MDM25 User’s Manual
4 97234 Issue 4, January 2018
INTRODUCTION
1.2 Display Units
The MDM25 measures relative humidity and temperature. The third output, when
displayed, is calculated from these two parameters.
The table below gives a description of all available units.
Signal Symbol Units Description
Relative Humidity RH %
The ratio of water vapor pressure to the
saturation water vapor pressure (the
maximum amount of water vapor that can
exist at a given temperature) over a liquid
water surface, expressed as a percentage.
Temperature Ta ºC or ºF
The temperature of a gas determines the
maximum quantity of moisture that can
be present as vapor in that gas.
Dew Point Td ºC or ºF
The temperature at which dew, or
condensation, forms, on cooling a gas.
This is, therefore, the temperature at
which air becomes saturated in equilibrium
with water.
Absolute Humidity Abs H g/m
3
The mass of water vapor present in unit
volume of moist air of a given temperature
and pressure. SI (metric) units are grams
of water per cubic meter of air (g/m
3
).
Wet Bulb
temperature
Tw ºC or ºF
Temperature indicated by a thermometer
sheathed in wet wicking, and infl uenced by
the rate of evaporation from the wicking.
Table 1 Unit Descriptions
1.3 Display Settings
The Display Settings Menu contains options to change the contrast ratio, backlight
intensity and orientation of the display.
1.4 Device Settings
The Device Settings Menu contains options to change the period of time before power-
off, temperature units and 3rd value units.
1.5 About
The About Menu gives information about the fi rmware version and serial number of the
hygrometer.
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