DigiVac Bullseye Precision Gauge Piezo Посібник користувача - Сторінка 6
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The Bullseye Precision Gauge Piezo incorporates an isolated Piezo resistive sensor. A piezo
resistive sensor is effectively a silicon strain gauge. When the pressure is placed upon the
silicon, the resistance of a MEMS measurement resistor changes which can be measured by
the sensor electronics.
The sensor itself incorporates a wheatstone bridge that is very
sensitive to absolute pressure levels from 1 Torr = 1 mbar to 775 Torr = 1033 mbar. Piezo
resistive sensors are often referred by their full scale or maximum pressure that they measure.
For example, our 775i sensor has a maximum pressure sensitivity of 775 Torr or 15PSIA. There
are other piezo-resistive types of sensors that are sensitive in other pressure levels. Other
pressure levels that are common with piezo resistive sensors are 30 PSI, 100 PSI, 200 PSI and
1000 PSI.
Piezoresistive sensors have exceptional durability combined with low cost and better than 1%
full scale accuracy. They are chosen when a combination of durability, cost and accuracy are
all important.
The accuracy of a piezo resistive sensors are typically given in % of full scale. For example, a
100 PSI piezo sensor that is 1% accurate would have an accuracy of ± 0.01 * 100 PSI or ± 1
PSI. Note this makes the % of reading accuracy very good near its full range of 100 PSI, but
not so good near its base pressure (lowest detectable pressure) of 1 PSI. The 775i sensor has
an accuracy of ± 2 Torr, or 0.25% of full scale. This would make it's accuracy at 10 Torr ± 2/10
or ± 20% of reading at an actual pressure of 10 Torr; and at 200 Torr, ± 2/200 or 1% of
reading.
The most important thing about accuracy is knowing what you need for your process. Do you
really care about the difference between 100 Torr and 102 Torr? All sensors have trade offs of
cost and convenience. There is a tendency of us science types to demand the best most
accurate instruments. However, if our process doesn't require the best accuracy available, it
may be more efficient to do work with a gauge that is slightly less accurate but more robust.
For example, a more accurate alternative to the Bullseye Piezo is a 1000 Torr capacitance
manometer. Some can be had at 0.15% of reading, and are also isolated. However, they tend
to be expensive, don't travel well (easily damaged), and require frequent maintenance to
maintain that accuracy, especially around 10 Torr. The Bullseye Precision Gauge Piezo was
built from the ground up to be rugged and thrive with field use.
The isolated part of the sensor is important. Isolated means that the sensing electronics are
separated and protected from the process gas pressure being measured. Who cares? When
measuring clean dry gasses, isolated doesn't matter. Non-isolated sensors where the silicon
die of the piezo resistive would come in contact with clean dry gasses like air and nitrogen
work just fine. These electronics can survive a lifetime exposed to clean dry air and even low
humidity and an occasional splash of water (when not energized). Corrosive gasses on the
other hand will corrode the silicon die and wires that make up a sensor just like the ocean