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There are
many requirements for pressure measurement in ranges of
less than 1 kPa to 15 kPa above and below atmosphere.
The calibration support of instruments in this
range presents special challenges in the areas both of
pressure measurement and control. DH Instruments, Inc. has
developed a new pressure controller/calibrator to address
the need for a low gauge pressure transfer standard. |
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Figure 1.
PPC2+ BG0002 TM (top) with thermally isolated
volume accessory (bottom) |
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The
instrument exploits recent developments in Paroscientific
Digiquartz® quartz crystal pressure transducers.
A new Digiquartz® Model 202BG provides
enhanced performance over the range of
– 15 to + 15 kPa.
A new fundamental standard
developed by DH Instruments, Inc. to cover low gauge and
absolute pressure is used to calibrate the reference
pressure transducer. The
standard, called a force balanced piston gauge
(FPG8601TM), is fully automated and has
an estimated measurement uncertainty (k=2) of ± (0.02 Pa + 30
ppm of reading). The
standard’s range and uncertainty are included in
the scope of DHI’s calibration laboratory A2LA
accreditation.
Combining the precision and stability of the
quartz crystal resonator transducer with the FPG8601
calibration capability gives the PPC2+ BG0002 one year
estimated measurement uncertainty (k=2) of better than + 0.01%
of span for the each of the three ranges, ± 5, 10
and 15 kPa (± 20, 40, 60 “H2O).
Figure 2 provides a plot of as left errors of a low
pressure controller’s three ranges relative to the force
balanced piston gauge. |
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2. As left errors of PPC2+ BG0002’s three ranges
relative to FPG8601 |
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The
low pressure calibrator’s pressure control system
operates on the positive shut-off pressure control
principle used in other DHI pressure
controller/calibrators. Measures taken to optimize
positive shut-off pressure control for very low pressure
operation allow pressures to be set with control precision
up to 0.001% of the active range so that the contribution
of control precision to the uncertainty in the
pressure controller’s delivered pressure is negligible.
The typical time to set and stabilize a pressure is
30 seconds.
The new low pressure controller/calibrator has been used
in both calibration laboratories and production
transmitter characterization. Figure 3 is an error
plot for a two cycle calibration of an industrial pressure
transmitter in the range of +5 kPa (+20 "H2O).
Note that the test repeatability, including
repeatability of the transmitter, is within ± 0.4Pa
(0.0015 “H2O) for all data points which is 40 ppm of the
transmitter range and 20 ppm of the controller range.
The test was fully automated using DHI COMPASS®
calibration assistance software and completed about 10
minutes. |
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Figure 3. Pressure transmitter
errors relative to PPC2+ BG0002 |
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| Related
Links |
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Author: This article was
submitted by DH Instruments.
DH
Instruments
4765 East Beautiful Lane
Phoenix,
AZ
85044-5318
USA
Tel: (602) 431-9100 |
