Vaisala DM500 - Manual (Page 67)

Chapter 12 ___________________________________________________ Self Diagnostics Settings
VAISALA _______________________________________________________________________ 65
CHAPTER 12
SELF DIAGNOSTICS SETTINGS
This chapter describes the self diagnostic functions settings. The
DM500 goes through a self diagnostics procedure when the power is
switched on. If an error is found, the error display tells about it.
However, there are some self diagnostics operations that the user can
set, such as Salt detection, Sensor circuit check and Cooling capacity
test.
Salt Detection
During the salt detection function the wet sensor impedance is
detected to see if there is hygroscopic contamination on the sensor.
The clean sensor value (salt detection limit) is checked as reference.
Therefore, please clean the sensor always before determining the clean
sensor value. The clean sensor value is preset at the factory, but re-
determination by the user is recommended for optimum results.
The automatic salt detection is off as a default. You can turn it on by
setting the interval and salt detection limit. The manual salt detection
can be done to check if the sensor is clean.
Salt detection works only when there is dew on the sensor. Frostpoint
measurement is very little affected by hygroscopic contaminants, due
to the limited mobility of the ions. Consequently, a typical effect of
hygroscopic contamination on measurements is a large positive offset
that appears after the moisture phase on the sensor changes from frost
to dew.
The salt detection, as well as the clean sensor value determination
require a gas flow with dewpoint above 0º C through the sensing unit.
Lower dewpoints can also be used as long as the moisture phase
remains liquid.