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RUSKA 7750i
Users Manual
Long Term Stability
Long Term Stability defines how the instrument drifts with time. This specification can
be utilized to define the calibration interval for the standard. Some manufacturers will
provide more than one stability specification for their instrument based on different
calibration time intervals. It is important to understand that you should not arbitrarily
vary the manufacturer's stability specification based on time without knowing the
characteristics of their device. Some manufacturers identify that their stability
specification is proportional with time. Therefore, if the calibration interval were reduced
in half, the magnitude of the stability specification would also be halved. This can be a
powerful tool when you are trying to improve the measurement performance of a
standard. By reducing the calibration interval, the expanded uncertainty would also
reduce. On the other hand, some manufacturers do not claim that their stability
specification is proportional with time. This would be the case for instance if the
instrument naturally drifted in a sinusoidal fashion. This would suggest that the sensor
could drift to its maximum stability limit at any time and therefore, reducing the
calibration interval would not improve the expanded uncertainty of the device.
Short Term Stability
Short Term Stability relates to the zero drift characteristics of the instrument.
This generally is classified as short term drift since the instrument can be re-zeroed
without performing a full calibration as required to correct for long term span drifts.
The magnitude of zero drift can be assessed based on the length of time between
re-zeroing the instrument.
Uncertainty of the Standard
Uncertainty of the Standard is used to calibrate the transfer standard. This is the expanded
uncertainty of the calibration standard that was used by the manufacturer to calibrate the
digital transfer standard. This should be the expanded uncertainty of the calibration
standard and include all sources of uncertainty that would influence the calibration
standard including the uncertainty from the National Standards Laboratory that the
standard is traceable.
It should also be noted that when the instrument is re-calibrated, the uncertainty of the
device is influenced by the uncertainty of the calibration standard that will be used to
perform the re-calibration. Therefore, the uncertainty analysis should be evaluated
following each re-calibration. If the instrument is re-calibrated using a different
calibration service provider than the manufacturer, the uncertainty of the standard that the
calibration service provider used to perform the calibration would need to be substituted
for the manufacturers calibration uncertainty that was used in the original uncertainty
analysis.
Environmental or Installation Influences
Environmental or installation influences that could cause errors in the transfer standard:
This includes influences such as ambient temperature, line pressure, head pressure, time
response, and controller effects. (It may include other influences that are very specific to
one manufacturer's instrument.) It is recommended that the intended application is
reviewed to assure that the environmental does not impact the instruments performance,
or that the impact from the environment is accounted for in the uncertainty analysis.
For instance, if an instrument has a 0.001% FS per °C temperature effect from a
calibrated temperature of 20 °C, and the instrument is to be used in an environment where
the temperature will vary from 15 °C to 25 °C, then a ± 0.005% of full scale uncertainty
should be included in the uncertainty analysis for ambient temperature effects.
Absolute Zeroing Reference Sensor
The two sigma expanded uncertainty of the vacuum sensor is used to zero the absolute
Ps sensor is estimated to be less than or equal to 10 mtorr (1.33 Pa) per year.
A-2
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