to a pressure difference between the 
vacuum area and the ambient pressure area, the snap element actuating an 
electric contact when moved by the piston in response to a sufficient 
pressure difference. 
A particular advantage of the invention is the use of a snap element. This 
snap element has a precisely defined snapping point which is determined by 
the geometry of the element. When the vacuum rises, the piston will 
initially exert a force on the snap element without activating the snap 
element. Only above a predetermined force will the snap element shift into 
the switching position and thereby generate a defined indicator signal. 
In accordance with a further embodiment of the invention, the snap element 
simultaneously serves as the spring which, when the vacuum is absent, 
exerts a restoring force to return the piston into the starting position. 
Advantageously, the snap element is a small rectangular metal leaf which 
has a planar circumferential edge and is provided with a trapezoidal 
through-bending in the center region. The advantage of this rectangular 
element is its precise producibility. Also, with such an element, it is 
possible to bend this element beyond the snapping point, the further 
bending occurring essentially linearly in accordance with a characteristic 
spring curve. 
In accordance with another embodiment of the invention, the electric 
contact for emitting a signal is a reed switch. This switch is switched by 
means of a magnetic snap element or a magnet arranged on the snap element. 
Alternatively, the electric contact may be a proximity switch which is 
uncoupled from this snap element when the snap element is in the 
inoperative position, and which emits a signal only after the operating 
stroke of the element or only within the phase in which the snap element 
has a characteristic linear spring curve. 
These and other features of preferred embodiments of the invention, in 
addition to being set forth in the claims, are also disclosed in the 
specification and/or the drawings, and the individual features each may be 
implemented in embodiments of the invention either individually or in the 
form of subcombinations of two or more features and can be applied to 
other fields of use and may constitute advantageous, separately 
protectable constructions for which protection is also claimed.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
FIG. 1 illustrates an indicating apparatus or a servicing switch for a 
filter element, such as the air filter element of an internal combustion 
engine. This indicating apparatus comprises a housing 10 on which a 
connection 11 for a vacuum source is provided. For example, the connection 
may communicate with the filtered air side of the air filter of an internal 
combustion engine, so that the indicator may indicate the degree of dirt 
accumulated on the filter by sensing the increase in the vacuum on the 
filtered air side resulting from blocking of the filter by accumulated 
dirt. The connection 11 also communicates with the housing interior 12. A 
filter element 13 is provided to prevent dirt from entering the housing 
interior 12. The housing is closed off by means of a cover 14. A membrane 
15 is fastened between the housing and the cover. In the illustrated 
embodiment, this membrane simultaneously serves as a seal between the 
housing 10 and the cover 14. As can be seen in FIG. 1, housing 10 and 
cover 14 are joined by snapping them together around their circumference. 
An axially movable piston 16 is arranged on the membrane 15. Ambient 
pressure exists in an ambient pressure area above the piston 16 and the 
vacuum applied through connection 11 is present in a vacuum area in the 
housing interior underneath the piston 16. A snap element 17 is also 
situated in the housing interior 12. 
The snap element 17 is also illustrated in the top view according to FIG. 
2. Snap element 17 has a rectangular design and, on the shorter sides, is 
fixed by noses 18 against a supporting body 19. The snap element comprises 
an essentially rectangular leaf or snap plate which has a planar 
circumferential edge and central depression formed by stamping having a 
trapezoidal configuration. An electric contact spring 20 is arranged below 
the snap element 17. In the deactuated position shown in the drawings, 
contact spring 20 is rests against the support 21. In the activated 
position, the contact spring 20 rests against the contact plate 22. The 
contact plate 22 is connected with a first connection pin 23 in an 
electrically conducting manner. A second connection pin 24 is electrically 
connected with the electric contact spring 20. As illustrated in FIG. 2, 
the snap element formed by stamping in such a manner that an essentially 
rectangular pressure area is formed in the center. As illustrated in FIG. 
1, the piston 16 is arranged centrally with respect to the pressure area. 
Since the snap element is supported only on the outer areas 25 and 26, the 
snap element can not only snap through, but can also continue to bend 
(i.e., "bend through") after it has snapped through. 
The relationship between the force of the vacuum and the movement of the 
snap element is illustrated in FIG. 3. On the horizontal axis, the path of 
movement S traversed by the snap element is shown; on the vertical axis, 
the force F is shown which acts upon the snap element. At the beginning, a 
very high force is required which, at the maximum point F1, has the result 
that the snap element snaps through. The snapping-through point is reached 
in position S3, and when the force is increased thereafter, the 
through-bending occurs linearly with respect to the supplied force. The 
switching point S2 is situated in this linear area. This switching point 
occurs, for example, at a vacuum of 15 mbar. The high precision of the 
indicating apparatus is achieved by the fact that the linear area which 
occurs after the snapping, can be determined in a very precise manner. The 
snapping force F1 can be defined by the depth of the profile stamped into 
the snap element. 
Thus, depending on the particular requirements of a given situation, a 
servicing switch can be provided which can be optimally adapted to the 
maximum permissible vacuum. 
The foregoing description and examples have been set forth merely to 
illustrate the invention and are not intended to be limiting. Since 
modifications of the described embodiments incorporating the spirit and 
substance of the invention may occur to persons skilled in the art, the 
invention should be construed broadly to include all variations falling 
within the scope of the appended claims and equivalents thereof.