NARDA EHP-50-G, User Manual

Page: 14 / 92

1-6 General Information
1.6.2 Explication Notes a) If we have the certificate with different values of the uncertainty of
calibration, in order to calculate the total expanded uncertainty U
TOT
,
the uncertainty of calibration has to be taken into account:
where is the standard uncertainty
………….
Cal
u
is the standard uncertainty
(
Cal
u
= U
Cal
/ k )
where U
Cal
is the expanded uncertainty reported on certificate
and k is the coverage factor reported on certificate (normally
k=2 or 1.96)
This quadratic sum is considered a precaution because the
uncertainty
Cal
u
is added quadratically to an uncertainty that is
already a combined uncertainty.
b) When the environmental temperature is higher than 23 °C, the contribute
due to the temperature can be added quadratically to the uncertainty.
Example: for magnetic measurements, if the temperature is 43°C, we get a
variation of 20°C in comparison to 23°C, corresponding to a variation of
0.26 dB ( 0.013 dB / °C ) equivalent to 3,04% and therefore, assuming a
rectangular distribution of probability, the standard uncertainty is 1.755%
( = 0,0304 /
3
).
Assuming an expanded uncertainty U
TOT
=2.4%,
( )
042 . 0 01755 . 0
2
024 . 0
96 . 1
2
2
≅ + 





• =
TOT
' U
is obtained.
The quadratic sum is considered a precaution because the uncertainty due
to temperature, is added quadratically to an uncertainty that is already a
combined uncertainty.
A similar calculation can be made in case the relative humidity overcomes
50%.