............................Page 1

Application Considerations ...........................................................................................Page 1
Mounting Considerations
Zero Adjustment.................................................................................................Page 2
Sensitivity Adjustment........................................................................................Page 3
Temperature Compensation
Overpressure Considerations ............................................................................Page 4
Frequency Response
Cable Effects
Stability ..............................................................................................................Page 5

Typical Applications ......................................................................................................Page 6
Fluid Power and Hydraulic Valves
Coal Mining Equipment
Exercise Equipment
Paint Spray Coating
Compressor Monitoring
Medical Equipment ............................................................................................Page 7

Signal Conditioning Circuits..........................................................................................Page 7
Bridge Amplifiers
Transmitter Circuit, 4-20 mA ..............................................................................Page 8
Set Point Switch Circuits....................................................................................Page 9







Advanced Custom Sensors, Inc.
45 Parker, Suite A
Irvine, CA 92618
Tel: (949) 855-6688
Fax: (949) 855-6685

0 Advanced Custom Sensors, Inc.

INTRODUCTION:

The Model 7000 is a series of high performance, low cost pressure sensors. It consists of a pressure sensing
diaphragm and a vacuum deposited thin film strain gage as shown in Figure 1. The diameter and the thickness of
the diaphragm can be adjusted to render consistent performance in pressure range from 100 psi to 10 000 psi.

The Model 7000 offers better reliability and stability by replacing the organic epoxy used in the bonded foil design
with highly stable molecular bonds between the gages and the sensing diaphragm. Thin film technology can
easily generate 0.001 wide strain gage grid line in comparison to the 0.010 grid line in foil gage. This allows
ACSI to offer designs with a smaller configuration and a higher resistance bridge for applications demanding
miniaturization and low current consumption.

Stainless steel diaphragm is the only material in contact with the media in Model 7000. It can handle particle,
contamination and corrosion much better than the micromachined silicon sensor. The standard Model 7000 can
operate up to 300
o
F which is higher than most of the silicon and bonded foil sensors. Its operating temperature
can be raised to 400
o
F for Model 7000 with a proprietary process. The temperature effects for Model 7000 are
much smaller than the silicon sensor. This makes it easier to perform temperature compensation in transducer
design. Applications with large temperature gradients and fast transients will find good use of Model 7000.

The Model 7000 is specifically designed to offer the best cost/performance mix for OEM pressure sensing
applications. These sensors cost less than 15% of a commercially available complete pressure transducer. The
users have the freedom to add the desired mounting design, pressure fitting and signal conditioning circuitry. The
performance of this approach can be as good as any strain gage pressure transducer on the market. However,
there are many pitfalls that should be avoided in order to preserve the true performances of Model 7000. This
application note is intended to provide the users all the needed information to incorporate Model 7000 into their
design. ACSI will be available to discuss any additional design issues that you may have.


0.25" TYPICAL
0.5" TO 1.0"
DIAMETER
Pressure Inlet


Figure 1


APPLICATION CONSIDERATIONS:

1. MOUNTING CONSIDERATIONS
The strain gage on Model 7000 will generate an electrical signal whenever there is a stress developed on the
sensing diaphragm. It is a true signal if this stress is introduced by the pressure of the media. It is a noise if the
1 Advanced Custom Sensors, Inc.

stress is introduced by the mount, temperature or other unwanted sources. The temperature effect will be
discussed later. This section will focus on the isolation on the mounting stress.

An improper mount will introduce unwanted stress to the diaphragm and result in an error signal. This built-in
stress can cause shift in zero balance, drop in sensitivity and, worst of all, non-repeatability. Following are
illustrations on several commonly used mounting schemes.

Shown in Figure 2 is the simplest arrangement. This can be used for all pressure ranges since the epoxy joint is
under compression. However, the best performance can be achieved by welding the sensor to a pressure fitting.
Some stress isolation scheme may have to build into the pressure fitting in order to get optimum performance.
The welding schedule needs to be developed to avoid damaging the sensor. Epoxy instead of welding can be
used in this arrangement if pressure range is less than 300 psi.

Standard tube fittings are used in Figure 4. These can be purchased from Parker with a price less than $5 in
quantity. Figure 5 is an extension of Figure 2. It provides the threaded connector for ease of installation.


PRESSURE INLET
MODEL 7000
Epoxy

Welding
MODEL 7000
PRESSURE INLET



Figure 2
Figure 3


PRESSURE INLET
Sleeve
MODEL 7000
O-Ring
Tube Nut
Tube Connector

MODEL 7000
PRESSURE INLET
Epoxy



Figure 4
Figure 5


2. ZERO ADJUSTMENT
The zero may have to be adjusted for various reason. For instance, the zero has to be adjusted to a small value
so that the auto-zero feature on the signal conditioner will work or meter can be set to a range to render required
resolution. Some of the circuits can only handle signal of one polarity. The zero has to be biased to one
direction to ensure that the output will not change polarity during service.

If an amplifier circuit is used, the zero can be adjusted by a potentiometer. This will be covered in the last section
of this application note. There are two ways to adjust the zero balance on sensor itself.

First approach is to use abrasive to thin down the thickness of the gages. It is done by rubbing a q-tip across the
desired gage. The q-tip is wetted by a compound of 0.05 micron aluminum oxide and mineral oil. Rub the gage
2 Advanced Custom Sensors, Inc.

between +Vin and -Vout, or between +Vout and -Vin will move the output to the positive direction. Rubbing the
other two gages will move the zero to the negative direction.

The other way to adjust the zero is to shunt a large resistor, Rs, across one of the gage as shown in Figure 7.
Shunting a resistor across the gage between +Vin and -Vout, or between +Vout and -Vin will move the output to
the negative direction and vice versa. This approach will introduce some temperature errors since the TCR of the
resistor is different from gages. The smaller the value of the resistor used, the larger the temperature error will
be.


SENSOR
RED(+Vin)
WHT(-Vout)
BLK(-Vin)
GRN(+Vout)

SENSOR
RED(+Vin)
WHT(-Vout)
BLK(-Vin)
GRN(+Vout)
Rs



Figure 6
Figure 7


3. SENSITIVITY ADJUSTMENT
The sensitivity of the sensor depends on the thickness of the element and the excitation voltage. The thickness
of the element is very difficult to change once it is made. By adjusting the excitation voltage to the bridge, the
sensitivity can be adjusted to be the same for all sensors and makes them interchangeable. Figure 8 shows a
simple setup for sensitivity adjustment. Two potentiometers are used here to keep the common mode voltage of
the output at � of the excitation voltage. If this is not required, one potentiometer can be removed.


SENSOR
RED(+Vin)
WHT(-Vout)
BLK(-Vin)
GRN(+Vout)
Excitation Voltage


Figure 8.


4. TEMPERATURE COMPENSATION
Figure 9 is the generic approach to temperature compensate Wheatstone bridge device. Only resistor
components are used in this scheme. Similar results can be achieved using transistor, diode and op-amp.

R10 and R11 are for sensitivity adjustment. R1, R2, RT1 and RT2 are used to compensate the thermal sensitivity
shift. RT1 and RT2 are thermistors. Their TCR is determined by the TCR of sensors sensitivity. R6 and R8 are
for zero adjustment, and R5 and R7 are for thermal zero compensation. R5 and R7 can be either resistors or
thermistors depending on the TCR of sensors zero balance.

3 Advanced Custom Sensors, Inc.

All these compensation resistors are inter-dependent. It may take several iterations to come up with a set of
acceptable values. ACSI will be able to help you with your particular concerns.


RED(+Vin)
WHT(-Vout)
BLK(-Vin)
GRN(+Vout)
R11
R1
RT1
G1
G2
G3
G4
R5
R6
R8
R7
R10
R2
RT2


Figure 9


5. OVERPRESSURE CONSIDERATIONS
Pressurizing the sensor beyond its design limits is a common problem. ACSI specifies three pressure ranges:
operating range, overpressure range, and burst pressure range. The overpressure range is the level that the
sensor can withstand without a subsequent change in performance characteristics. Exceeding the overpressure
range w