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LABVIEW DAQ 压力传感器资料 Pressure Measurements with DAQ Systems
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Pressure Measurements with DAQ Systems
Overview
This application note describes pressure sensors and explains how they work. It also details the requirements for
measuring pressure and the signal conditioning required for the measurement. It explains how to use NI data acquisition
(DAQ) systems to measure bridge-based sensors, and recommends three different starter kits for pressure
measurements. Finally, an appendix is included explaining other less common pressure sensors and their areas of
application.
For in-depth guidance on making a pressure measurement, visit the how-to guide.
What Is Pressure?
Pressure is defined as force per unit area that a fluid exerts on its surroundings.[1] There are two pressure measurement
types: absolute and gauge. Absolute pressure is referenced to the pressure in a vacuum, whereas gauge pressure is
referenced to another known pressure level, usually the ambient atmospheric pressure. The SI unit for pressure is the
Pascal (N/m2), but other common units of pressure include atmospheres, bars, inches of mercury, millimeters of
mercury, and Torr.
Pressure Sensors
Because of the great variety of conditions, ranges, and materials for which pressure must be measured, there are many
different types of pressure sensor designs. Often we measure pressure by converting it to some intermediate form, such
as displacement, which we can then measure with a sensor. Different methods are available for measuring pressure
depending on whether the pressure being measured is greater or less than atmospheric pressure. Of all the pressure
sensors, Wheatstone bridge (strain based) sensors are the most common, offering solutions that meet varying accuracy,
size, ruggedness, and cost constraints. For information on other types of sensors, refer to the Appendix at the end of this
document.
Bridge sensors are used for high- and low-pressure applications, and can measure absolute, gauge, or differential
pressure (the difference in pressure between two different points of interest). All bridge sensors make use of a strain
gauge and a diaphragm (see Figure 1).
©National Instruments. All rights reserved. LabVIEW, National Instruments, NI, ni.com, the National Instruments corporate logo, and the Eagle logo are trademarks of National Instruments. See
ni.com/trademarks for other NI trademarks. Other product and company names are trademarks or trade names of their respective companies. For patents covering National Instruments products, refer
to the appropriate location: Help>>patents in your software, the patents.txt file on your CD, or ni.com/patents .
Document Version 10
Figure 1. Cross Section of a Typical Bridge-Based Pressure Sensor.[2]
When a change in pressure causes the diaphragm to deflect, a corresponding change in resistance is induced on the
strain gauge, which can be measured by a DAQ system. Sensors that use foil strain gauges can be bonded directly to a
diaphragm or bonded to an element that is connected mechanically to the diaphragm. There are also silicon-based
sensors, wherein the strain gauge is etched onto a silicon substrate and a transmission fluid is used to transmit the
pressure from the diaphragm to the substrate. In general, foil strain gauges are used in high-pressure (70-700 MPa)
applications. They also have a higher operating temperature than silicon strain gauges (200° C versus 100° C,
respectively), but silicon gauges do offer the benefit of larger overload capability. A silicon strain gauge can sustain up
to 400 percent of its rated pressure without being damaged, whereas a foil strain gauge can only typically sustain a 50
percent overload. Because they are more sensitive, silicon gauges are also often preferred in low-pressure applications
(~2kPa).[2]
Once you have chosen the type of material for your pressure sensor, you must determine the type of pressure
measurement you will be making. There are three types of pressure measurements: absolute, gauge, and differential. As
mentioned earlier, an absolute pressure measurement includes atmospheric pressure in the measurement; its reference
point is 0 Pa, the pressure in a vacuum (see Figure 2).
2 www.ni.com
Figure 2. Diagram of a Absolute Pressure Sensor.[3]
Gauge pressure is measured relative to ambient atmospheric pressure, so it does not include atmospheric pressure (see
Figure 3).
Figure 3. Diagram of a Gauge Pressure Sensor.[3]
Differential pressure is similar to gauge pressure, but instead of measuring relative to ambient atmospheric pressure,
differential measurements are taken with respect to a specific reference pressure (see Figure 4).
3 www.ni.com
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