Measuring and testing – Fluid pressure gauge – Vibration type
Reexamination Certificate
2000-09-26
2003-03-18
Williams, Hezron (Department: 2855)
Measuring and testing
Fluid pressure gauge
Vibration type
C073S728000, C073S722000, C073S778000
Reexamination Certificate
active
06532822
ABSTRACT:
BACKGROUND OF INVENTION
1. Field of Invention
The present invention relates generally to gas pressure sensors. More particularly, the present invention relates to a gas or fluid pressure sensing device operating in a resonant mode, and particularly adapted for sensing a wide range of pressures with a high degree of accuracy and repeatability. The resonant motion of the pressure sensor is at right angles to the effect of fluid pressure within the sensor and thus has a high degree of accuracy independent of the type of fluid.
2. Description of Prior Art
The need for accurate, low cost, compact pressure sensors having a broad range of measurement is becoming widely appreciated. Measurement of broad ranges of pressures in systems is particularly challenging because of the enormous range of pressures that can be realized. Many systems have two or more types of gauges, each with its particular range of usefulness. The need to switch between different gauges is tiresome and produces reading discontinuities where the gauge ranges meet. Gauges that are capable of accurate measurement in a broad pressure range are attractive because they can reduce the number of different types of gauges needed to monitor a particular vacuum system.
In an attempt to satisfy this need, a number of devices have been proposed. One type of gauge known to those familiar with vacuum techniques as the Langmuir gauge utilizes the molecular drag effect. These devices use the fact that the presence of air or other gases surrounding a mechanical vibrator exerts a damping effect upon the vibration. As the pressure of the surrounding gas is diminished, the damping effect is reduced. In this type of gauge, a fine quartz fiber anchored at one end and the other end is free to vibrate. Vibration is excited by striking the fiber (internally) and the time for the free vibrations to decay to half amplitude is monitored. Essentially, the decay time is as an indicator of vacuum. As the gas pressure is reduced, the decay time of the vibrator increases.
In the Langmuir gauge, measuring the time for the oscillations to decay (e.g. to one half amplitude) after the driving oscillator has stopped is used to determine the pressure. A second method is a measurement of the bandwidth of the vibrator by driving it with a variable frequency oscillator and tuning over the frequency band of the vibrator. Neither of these two methods lends itself to a simple direct measurement suitable for a commercial gas pressure gauge.
Another class of gauges making use of the molecular drag phenomenon uses a freely swinging fiber or vane as a means of measuring pressures in the range of 10.sup.−3 Torr to 10.sup.−5 Torr. The fiber or vane pendulum is started swinging mechanically and time for the pendulum to damp to one half its original amplitude, or half-life, is measured. This method of measuring pressure, however, is quite limited in range. It is also cumbersome and takes on the order of one hour to make a measurement at low pressure.
Another type of pressure gauge that makes use of the drag forces of a gas is called a spinning rotor gauge. This gauge measures the deceleration of a magnetically levitated spinning metal sphere inside a stainless steel chamber that its, in turn, immersed in the gas that is to have its pressure measured. The ball is electromagnetically spun up to a target rotation rate and then allowed to decelerate. The rate of the ball's deceleration-is proportional to the number of gas molecules that come in contact with the ball per unit time which is, in turn, proportional to gas pressure. This gauge can measure pressures in the range of 10.sup.−2 Torr to 5.times.10.sup.−7 Torr. Spinning ball gauges are very accurate, however their use, is restricted by their size, high cost and limited range of measurement capability.
Another type of resonant gauge uses tuning fork quartz crystal oscillators to measure gas friction. In this gas pressure sensor, a tuning fork quartz is exposed to the measuring environment and the resistance of the tuning fork quartz at resonant frequency is measured. By means of a phase-coupled, electric oscillation system (PLL circuit) and an evaluation circuit, the change of resistance of the tuning fork is measured and appropriately indicated.
Another type of gauge employing this principle uses a tuning fork made from piezoelectric material as the sensing element. The tuning fork is made to oscillate and its resonance resistance, which is directly proportional to gas pressure when the pressure is low enough to be in the molecular flow region, is measured. When the pressure rises to a level where the flow begins to become viscous, the resonance resistance continues to increase with pressure, but at a much reduced rate. To make a pressure measurement using the tuning fork oscillator, the tuning fork is placed where the pressure is to be measured and caused to oscillate by means of an oscillator circuit. The pressure is determined by measuring the difference between the resonant resistance where the pressure is being measured and the natural resonance resistance of the tuning fork. One of the drawbacks of this device is that its range is limited at the low end when the resistance caused by the gas is of the same order as the natural resonance resistance of the tuning fork. The sensitivity is also limited at the high end by the shift from molecular resistance to the transition between molecular and viscous resistance.
In yet other gauges, a resonator is formed from a single crystal of silicon. In these devices a resonating element is maintained in a state of oscillation, the oscillation frequency providing a measure of a pressure or strain applied to the transducer.
The practical use of such tuning fork quartz sensors has hitherto been restricted or made impossible by numerous different problems. In the conventional method that uses a crystal resonator to measure pressures, such as a fork oscillator, the resonator usually has a temperature that is indefinitely varying during the measuring process. This is a major problem in connection with this measuring method is that the measured damping value is not only dependent on the pressure, but also on the temperature. This is negligible at high pressures (>1 mbar). However, the desired measurement can be greatly impaired by the temperature dependence at lower pressures, causing large errors when measuring the pressures in the lower pressure range. This disadvantageously makes accurate pressure measurement with the fork oscillator alone nearly impossible.
In order to reduce the interference effects, a number of possibilities have been proposed, such as using a special, tailor-made oscillating quartz in order to keep the temperature influences as low as possible. Use of a special quartz geometry has lead to an improvement in the measurement characteristics of the quartz, but does not satisfy practical requirements. The use of such special quartzes also leads to higher manufacturing costs.
It has also been proposed to use a resistor for temperature compensation or even thermostatically controlling the quartz. Compensation of the temperature influences by means of a NTC-resistor connected in series with the quartz oscillator is inadequate because there are only a few degrees of freedom of compensation and in particular higher order components remain uncompensated. Furthermore, thermostatic control leads to very high, unacceptable costs. Thus, the proposals made up to now only lead to a very limited error or fault compensation, while also being complicated and costly.
A major problem with conventional devices is that they do not have sufficient accuracy where a high degree of precision is required.
Another problem with conventional devices is that measuring a pendulum half-life in the molecular drag method is time consuming.
Another problem with conventional devices using the molecular drag method is their large size, high cost and limited range of measurement capability.
A problem with conventional resonant devices is that they are inaccurate
Bolduc David J.
Jenkins Jermaine
Williams Hezron
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