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Combining the Uncertainty Components into an Expanded Uncertainty Statement
Once all of the uncertainty components are identified, they can be combined into an
overall Expanded Uncertainty Statement for the instrument. The vast majority of the
measurement community has adopted a statistical approach to uncertainty analysis such
as the International Organization for Standards (ISO) Guild to the Expression of
Uncertainty in Measurement. This statistical approach combines all of the uncertainty
components in quadrature (RSS).
In this approach, it is important to define the level of confidence that all of the
uncertainty components are defined. For instance, all of the uncertainty specifications
that Fluke publishes are at the 2 sigma level which would be at a 95% confidence level.
Some manufacturers may state their uncertainty at different confidence level and
therefore, you would need to be able to convert from one to the other in order to compare
the two devices. For instance, if a manufacturer states that they have an uncertainty of
0.005% FS and that the uncertainty is expressed at the 1 sigma level, then it would be
doubled to express it at the 2 sigma level (i.e. it would be an 0.01% FS device at a
2 sigma or 95% confidence level.)
Controller Specification
When the system is in control mode, the control noise is defined as how close the
controller can maintain the control pressure to the commanded set-point. In the active
control mode, the RUSKA 7750i has a control noise specification of 0.001% of
Full Scale.
In the Passive control mode, since the controller shuts off when it reaches a user-defined
band about the commanded set-point, the uncertainty of the pressure measurement is not
influenced by the controller.
The following tables are uncertainty analysis examples for a RUSKA 7750i based on a
three-month and a yearly calibration.
Table A-1. Uncertainty Analysis for a RUSKA 7750i — 3 month and Yearly Calibration
Uncertainty Analysis —
RUSKA 7750i
A.1 Performance — (Linearity, Hysteresis,
Repeatability and Temperature)
A.2 Span Stability
A.3 Zero Stability — (re-zero at 8 hours)
A.4 Calibration Standard – (Abs. Mode)
(RUSKA 2465 or 2468 DWG)
A.5 Environmental —
Temperature (Included in A.1)
Head Pressure (± 0.25 in)
Control — (in Passive Control)
A.6 Vacuum Reference Zeroing Sensor
A.7 Two Sigma Expanded Uncertainty
90 Day Uncertainty
32 inHg Ps Sensor
(2 sigma)
Greater of 0.005% of RDG
or 0.0004 inHg
0.0019% of RDG/90 days
0.0004 inHg
0.001% RDG & 0.00039 inHg
0.000% of RDG
0.0007% of RDG
0.000% of RDG
0.00039 inHg
RSS (0.0055% RDG &
0.0008 inHg)
Summary of Specifications
Uncertainty Analysis
1 year Uncertainty
32 inHg Ps Sensor
(2 sigma)
Greater of 0.005% of RDG
or 0.0004 inHg
0.0075% of RDG/year
0.0004 inHg
0.001% RDG & 0.00039 inHg
0.000% of RDG
0.0007% of RDG
0.000% of RDG
0.00039 inHg
RSS (0.0091% RDG &
0.0008 inHg)
A
A-3
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