Vaisala DM70 - Manual (Page 68)

USER'S GUIDE____________________________________________________________________
Adjusting DM70
The reference condition of the dewpoint must be traceable to the
appropriate standards. The user adjustment requires a stable humidity
generator capable of producing the required humidities and a
calibrated reference dewpoint meter. For adjustment, the probe and
the reference dewpoint meter are connected to the humidity generator
output, the reference condition is adjusted and let to stabilize. After
the probe and the reference dewpoint meter are stabilized, the DM70
reading is adjusted to correspond the reference value.
For the adjustment of the DMP74A probe, the dewpoint reference
temperature should be between -40 °C and -60 °C (-40 °F ... -76 °F),
and for the DM74B and DMP74C probes, the dewpoint reference
temperature should be between
-57 °C and -67 °C (-70.6 °F ... -88.6 °F). The optional sampling cell
DSC74 (see Figure 20 on page 48) can be used as a calibration
chamber to be connected with a humidity generator.
To ensure the correctness of the adjustment, the reference dewpoint
meter must be calibrated at a recognized laboratory with a known
uncertainty and traceability to national or international standards
The T
d
and H
2
O parameters can be viewed in the adjustment mode to
see the effect of the adjustment.
To see when the adjustment was made last, select Last adjustment
date or check the date from the Device information (see section
Device Information on page 41).
Adjusting DMT340 Series Transmitters Using
DM70 as Reference or Terminal
Vaisala DRYCAP
®
Dewpoint and Temperature Transmitter Series
DMT340 can be adjusted using a correctly adjusted DM70 as a
reference dewpoint meter. Note that dewpoint adjustment should be
carried out in laboratory conditions and not in the field. The
adjustment procedure for DMT340 using DM70 as a reference is very
similar to the adjustment of the DMP74B/C probes. You can also use
the MI70 indicator without the reference probe as a terminal to display
the measurement readings and to control the adjustment functions
when making adjustment of DMT340. This is especially useful with
transmitters without display. Instructions are provided in section Td/f
Adjustment of DMT340 Series Transmitters using DM70 on page 71.
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