Quartz barometer

In a quartz barometer untilizing the temperature dependence of the resistance of a quartz oscillator at resonance, the present invention is directed to provide a circuit which compensates for the temperature change of the resistance at resonance by connecting a temperature-dependent resistor in series with a quartz oscillator thereby enabling said quartz barometer to measure gas pressure much more accurately.

BACKGROUND OF THE INVENTION 
This invention relates to a barometer for measuring gas pressure around a 
quartz oscillator using the quartz oscillator. 
There is an urgent need in industrial applications to measure continuously 
gas pressure ranging from ambient pressure to 10.sup.-3 Torr with a single 
sensor. 
A quartz barometer which utilizes the phenomenon that the frequency at 
resonance of a quartz oscillator increases with a decreasing gas pressure 
surrounding the oscillator satisfies to some extent the industrial 
requirement described above. However, this barometer involves a critical 
problem in that the lower limit of measurement is about 10 Torr. Though a 
heat conduction vacuum gauge such as the Pirani gauge has a lower limit 
value of measurement of about 10.sup.-4 to 10.sup.-3 Torr, it is not free 
from the same problem as that of the quartz type barometer because its 
upper limit of measurement is about 10 Torr. 
It has been shown recently that the resonance resistance of a quartz 
oscillator depends upon ambient gas pressure over an extremely wide range, 
and that a barometer which can continuously measure pressure ranging from 
ambient atmospheric pressure to 10.sup.-3 Torr can be realized by 
utilizing this property. This is reported, for example, in "Development of 
Ultra-Miniature Vacuum Sensor Using Quartz Oscillator" in the magazine 
"Instrumentation", 1984, Vol. 27, No. 7. 
However, in a quartz barometer having the prior art construction which 
utilizes the temperature depedence of the resistance of a quartz 
oscillator at resonance described above, a problem has been left unsolved 
in that precision measurement can not be readily effected because the 
resistance of the quartz oscillator at resonance varies markedly with 
temperature, particularly in the low pressure range of roughly 10.sup.-3 
to 10.sup.-2 Torr. 
SUMMARY OF THE INVENTION 
To eliminate this problem, in a quartz barometer utilizing the temperature 
dependence of the resistance of a quartz oscillator at resonance, the 
present invention is directed to provide means which compensate for the 
temperature change of the resistance at resonance by connecting a 
temperature-dependent resistor in series with a quartz oscillator thereby 
enabling said quartz barometer to measure gas pressure much more 
accurately.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
Hereinafter, the present invention will be described with reference to the 
accompanying drawings. 
FIG. 1 is a diagram showing the relation between gas pressure and the 
characteristic values (resistance at resonance, current at resonance and 
resonant frequency) of a quartz oscillator. The resonant frequency starts 
changing when pressure exceeds 10 Torr, but the sensitivity to pressure is 
virtually nil below 10 Torr. However, the resistance of a quartz 
oscillator at resonance is sensitive to pressure ranging from ambient 
atmospheric pressure to 10.sup.-3 Torr. When this quartz oscillator is 
driven at a constant voltage, a resonance current-v-gas pressure curve can 
be obtained as represented by symbol i.sub.o in the diagram. It is 
sensitive to pressure ranging from ambient atmospheric pressure to 
10.sup.-3 Torr in the same say as the resistance at resonance described 
above. Therefore, it is easier to measure the current at resonance or the 
voltage at resonance than to measure the resistance at resonance. 
FIG. 2 is a block diagram of a quartz barometer electronic circuit to which 
the present invention is directed. Its principal components are a PLL 
circuit, a display conversion circuit and a display. The PLL circuit 
consists of a variable frequency oscillator 1 which is controlled by a 
voltage or a current, an amplifier 2 which amplifies the current of a 
quartz oscillator 5 at resonance as a voltage, a phase comparator 3 which 
compares the phase of the output signal of the amplifier 2 with that of 
the output signal of the variable frequency oscillator 1 and produces a 
signal proportional to the phase difference, and a low-pass filter 4 which 
converts the pulse-like output signal of the phase comparator 3 to a d.c. 
voltage. The output voltage of the low-pass filter 4 controls the 
oscillation frequency of the variable frequency oscillator 1. The 
pressure-sensitive quartz oscillator 5 is connected to the output terminal 
of the variable frequency oscillator 1 and the input terminal of the 
amplifier 2. 
The principle of operation of the PLL circuit is well known already, it is 
not described herein. The output signal of the variable frequency 
oscillator 1 is always controlled such that the phase difference between 
the output signal of the variable frequency oscillator 1, that is, the 
driving voltage of the quartz oscillator 5, and the output signal of the 
amplifier 2, that is, the current flowing through the quartz oscillator 5, 
is zero. Therefore the quartz oscillator 5 is always driven at its 
resonance frequency. This is a significant factor in practical application 
of a quartz barometer since the resonant frequency of the quartz 
oscillator varies with pressure as shown in FIG. 1. 
Next, the display conversion circuit portion consists of a main amplifier 6 
which further amplifies the signal from the amplifier 2, a rectifier 7 
which changes the output signal of the main amplifier 6 to d.c. an 
inverter 8 which inverts the polarity of the output viltage of the 
rectifier 7, and a buffer 9 which biases the output voltage of the 
inverter 8. The bias level can be controlled by a variable resistor 9a. 
The display may be either digital or analog. In this embodiment, it 
consists of a meter 10; pressure is read from the deflection angle of said 
meter. 
The pressure characteristics of the resonant current of the quartz 
oscillator are such that said current increases as ambient pressure 
decreases, as shown in FIG. 1. Therefore, if the current at resonance is 
amplified as a voltage and is changed to d.c. to drive the meter, the 
deflection angle of the meter will increase with decreasing pressure; 
consequently, the display will be the opposite of the detected pressure. 
This is obviously undesirable from the common sense point of view; 
therefore, the inverter 8 inverts the polarity of the d.c. voltage, and 
the buffer 9 then applies the bias voltage so that the meter dirving 
voltage shown in FIG. 3 can be obtained. In the embodiment shown in FIG. 
3, the bias quantity is adjusted so that the meter driving voltage is 10 V 
at ambient atmospheric pressure. In this manner, a conventional pressure 
display can be effected in which the meter indicator angle of deflection 
increases as ambient atmospheric pressure increases and decreases as 
ambient atmospheric pressure decreses. 
FIG. 4 shows the temperature characteristics of the resistance of the 
quartz oscillator at resonance. The degree of change of the resistance at 
resonance due to temperature is great in vacuum, and the resistance at 
resonance increases with as temperature increases. Since most of the 
resistance at resonance in the ambient atmosphere is frictional 
resistance, the resistance at resonance does not vary greatly with 
temperature. As a result, the prior art technique involves the problem 
that the effect of temperature increases markedly as gas pressure 
decreases, thereby introducing an error into the measured value. 
The present invention provides means for minimizing the error just 
described. 
FIG. 5 shows one embodiment of the present invention. Component 5 is a 
quartz osoillator, 11 is a thermistor and 12 is a resistor. In FIG. 6, a 
is the curve of the temperature characteristics of the resistance at 
resonance of the quartz oscillator at resonance, b is the curve of the 
temperature characteristics of the combined resistance of the thermistor 
and the resistor, and c is the curve of the temperature characteristics of 
the combined resistance of the quartz oscillator, the thermistor and the 
resistor. The resistance of the quartz oscillator at resonance has a 
positive temperature coefficient, whereas the resistance of the thermistor 
has a negative temperature coefficient. If they are connected in series, 
therefore, the combined resistance value becomes a curve which has a 
valley with respect to temperature. The resistor described above is a 
variable resistor so that the temperature range of the valley is that of 
room temperature (20.degree.-30.degree. C.). 
The resistance of the thermistor is not affected by the gas pressure around 
it; hence, it can compensate for temperature without regard to pressure. 
In the prior art technique in which the thermistor does not compensate for 
temperature, the measured value of 1.times.10.sup.-2 Torr at 25.degree. C. 
varies to a maximum of 4.times.10.sup.-2 Torr if the ambient temperature 
varies from 10.degree. C. to 40.degree. C. If the thermistor compensates 
for temperature, the measured value of 1.times.10.sup.-2 Torr at 
25.degree. C. will fall within the range of a maximum of 2.times.10.sup.-2 
Torr. Thus, the measurement error due to temperature can be reduced by 
half. The practical temperature range in the environment of measurement is 
roughly 25.degree..+-.5.degree. C. Since the embodiment of the present 
invention can easily set the minimal point of the resistance value 
relative to the temperature to 25.degree. C., measurement error due to 
varying temperature can be drastically reduced in practice. 
As described above, the present invention can minimize the adverse effect 
of varying temperature upon the resistance of the quartz oscillator at 
resonance by extremely simple means, and can improve accuracy, 
particularly, in the low pressure range. Since the barometer of the 
present invention is simply constructed, any increase in the production 
costs will be minimal. 
Though the embodiment described above uses the thermistor as a device to 
compensate for temperature characteristics of resistance of a quartz 
oscillator at resonance, devices other than the thermistor can be of 
course employed if they have a temperature coefficient opposite to that of 
the resonance of the quartz oscillator at resistance.