Applying User's Calibration Uncertainties; Operating And Calibration Temperature Ranges; Applying Temperature Coefficient Specifications - Fluke 8508A Service Manual

8508A
Service Manual

Applying User's Calibration Uncertainties

When the 8508A is calibrated by another laboratory the uncertainties of the calibration
standards used may be applied by combining those uncertainties with the 8508A's
Relative to Standards specifications. The applicable calibration uncertainties and the
8508A relative specifications must both be expressed at the same confidence level, and
be combined in a RSS (Root Sum Square) summation. Accepted metrology practice
mandates that calibration uncertainties are stated at 95 %. Check the applicable
calibration uncertainties are stated at 95 % and then combine them with the 8508A 95 %
Relative specifications. For example, if the 8508A is calibrated at 10 V DC with an
uncertainty of 1.5 ppm at 95 %: The absolute uncertainty at 10 V for a period of 90 days
and 1 C from calibration is:

Operating and Calibration Temperature Ranges

As a metrology tool, the 8508A will commonly be used in a calibration laboratory where
the temperature would be controlled to 1 C, and the 8508A 1 C specifications are
applicable to those situations. The majority of electrical calibration laboratories operate at
a nominal temperature of 23 C, the temperature at which the 8508A is calibrated by
Fluke during manufacture and service. The 8508A is also capable of being calibrated at
any temperature between 20 C and 25 C and the 1 C specifications will apply to
operation within 1 C of that calibration temperature. In the 8508A specification tables
the temperature of calibration is referred to as TCal. Specifications for 5 C are
provided for situations where the 8508A is operated in environments with wider
temperature variations up to 5 C. For applications where the knowledge of the effect of
temperature on 8508A performance is important, temperature coefficients are listed in the
8508A specifications. If the operating temperature is within the range 15 C to 30 C the
15 C to 30 C temperature coefficient specifications are applicable otherwise use the
5 C to 15 C/30 C to 40 C figures, provided the temperature lies within that range. The
8508A may be operated at temperatures between 0 C and 50 C, but performance is not
specified outside the range 5 C to 40 C.

Applying Temperature Coefficient Specifications

The 8508A specification tables include information for the typical operating conditions
of 1 C for calibration laboratories with tight temperature control, and 5 C for
calibration laboratories with looser temperature control or uncontrolled environments
within that temperature range. For the majority of applications choosing the Absolute
specifications for the most appropriate operating temperature range will be adequate.
However performance at other temperatures may be determined by including an
allowance for temperature coefficient over the additional temperature range. Care should
be taken when making this calculation as an amount of temperature coefficient is already
included in the 8508A specifications and those specifications are themselves based on
combining contributions using techniques similar to those employed in uncertainty
analysis. For example, consider operating at 33 C, 10 C from the 23 C calibration
temperature. The 5 C specifications already include a contribution for 5 C of
temperature difference, so this amount of temperature effect must be removed before the
effect of the 10 C difference is added. Consider 10 V on the 20 VDC range: 365 day
absolute specification (95 %) at 33 C expressed in parts-per-million of 10 V is:
1-20
⎛ ⎞
20
2
⎜ ⎟
1 5 . 1 4 . 0 2 .
⎝ 10 ⎠
2
⎛ ⎞
20
⎜ ⎟
3 5 . 0 2 . 5
⎝ 10 ⎠
2
2 3 . ppm of 10 V
2 2
x 0 3 . 10 x 0 5 . . 6 16 ppm of 10 V