4 Transmitter
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4.1.4.
Technical data
Parameter
• Dewpoint
temperature
1
Measuring range at 25 °C
•
-90 to -20 °C
td
Meas. uncertainty
2
-20 °C
td
to -40 °C
td
: +/-1,5 K
-40 °C
td
to -60 °C
td
: +/-2 K
-60 °C
td
to -75 °C
td
: +/-2,5 K
3
In case of direct installation of the 6781 in the process the
ideal face velocity is between 0.5 and 2 m/s. The
measurement uncertainty increases with deviating flow
values.
1
Above 0 °C
td
the dewpoint temperature is shown and below 0 °C
td
the frost
point temperature is shown. In the instruction manual no differentiation is made
and both are referred to as dew point temperature. In rare cases at dewpoint
temperatures between -35 °C
td
and 0 °C
td
differences between the 6781 and a
dew mirror hygrometer may occur.
2
Expanded basic uncertainty (K = 2) at a process temperature of 25 °C.
Measuring uncertainty according to GUM (Guide to the Expression of Uncertainty in Measurement):
ISO guideline for determining the measuring uncertainty in order to render global measurement results
comparable.
The following uncertainties are used during the inquiry:
• Hysteresis
• Linearity
• Reproducibility
•
Adjustment area/factory calibration
• Test
location
An additional uncertainty factor depending on dewpoint temperature and
process of ±0.03 K x reading (in °C
td
) + 0.2 K x (25 °C - process temperature in
°C) results from this overall view
Below -75° to -90°Ctd, an assmmetrical measurement uncertainty of +8.5K/
-2.5K applies.
3
At process temperatures > 25 °C the lower limit of the measuring range
increases by 0.5 K/K. The uncertainty values are valid for the respective
measuring range.
