Vaisala HMP240 - Manual (Page 7)

HMP240 series
M210300en User's Guide
5
1. PRODUCT DESCRIPTION
The HMP240 transmitter is a microprocessor based instrument for the
measurement of dewpoint temperature especially in high humidities and/or
fast changing temperatures. The dewpoint temperature is measured through
relative humidity and temperature. The dewpoint temperature, although a
calculated variable, is the primary reading obtained with the HMP240. As the
probe is equipped with the warming function, the relative humidity reading
obtained is not correct as such whereas the dewpoint temperature is. If the
temperature is below 0
°
C, the user can select whether the transmitter
calculates dewpoint or frostpoint reading; as default, the transmitter calculates
frostpoint.
The transmitter can be ordered with one or two sensor heads. If the transmitter
has only a humidity sensor head, it can output the dewpoint temperature or the
mixing ratio. If the transmitter is ordered with an additional temperature head,
the user can choose the output from the following readings: dewpoint, relative
humidity, ambient temperature, dewpoint difference (= ambient temperature -
dewpoint), mixing ratio, absolute humidity, and wet bulb temperature. The
configuration that the user completes in the order form determines the
available readings. The transmitter has two analogue outputs and can be
connected to a serial bus via the RS 232C interface or through an RS 485/422
serial module or a digital current loop module.
There are various possibilities for the configuration of the transmitter. It can
have either a blank cover, or a cover with a local display and keypad with
which to operate the transmitter. Two analogue output signals are selected
from the measured and calculated quantities; the signals can be scaled and the
measurement ranges changed. The HMP240 can be supplied with two, five or
ten meter sensor head cables.
The dewpoint measurement range is -40...+100
°
C. The range depends on the
desired accuracy because the dewpoint is calculated through the RH and the T
readings (see Chapter 8). The analogue temperature output can be scaled quite
freely, for example -20...+60 °C can be set to correspond to 0...10 V. The
relative humidity, absolute humidity, dewpoint difference, mixing ratio and
wet bulb temperature ranges are also scalable.
In some specific applications, the sensor gain may gradually decrease because
of an interference caused by some particular chemicals present in the ambient.
These changes can be recovered with an optional re-gaining function.
The transmitter is equipped with a composite humidity and temperature
sensor; the operation of the HUMICAP
® KC humidity sensor is based on
changes in the capacitance of the sensor as its thin polymer film absorbs water
molecules.