Atmospheric Pressure Compensation; Cold Junction Compensation - Ahlborn ALMEMO 2590-2 Operating Instructions Manual

10. Measuring menus

10.2.5 Atmospheric pressure compensation

Measured variables dependent on the ambient atmospheric pressure (see
Section 12.9 Measuring range list ´with PC´) may, in the event of large devia-
tions from normal pressure (1013 mbar), involve certain measuring errors.
e.g. Error per 100 mbar
Rel.humidity psychrometer
Mixture ratio, capacitive
Dynamic pressure
O saturation
2
It is advisable therefore, especially when taking measurements at appreciable
heights above sea level to take due account of the atmospheric pressure (ap-
prox. -11 mbar / 100 meters above mean sea level, MSL). On all sensors re-
quiring atmospheric pressure compensation the
function
atmospheric pressure
The appropriate atmospheric pressure can be entered either in
or in device programming (see 13.6) or it can be measured using an atmo-
spheric pressure sensor (reference sensor with designation ´*P´, see 12.2,
Manual 6.7.2). While the atmospheric pressure is being used for compensation
both in the function
symbol appears; if it is being measured a point flashes behind
CP

Please note that as soon as a reference sensor is disconnected
normal pressure, 1013 mbar, is used.

Flow values (also volume flow with rotating vanes) acquired with at-
mospheric pressure compensation can by means of ´#N´ in the
designation (see 12.2) be converted to standard conditions of 1013
mbar.

10.2.6 Cold junction compensation

Cold junction compensation for thermocouples is normally performed automati-
cally by means of an NTC sensor in measuring socket M2. This cold junction
temperature is displayed in the device configuration as an operating parameter
(see 13.8). This can if necessary be incorporated in measured data acquisition
as device temperature with function channel ´CJ´ (see 12.10). Instead of this
form of cold junction temperature measurement it is also possible to use an ex-
ternal measuring sensor (Pt100 or NTC) in an isothermal block (see Manual
6.7.3); this must be positioned upstream from the thermocouples and ´*J´ must
have been programmed in the first two positions in the designation (see 12.2).
For especially exacting requirements (e.g. for thermocouples for which there is
no connector with thermo-contacts or for large temperature differences caused
by thermal irradiation) special connectors are available, each with its own inte-
grated temperature sensor (ZA-9400-FSx) for cold junction compensation.
These can be used for all thermocouple types; however, they require 2 mea-
suring channels. Having "#J" programmed in the first two positions in the des-
ignation for the thermocouple ensures that the temperature sensor integrated
in the connector is indeed used for cold junction compensation.
26 ALMEMO
Compensation range
approx. 2%
approx. 10 %
approx. 5%
approx. 10%
:
atmospheric pressure
® 2590
500 to 1500 mbar
Vapor pressure VP up to 8 bar
800 to 1250 mbar (error < 2%)
500 to 1500 mbar
sensor display includes
atm pressure: CP
and at the measured value the
the
1013 mbar.
sensor display
CP.