The present invention relates to a device for measuring differential pressures with the aid of pressure sensors that have separate control circuits.

There are many types of measuring devices that measure pressure with the aid of electric sensors, strain gauges or like indicators. A common feature of such devices is that the majority include a membrane or diaphragm whose movements in response to pressure changes can be registered, e.g., piezoelectrically, by resistivity changes in response to axial stresses on metal wire, by changes in induction caused by positional shifts, etc. Provided that the conditions in the fluid whose pressure changes are to be registered are substantially favourable in this respect, a measuring result of acceptable reliability will also be obtained.

Unfortunately, the measuring conditions are very seldom ideal. Examples of typical error sources are temperature variations, pressure fluctuations, chemically aggressive fluids, contaminated fluids, and coatings on the pressure sensors. The conditions can be particularly unfavourable for obtaining correct results when registering pressure gradi¬ ents, since two measuring positions are normally used and occurrent measuring errors thus stand the risk of being summated.

The device, or apparatus, fundamental to the present invention has the advantage of constantly registering a pressure difference in the form of an absolute value, i.e. error sources resulting from more or less inexact conversion numbers are eliminated. Because the pressure difference is an absolute value within a specific path from the measuring points, the measured pressure gradient will be independent of fluctuations in the total pressure. Measuring of the fluid pressure in a system at two points located at mutually different depths in the system is an example in this regard. If the absolute value of the pressure is not measured, the result obtained with respect to the pressure gradient will be significantly unreliable, not least when the level of liquid varies.

U.S. Patent Specification 4,614,118 describes a pressure measuring cell in which movements of a diaphragm are registered by a sensor, which delivers a signal that, after being amplified via a control device, actuates a valve for a servomedium which is pumped into the cell so as to adjust the diaphragm to zero deflection. Certain intrinsic errors of the measuring cell are eliminated thereby, for instance such errors as the lack of linearity as the diaphragm stretches, etc. The inner pressure of the cell is established on the basis of the pressure of the servomedium, this pressure corresponding to the pressure of the medium outside the cell. However, this prior publication suggests no method with which there is obtained an outgoing measuring signal which is directly proportional to the pressure gradient to be measured and which is independent of pressure fluctuations or other variables, e.g. temperature.

SE Published Specification 395188 teaches a measuring device that includes a venturi tube between two pressure sensors. Each sensor is supplied with a pressure controlled fluid and includes a fluid inlet and two fluid outlets. The fluid outflows from the sensor are also pressure controlled. One outlet is controlled by a flap which is connected to the pressure sensing membrane of the sensor. The other outlet undergoes a pressure reduction subsequent to passage of the fluid through a cavity in the sensor. The pressure sensing movements via the membrane will thereby be very small. It is stated on page 8 of the published specification that the sensors can be used for, e.g., sensing pressure and liquid levels in reservoirs, for instance. Distinct from the inventive device, however, the prior publication makes no reference to mamtaining a constant pressure within respec¬ tive sensors, which excludes the use of measuring devices that include differential pressure sensors or indicators.

The sensors used in die present measuring system are balanced-out during a measuring operation by means of a servomedium in separate control circuits and, distinct to earlier techniques, a pressure difference between two measuring points can be registered via a measuring device subsequent to regulating to equilibrium between outer and inner pres- sures of respective sensor membranes. Deflection of the membrane, i.e. the initial inward bulging of the membrane in normal cases, is controlled on the basis of

inductance from a sensing element in the sensor, although other sensing possibilities are feasible, such as piezoelectric sensors and strain-gauge related resistivity. The control is preferably effected to zero deflection, even though other degrees of deflection are feasible for achieving equilibrium. The servomedium used to balance-out the sensors may be an inert gas or a low viscous liquid that are not electrically conductive. The servomedium may conveniently comprise an inert thinly fluid oil, particularly a hydrocarbon based oil. In particularly, the servomedium may comprise an inert, thinly fluid oil, preferably a hydrocarbon based oil.

Small area membranes are preferred, since the membrane is relatively insensitive to the build-up of foreign material that changes the mass of the sensor elements. The membrane can be considered to be rigid, i.e. non-resilient in operation, which counteracts errors due to creepage.

Figure 1 is a schematic illustration of the inventive measuring system, and shows the functional principles in broad outlines. Each measuring sensor (7, 8) is fixedly mounted at a fixed distance between the two measuring points. It is assumed that a pressure difference exists over this constant distance. Respective sensor membranes (9, 10) are balanced-out by separate pressure regulators (11, 12) with the aid, for instance, of an hydraulic valve in the regulator, and the pressure difference dP between the servomedia in respective circuits is registered by a measuring device (13).

The pressure regulators are, in principle, supplied with a pressurized servomedium from a common connection (1). When the sensors are balanced with respect to one another, i.e. after achieving equilibrium between the pressure in the servo medium (2) and the pressure on the outside of the sensor, the counterpressure in each sensor will correspond to the pressure prevailing on the outside of the membrane. By connecting the measuring device for differential pressure (13) to the servomedium (2) in respective sensors, there is obtained an output signal which is directly proportional to the pressure level at each measuring point subsequent to balancing-out the sensors.

The pressure in respective sensors is registered by separate movement sensors (5, 6) and respective signals (3, 4) are used to control the pressure regulators (11, 12).

The measuring device enables primarily the densities of liquids to be measured under severe conditions, e.g. corrosive conditions or of liquids whose properties cannot be readily handled, such as liquids of high viscosity, tacky liquids, liquids that have high sedimentation tendencies, etc., with which different types of mass-flow measuring devices, weighing apparatus, etc., function less well. The inventive device has no open intakes, which contributes towards why drawbacks relating to clogged flow passageways and pressure absorbing zones can be greatly reduced, thereby lengthening the periods between which inspection is necessary. The adjustment needs of the structurally very simple measuring system are minimal in comparison with known density measuring apparatus.

It will be understood, however, that the inventive device can be used advantageously in fields other those concerned with measuring the density of liquids. For instance, the device can be used to measure rates of flow through venturi tubes, liquid levels in systems, investigation of aerodynamic and hydrodynamic flow sequences, etc.