GB2278451A

GB2278451A - Measuring devices exposed to fluid flow

Info

Publication number: GB2278451A
Application number: GB9410722A
Authority: GB
Inventors: Andreas Wildgen
Original assignee: Siemens AG
Current assignee: Siemens AG
Priority date: 1993-05-28
Filing date: 1994-05-27
Publication date: 1994-11-30
Anticipated expiration: 2014-05-27
Also published as: DE4317952C1; JPH07117437B2; FR2705777A1; JPH06347302A; GB2278451B; FR2705777B1; GB9410722D0

Description

2278451 Y MEASURING DEVICES EXPOSED TO FLUID FLOW The present invention

relates to measuring devices exposed to fluid flow.

When an electrical measuring device, for example a resistance layer disposed on a substrate, is exposed to a fluid medium such as air, for example to measure the temperature of a flow of air, a deposit in the form of a layer of dirt may gradually form on the measuring device and may result in displacement (drift) of a characteristic curve of the device. Since air pollution, particularly in internal combustion engines, consists mostly of electrically conductive soot, deposits of this kind can result in parallel resistances being formed which are particularly troublesome if the measuring device employs high- impedance components. For this reason, it is desirable to attempt to prevent a deposit in the form of a complete layer of dirt from forming on such a measuring device. 20 For example, a body facing the direction of flow could be disposed in front of an air mass meter, disposed in a measuring channel, so as to shield the measuring region of the air mass meter somewhat and prevent the flow striking it directly. A deposit of dirt may be prevented or at least reduced by this means.

A dirt deposit that provides a parallel resistance as mentioned above can cause a change in the effective resistance between two connectors (anode and cathode) of the measuring device. This effect may be increased by air humidity. After long operation or high contamination, for example by soot from a diesel engine, the dirt combined with the air humidity can result in electrolysis occurring between the connecting contacts (anode and cathode) so that, particularly in the case of platinum resistors between the connecting contacts, they come loose from a supporting substrate and consequently render the measuring device unreliable.

To obtain a highly sensitive response, for example in air mass meters for internal combustion engines, the measuring device may be exposed directly to the fluid medium. One previously-considered method for preventing electrolysis consists of at least partial application of a protective layer of lacquer. This method cannot readily be automated and is therefore expensive and subject to error. Drift may be reduced only if the lacquer extends to the nearest insulator, which may require special construction. However, tests have now shown that, in the case of fluid flow having a clearly preferred direction of motion, drift can be prevented if the layer of dirt is interrupted at just one place. The interruption can be made easily and at minimal expense by means of an obstacle disposed in front of a support portion of a measuring device.

According to the present invention, there is provided a device for use in measuring a parameter of a medium flowing in a predetermined direction through a predetermined location, which device comprises temperature-dependent electrical resistance means, connected between two electrical connectors and extending across the said location, and an obstacle mounted so as to extend adjacent to the electrical resistance means, upstream thereof, and dimensioned so as to shield a relatively small part of the length of the said resistance means, between the two electrical connectors, from the flowing medium, thereby to ensure the presence of a gap in a layer of dirt formed, by deposition from the flowing medium, between the two electrical connectors when the device is in use.

An embodiment of the present invention can effectively reduce the danger that distortion of measured results may occur, as a result of drift of the resistance values, and also the danger that components of the device may become detached from a support portion thereof. 5 Reference will now be made, by way of example, to the accompanying drawings, in which: Fig. 1 is a diagrammatic side view of parts of an air mass meter embodying the present invention; and Fig. 2 is a plan view of the parts shown in Fig.

1.

An air mass meter, as shown In Fig. 1 and 2, includes holder means 1, which can be made of a poor thermal conductor, for example a plastics material or glass. A support 2 is connected to the holder 1 and 15---maybe made of a ceramic material.

A temperature-dependent precision measurement resistance (not shown) is mounted on the support 2 and comprises a plurality of resistance elements, which are connected, for example by wires, to electrical connectors (not shown) arranged on the holder means 1.

An obstacle 3 formed by a narrow metal strip is positioned, near a holder 1, adjacent to and upstream (in front) of the support 2, bearing the temperaturedependent precision measurement resistance, and projects beyond the support 2 at least on the side of the measuring region and is narrow compared with its length. The longitudinal axis of the strip 3 extends transversely with respect to the adjacent longitudinal side edge of the support 2, and the strip is arranged edgewise on to that side edge.

A fluid medium striking the air mass meter is denoted by arrows L showing the direction of flow. As shown in Fig. 2, the obstacle 3 is disposed to the upstream side of the support 2.

After prolonged operation of the air mass meter, layers of dirt 4 and 5 become deposited on the support 1 2 and the obstacle 3 respectively, as a result of impurities in the fluid medium. As Fig. 2 shows, the layer of dirt 4 on the support 2 is interrupted by a relatively small gap in the vicinity of the obstacle 3.

Thus the obstacle 3 shields a small part of the length of the support 2 (and the measurement resistance thereon) from the fluid flow so as to prevent a complete parallel resistance from being formed on the substrate between the electrical connectors. In this way drift due to such parallel resistance can be effectively prevented.

Claims

CLAIMS:

1. A device for use in measuring a parameter of a medium flowing in a predetermined direction through a predetermined location, which device comprises temperature-dependent electrical resistance means, connected between two electrical connectors and extending across the said location, and an obstacle mounted so as to extend adjacent to the electrical resistance means, upstream thereof, and dimensioned so as to shield a relatively small part of the length of the said resistance means, between the two electrical connectors, from the flowing medium, thereby to ensure the presence of a gap in a layer of dirt formed, by deposition from the flowing medium, between the two electrical connectors when the device is in use.

2. A device as claimed in claim 1, wherein the said obstacle is elongate in form and extends transversely with respect to a longitudinal side edge of the resistance means, which side edge is opposed to the said predetermined direction.

3 A device as claimed in claim 1 or 2, wherein the said obstacle is a metal strip.

4. A device as claimed in claim 3, wherein the said metal strip is arranged edgewise on to the said resistance means.

5. A device as claimed in any preceding claim, wherein the said device is part of an air mass meter for use in an internal combustion engine.

6. A device as claimed in any preceding claim, wherein the said electrical resistance means comprise an electrically resistive layer on a supporting substrate.

7. A device as claimed in claim 1, substantially as hereinbefore described with reference to the accompanying drawings.

8. An internal combustion engine fitted with a v device, as claimed in any preceding claim, arranged for use in measuring a parameter of a medium flowing In the engine when it is in use.