Vaisala HMP228 User Manual
Page 7
HMP228
M210282en-A
Operating Manual
1
1.
PRODUCT DESCRIPTION
1.1
General characteristics
The HMP228 transmitter is a microprocessor based instrument for the
measurement of moisture in terms of water activity e.g. in the lubrication of
circulation systems or in transformer oil. The transmitter incorporates a
capacitive thin film sensor. The operation of the sensor is based on changes in
its capacitance as the thin polymer film absorbs water molecules.
The HMP228 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 current loop module.
The transmitter can be configured in many ways. It can have either a blank
cover or a cover with a local display and keypad with which the user can
operate the transmitter. The power supply voltage can be selected from three
alternatives. Two analogue output signals can be scaled and the measurement
ranges changed within certain limits. The HMP228 transmitter can be supplied
with two, five or ten metre sensor head cable.
The HMP228 also provides for accurate temperature measurement. It is an
easy-to-install on-line transmitter which can be calibrated against traceable
salt solutions.
Options
Power supply
24 VDC (standard); (24 VAC: see Chapter 2.3.2),
115/230 VAC
Serial interface
RS 232C (standard), RS 485/422, current loop
Display cover
cover with or without local display & keypad
Alarm output unit
not with 115/230 VAC power supply
Cable length
2, 5 or 10 metres
Cable connectors
for 24 VDC supply, for analogue outputs, for RS
232C line and for RS 485 single loop line; see
Appendix 8 for details.
1.2
Typical applications
1.2.1
About the method used for measuring moisture in oil
The HMP228 transmitter measures water in oil in terms of water activity (aw)
which can be determined as follows: water activity indicates the amount of oil
in the scale of 0 - 1 aw. In this scale, 0 aw is an indication of completely water
free oil and 1 aw an indication of oil fully saturated with water. Water is
present in free form.
The most important feature which distinguishes the measurement of water
activity (aw) from the traditional measurement of absolute water content (in