Specifications

35
FEATURES/BENEFITS
 Latest generation of HUMICAP® sensor
 Withstands and recovers from condensation
 Adjustable probe head ideal for hot tapping
 East to install, calibrate and maintain
 User-programmable, versatile and easy to use
 NIST traceable (certificate supplied)
 Optional alarms relays, sensor re-gaining, local
display, sampling systems
CUSTOMIZE YOUR INSTRUMENT
Vaisalas unique microprocessor design and modular-
ity allow you to customize the HMP 238 at time of
purchase:
 Selection of output parameters:
- relative humidity - absolute humidity
- dewpoint - mixing ratio
- temperature - enthalpy
- wet bulb temperature
 Selection of:
- Alarm outputs
- Local display and keypad
- Serial bus type (RS 232, 422/485 or digital current loop)
- Power supply type (24/VDC, VAC; opt. 115 VAC, 230 VAC)
After purchase, program directly via computer,
or local display and keypad:
 Units (metric or non-metric)
 The two output parameters corresponding to
analog channels 1 and 2 (note: all output param-
eters selected by users are also available via serial
output and display)
 Selection of the following output signals:
 4 to 20 mA  0 to 1V
 0 to 20 mA  0 to 5V  0 to 10V
 Scaling of measurement parameters and output
signals (e.g. -10°C to +50°C corresponds to 1 to 5V)
The HMP 238 incorporates the latest generation of
Vaisalas patented thin-film HUMICAP
®
sensors.
Vaisala has pioneered the field of humidity since it
developed the first HUMICAP
®
humidity sensor more
than 25 years ago. Intensive in-house R&D produced
a sensor which measures humidity more accurately
(up to ±1%) and reliably at temperatures higher than
other sensors. The sensors tolerance to high tem-
perature; its ability to operate in condensing environ-
ments; its resistance to harsh chemicals and contami-
nants; and its excellent long-term stability are of great
value in monitoring and controlling critical environ-
ments.
PRESSURIZED ENVIRONMENTS WITH
VARYING HUMIDITY/DEWPOINTS
The HMP 238 probe head withstands pressure up to
600 psig. The probe depth may be adjusted under
pressures up to 150 psig. Under greater pressures it
becomes too difficult to slide the probe and tighten
the hex lock nut. This versatile probe can be installed
through a process wall, inserted through a ball valve
(hot tapping), on the back side of a ball valve (leak
position), or with a sample system for additional
filtration or cooling.
THE HMP 238 HOT TAPPING FEATURE
Hot tapping is used to enter a system without
exposing the process to ambient air, venting process
gas, or shutting the process down. The HMP 238
probe head, designed with an adjustable locking hex
nut, is hand screwed onto an existing ball valve
assembly that is already secured to the process pipe/
wall. Hand tighten the hex nut in the up position,
open the ball valve, adjust the probe to the proper
depth in the down position, and then tighten the hex
nut with a wrench. The reverse procedure is fol-
lowed when removing the probe from the process.
Hot tapping can be carried out in process pressures
up to 150 psig. It is ideal for servicing and making
portable measurements on compressed air lines
without shutting the process down.
ON-SITE, ONE-POINT CALIBRATION
By utilizing a reference RH (or temperature) probe
for comparison, routine maintenance and/or calibra-
tion of the HMP 238 can be performed on site within
a matter of minutes. This unique one-point calibra-
tion feature will save down time, reduce service
costs, and ensure high accuracy.
HMP 238 Smart Humidity/Dewpoint
Transmitter with Adjustable Probe Head for
Ease of Installation into Pressurized Spaces
HMP 238