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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a quartz crystal resonator pressure
transducer assembly suitable for use downhole in oil, gas, geothermal and
other wells, at the wellhead, in industrial applications, for portable
calibration devices and in laboratory applications. More specifically by
way of example and not limitation, the invention relates to a
piezoelectrically-driven temperature-compensated quartz crystal resonator
pressure transducer assembly.
2. State of the Art
The general type of quartz crystal pressure transducer assembly as
disclosed herein includes a first pressure sensitive quartz crystal
resonator, a second temperature sensitive quartz crystal resonator, a
third reference frequency quartz crystal resonator, and supporting
electronics. For convenience, the terms "crystal" and "resonator" may be
used interchangeably herein in referencing a resonating quartz crystal
element. The first crystal changes frequency in response to changes in
applied external pressure and temperature, while the output frequency of
the second crystal is used to temperature compensate temperature-induced
frequency excursions in the first and third crystals. The third crystal
generates a reference signal, which is only slightly temperature
dependent, against or relative to which the temperature and
pressure-induced frequency changes in the first crystal and the
temperature-induced frequency changes of the second crystal can be
compared. Means for comparison as known in the art include frequency
mixing or using the reference frequency to count the signals from the
other two crystals. The first resonator is exposed via a fluid interface
to the external pressure sought to be measured, and all three resonators
are thermally coupled to the fluid to provide a rapid thermal response
time. The transducer (crystals plus electronics) is calibrated as a
complete unit over the intended pressure and temperature range so that all
temperature and pressure related effects can be compensated for in the
resulting calibration curve-fig coefficients. Exemplary patents for
transducers using three crystal resonators, each assigned a function as
described above, are U.S. Pat. No. 3,355,949 to Elwood, el al, and U.S.
Pat. No. 4,802,370 to EerNisse, et al.
In the Elwood patent, the temperature crystal is used to provide a
temperature readout and to compensate for temperature induced frequency
changes in the pressure crystal. However, Elwood did not realize that
pressure sensitivity of the pressure crystal is a function of the
temperature of the crystal. Moreover, all three crystals in Elwood are
disclosed as being relatively temperature sensitive, an attribute which
makes it more difficult to compensate for temperature dependency of the
pressure crystal. Finally, Elwood had no appreciation for the need to have
the crystals free, or at least substantially free, of frequency anomalies
or activity dips over the intended temperature range of the transducer and
the need to have the three crystals have substantially no change in
resistance with temperature. Activity dips can cause apparent pressure
errors and resistance changes can cause the electronics to cease
operation, to operate incorrectly, or require high drive levels.
The aforementioned EerNisse '370 patent isolates a temperature and a
reference crystal from the applied external pressure, but all three
crystals are temperature-sensitive, and EerNisse does not specifically
define desired crystal cuts, (although the temperature resonator is
specified as a torsional tuning fork resonator), nor did he specify
required or preferred individual crystal performance specifications. The
emphasis of the '370 patent is on mounting all three of the crystals in
the pressure-transmitting fluid, and matching the heat transfer or
conductivity characteristics, and thus the temperature response times, of
the temperature and reference crystals to that of the pressure crystal to
substantially eliminate temperature gradients produced either by external
heating or by pressure-volume heating.
U.S. Pat. No. 4,660,420 to EerNisse recognizes the desirability of
selecting a pressure crystal with a crystal cut having substantial
independence from temperature-induced frequency changes over the intended
range of temperatures, as well as a relatively large scale factor, i.e.,
greater frequency sensitivity to pressure changes in the range to be
measured. For the pressure and temperature ranges experienced in oil and
gas wells, an AT-cut quartz crystal is disclosed in EerNisse '420 to
possess these attributes.
Another EerNisse patent, U.S. Pat. No. 4,550,610, attempted to select a
crystal cut for a pressure crystal which minimized temperature effects,
and recommended an SC-cut. However, it was subsequently discovered and
disclosed in EerNisse '420 that the SC-cut impeded the ability of the
transducer to respond to pressure stresses applied to the resonator
housing.
Yet another EerNisse patent, U.S. Pat. No. 4,754,646, discloses the use of
an integral housing and resonator section preferably formed from AT-cut,
BT-cut, SC-cut or rotated X-cut quartz, but does not distinguish the
performance characteristics of any of the various cuts, or recommend a
particular cut. Rather, EerNisse '646 seeks to reduce resonator resistance
and pressure hysteresis via particular physical configurations of the
resonator and its area of joinder to the outer cylindrical shell.
U.S. Pat. No. 3,561,832 to Karrer, assigned to Hewlett-Packard Company,
discloses the use of AT-cut and BT-cut quartz crystal thickness-shear mode
resonators as pressure and reference crystals, but no temperature
compensation is disclosed. In fact, the preferred methodology of the
Karrer patent is to maintain the resonators at a constant temperature.
U.S. Pat. No. 3,617,780 to Benjaminson, also assigned to Hewlett-Packard
Company, discloses AT-cut and BT-cut resonators for pressure transducers.
It is known to the inventors that certain third party suppliers, such as
Clark Oilfield Measurement, Inc., of Tulsa, Okla., modify Hewlett-Packard
pressure transducers by the addition of a temperature compensating device,
commonly an RTD.
While prior art devices have attempted to address various deficiencies in
individual elements of quartz resonator transducer design, those skilled
in the art have failed to recognize that the overall design can be
enhanced in a synergistic fashion through a judicious selection of quartz
crystal characteristics for combined use in the transducer.
SUMMARY OF THE INVENTION
In contrast to the prior art, the present invention provides a superior
quartz crystal resonator pressure transducer assembly comprised of three
crystal resonators, each carefully selected for its role in the transducer
and with the end result of the lowest probability of activity dips and
resistance change while maintaining high precision and resolution in
pressure measurement.
In the pressure transducer assembly of the present invention, the pressure
crystal comprises a thickness-shear mode quartz crystal resonator selected
to have small or no activity dips and to have its crystallographic
orientation, or "cut", along the zero-temperature coefficient locus for
the shear modes as defined by Bechmann, et al. "Higher-Order Temperature
Co-efficients of the Elastic Stiffness and Compliances of Alpha-Quartz,"
Proc IRE, Vol. 50, August, 1962. The temperature crystal is a
thickness-shear mode crystal selected from cuts having high stability with
time and temperature, adequate temperature sensitivity over the intended
range for the transducer, and no (or small) activity dips. The reference
crystal is similar in characteristics to the temperature crystal, except
that it should be of a cut selected to possess a minimal
frequency-temperature response over the transducer design range, or a
so-called "zero temperature coefficient" cut, and no (or small) activity
dips.
In a preferred embodiment of the invention, the pressure crystal is
selected from the AT-cut and BT-cut, the temperature crystal is selected
from the AC-cut and BC-cut, and the reference crystal is selected from
cuts in the vicinity of the SC-cut, including the SC-cut, the IT-cut, and
the rotated X-cut. The reference crystal cut selection is somewhat
dependent upon the temperature design range of the transducer.
In a most preferred embodiment of the present invention, the pressure
crystal is an AT-cut crystal, the temperature crystal is an AC-cut
crystal, and the reference crystal is an SC-cut crystal.
The most preferred physical configuration for the transducer of the present
invention employs a pressure crystal assembly comprising a disc-shaped
pressure crystal resonator section enclosed by two hollowed-out end caps,
resonators of such design being disclosed in the aforementioned U.S. Pat.
Nos. 3,561,832; 3,617,780; 4,550,610; 4,754,646; 4,660,420; 4,802,370 and
in EerNisse, "Quartz Resonator Pressure Gauge: Design and Fabrication
Technology," Sandia Laboratories Report No. SAND78-2264, (1978). The
pressure crystal assembly is immersed in a pressure and
temperature-transmitting fluid, while the temperature and reference
crystals are thermally coupled to the fluid but isolated from the pressure
by being mounted in a pressure-proof enclosure. Electronics, as well known
in the art, are employed to drive the crystals, respond to their resonant
frequency changes, and provide mixed frequency outputs representative of
pressure (and temperature) data.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will be more fully understood by one of ordinary
skill in the art through a review of the following detailed description of
the preferred embodiments in conjunction with the accompanying drawing,
wherein:
FIG. 1 comprises a block diagram of a transducer according to the present
invention for pressure and temperature measurement; and
FIG. 2 comprises a schematic cross-sectional representation of the sensor
arrangement of a transducer according to the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring now to FIG. 1 of the drawings, there is depicted a transducer 10
for sensing pressure and temperature, and having utility, as noted above,
for both downhole and surface applications. Transducer 10 includes
electronics which drive and respond to the output of quartz crystal
resonators. The electronics and resonators are preferably contained within
a common housing, such as pressure housing 12, although this is not a
requirement of the invention.
There are three resonators, including a pressure crystal 14, a temperature
crystal 16 and a reference crystal 18. The pressure and temperature to be
sensed are of a fluid (for purposes of this invention, fluid is defined as
either the liquid or gaseous state) medium which is in thermal contact
with all three crystal assemblies, but only the pressure crystal 14 is
subjected to the pressure of the fluid: temperature crystal 16 and
reference crystal 18 being intentionally isolated from pressure effects by
their packaging and location.
The transducer electronics includes features well known in the art, such as
oscillator circuits 20, and mixer circuits 22. The oscillator circuits 20
are coupled to electrodes associated with each of the quartz crystal
resonators, piezoelectrically drive the quartz crystal resonators to
vibrate in a manner well known in the art, and transmit the frequency
output of the oscillators to mixer circuits 22. The mixed outputs are then
transmitted to processor circuits, such as frequency counter 24, exterior
to the transducer 10. The processor circuits receive the mixed frequency
outputs of the crystals, and convert same to values representative of
pressure and temperature data. The temperature output frequency is also
employed to enhance the accuracy of the pressure data by compensating for
temperature-induced frequency deviations of the pressure and reference
crystals. Actual pressure and temperature data is developed via a computer
26 receiving output from frequency counter 24. Power for the transducer is
supplied by a DC power supply 28, which may comprise batteries of any
suitable power, voltage and temperature stability. In industrial or
laboratory applications, the power supply may, of course, comprise a
conventional DC power supply.
FIG. 2 depicts a suitable physical arrangement of crystals 14, 16 and 18 in
transducer 10 wherein, as previously noted, pressure crystal 14 is exposed
to fluid pressure via inlet 30, while temperature crystal 16 and reference
crystal 18 are isolated from the pressure by housing 12, electrical
feedthrough 32 for pressure crystal 14 being pressure proof. If desired,
an electrically insulating spacer 34 may be placed about pressure crystal
14 to generally support same while permitting pressurized fluid access to
substantially the entire exterior thereof.
The pressure crystal 14 is cut along the zero-temperature coefficient locus
for the shear modes, as previously noted. This locus includes the AT, IT,
SC, rotated-X, RT, and BT-cuts. Choice of a particular cut along this
locus is dictated by a desire to optimize the frequency-temperature
behavior of the resonator of the pressure crystal assembly over a given
operating temperature range. For example, the AT-cut generally exhibits
the smallest frequency excursion for temperature ranges centered about
room temperature (20.degree. C.) operation, while the IT, SC or rotated
X-cuts are best for operating about 80.degree.-90.degree. C. However, as
noted in the EerNisse '420 patent, the SC-cut is less preferred since it
is less pressure-sensitive. As noted above, the pressure crystal is
exposed to a pressurized fluid, preferably a clean fluid surrounding the
pressure crystal to isolate it from the adverse effects of a corrosive
environment, such as is experienced in a well bore of an oil and gas or
geothermal well. The pressure crystal should not have any significant
activity dips or frequency anomalies throughout the intended pressure and
temperature operating range of the transducer, "significant" in this sense
being defined as greater than 0.1 ppm (parts per million) frequency
deviation. In addition, the pressure crystal should not exhibit large
increases (factor of 1.5 or more) in resistance over the temperature
range. The preferred orientation or cut for the pressure crystal would be
the AT-cut or BT-cut, experience having shown that these cuts generally
possess the best (lowest resistance) resonance characteristics, even
though their frequency-temperature characteristics may not be optimum for
a given temperature range. The most preferred orientation would be the
AT-cut, as it exhibits less frequency change than the BT-cut over a large
(>100.degree. C.) temperature range. While the temperature coefficient of
span (change in frequency versus pressure at different temperatures) of
the AT-cut is worse than that of the BT-cut, this is not a problem with
the ability of the transducer to accurately measure and compensate for
temperature. Activity dips of the AT-cut crystal, while occasionally
discernable, are not usually substantial. Experience has shown that
AT-cuts manufactured from man-made quartz exhibit smaller resistance
changes over temperature than when made from natural quartz. However,
sweeping natural quartz, as suggested and practiced by Hewlett-Packard,
reduces the change in natural quartz. See for example, Kaitz, "Extended
Pressure and Transducer Operation of BT-cut Pressure Transducers", Proc.
38th Annual Frequency Control Symposium (1984) and Kusters, et al.,
"Characteristics of Natural, Swept Natural, and Cultured X-and Z- Growth
Quartz Material in High Temperature, High Stress Applications", Proc. 39th
Annual Frequency Control Symposium, (1985).
The temperature crystal assembly 16 includes a temperature sensitive
crystal, the frequency output of which is employed to temperature
compensate the pressure crystal. The temperature crystal must exhibit high
stability with time and temperature, have an adequate temperature
sensitivity (normally greater than 10 ppm/.degree.C.) and have no, or no
substantial, activity dips and have small or no resistance change with
temperature. It is also desirable that the temperature crystal be of small
size and low cost, but these criteria do not form a part of the present
invention. The AC-cut has a temperature sensitivity of about 20
ppm/.degree.C. at about room temperature, which increases to about 30
ppm/.degree.C. at around 200.degree. C., so that an activity dip of 1 ppm
would create a localized temperature error of less than 0.05.degree. C.
Both the AC and BC-cuts have zero coupling to the low-frequency face-shear
modes, making them relatively free of frequency perturbations. Empirical
testing via numerous frequency-temperature scans on AC-cut crystals have
revealed no activity dips (to a resolution of >0.1 ppm) from
0.degree.-200.degree. C. as predicted by U.S. Pat. No. 2,173,589 to Mason.
In contrast, the well known U-cut temperature sensor crystal has a very
large temperature sensitivity--86 ppm/.degree.C.--but is known to be
plagued by activity dips. The AC and BC-cuts thus appear to be unique as
the only thickness-shear mode crystal cuts having a large temperature
sensitivity but no apparent activity dips, and are thus the preferred cuts
for a temperature crystal for use with the transducer of the present
invention. Stated another way, the AC- and BC-cuts appear to have zero
coupling to every piezoelectrically driven mode except the desired
thickness-shear mode, with the resulting phenomenon of a total lack of
activity dips (unless mechanically induced) at any temperature. The most
preferred cut at present is the AC-cut, based upon results obtained in
empirical testing, BC-cut crystals not having been tested.
The reference crystal assembly 18 should include a crystal of the same
characteristics of the temperature crystal, except that it should exhibit
a minimal frequency-temperature response over the intended temperature
range of use. For the range of 20.degree.-180.degree. C. the cut or
orientation of choice is the SC-cut, having the ability to be selected
(oriented) to have a maximum deviation of frequency versus temperature of
.+-.20 ppm. Other cuts along the zero temperature coefficient locus and in
the vicinity of the SC-cut have such temperature stability over similar
temperature ranges. For example, the IT-cut could be optimized over
0.degree.-150.degree. C., and the rotated X-cut could be optimized over
0.degree.-310.degree. C. However, the SC-cut has proven over
20.degree.-180.degree. C. to be extremely immune to activity dips over the
aforementioned temperature range, as they seldom occur and are typically
less than 0.1 ppm in amplitude. Moreover, the SC-cut is the only cut in
this family that is stress-compensated, a characteristic which helps
reduce any thermal transient induced frequency errors. AT-cuts, when used
as reference crystals, are plagued with activity dips, which makes the cut
a poor choice for a reference crystal about room temperature, even though
its frequency-temperature deviation about room temperature can be made
superior to that of the SC-cut (when configured as a pressure sensitive
crystal, the AT-cut does not exhibit substantial activity dips). Thus, the
SC-cut is the preferred cut for a reference crystal to be employed in the
transducer of the present invention.
While various combinations of thickness-shear mode crystals of different
orientations are possible in a working transducer of the design disclosed
herein, it is most preferred that the pressure crystal be of an AT-cut,
the temperature crystal be of an AC-cut, and the reference crystal be of
an SC-cut. This combination of crystals, each serving a special role, has
been found to provide the lowest probability of activity dips and
resistance change with temperature and therefore maximum performance of
the transducer with the least amount of testing.
While the present invention has been described in terms of certain
exemplary preferred embodiments, it will be readily understood and
appreciated by one of ordinary skill in the art that it is not so limited,
and that many additions, deletions and modifications to the preferred
embodiments may be made within the scope of the invention as hereinafter
claimed. In particular, while only specific combinations of pressure,
reference and temperature crystals having cuts selected from those
disclosed have been mentioned, it is contemplated that other combinations
from the disclosed cuts may also be utilized.
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