Van Air Systems High Dew Point Alarm Box User Manual
THE HUMISTAT MODULE USES 115 VAC. AVOID CONTACT
WITH EXPOSED TERMINALS WHEN PERFORMING SET
NEVER TEST THE HUMIDITY SENSOR WITH AN OHMETER
OR OTHER DC MEASURING DEVICES, DOING SO WILL
PERMANENTLY DAMAGE THE SENSOR.
DO NOT EXPOSE THE SENSOR TO TEMPERATURES
IF SENSOR IS EXPOSED TO LIQUID MOISTURE IT WILL BE
5.1 HUMISTAT MAINTENANCE
Periodically DISCONNECT POWER and clean dust and lint from the
PC board with a soft brush.
5.2 DEW POINT SENSOR MAINTENANCE
Periodically check the humidity sensor for accumulation of dust and
lint. If necessary, use a soft brush to remove accumulated dust and
lint from the exterior jacket, but DO NOT ATTEMPT TO REMOVE
ALL TRACES OF LINT AND DUST WITHIN THE SENSOR, since
the introduction of the brush, etc, may damage the sensor. A slight
accumulation of dust will not impair the normal function of the sensor
as long as it is not heavily contaminated.
It is recommended that the sensor be replaced every two (2) years.
ALWAYS CALIBRATE THE HUMISTAT AFTER THE DEW
POINT SENSOR IS CLEANED. REFEFENCE SECTION 5.4.
4.1 START UP
Once the unit has been properly installed it is ready for operation.
Turn on the power supply to the unit. Open the sample cell isolation
needle valve. Air must be exhausting from the sample cell when the
sample source is pressurized.
The sample air passes over the sensor in the sample cell. The
sample air is then exhausted through the bleed orifice fitting, which
is used to maintain line pressure in the sample cell.
The isolation needle valve should be fully open during operation.
This valve is only used to shut-off the sample air in the event that
the sensor needs to be removed.
The operation of the High Dew Point Alarm is based on relay actuation
at adjustable resistance values. The resistance of the sensor varies
in inverse proportion to the relative humidity to which it is exposed. When
the relative humidity rises above the set point (approx. 0
F pressure dew
point @ 80
F) , the controller detects the corresponding resistance
change and activates a single pole, double throw relay.
When the relay is closed the “HIGH DEW POINT” light is illuminated. At
the same time the control relay CR1 is energized, this closes the alarm
output contact and latches the alarm until the “ALARM RESET PUSH-
BUTTON” is pushed.
4.3 EQUIPMENT DESCRIPTION
The following sections include detailed descriptions of the individual
components in this alarm. REFERENCE FIGURE 2A.
4.3-1 SENSOR (hygrosensor)
The sensor element consists of a dual winding of precious metal wire,
on a plastic cylinder, with a moisture sensitive compound (DUNMORE-
TYPE lithium chloride element). A perforated metal or plastic cover
protects the sensor element from physical damage.
The operation of the sensor is based upon the ability of the coated
hygroscopic film to change it’s resistance instantly with microchanges
in the relative humidity of the sample air. The resistance of the sensor
is inversely proportional to the relative humidity to which it is exposed.
The sensor is mounted in the sample cell, which is located on the
bottom of the enclosure.
4.3-2 DEW POINT CONTROLLER (humistat)
The controller is designed to monitor electric current flowing through
the sensing element and detect resistance changes and automati-
cally actuate a single pole double throw relay when the resistance
varies above the set point.
The controller set point is calibrated using the set plug.
4.3-3 SET PLUG
The set plug is provided with the controller. The set plug is used to
calibrate the controller. The plug is simply plugs into the cable instead
of the sensor. The plug will emulate a set humidity condition. The
controller then is adjusted accordingly. The calibrating instructions
are outlined in Section 5.4.
5.3 MISC EQUIPMENT MAINTENANCE
Periodically check the following:
• Check the bleed orifice on the sample cell to make sure it is not
plugged or damaged. If plugged or damaged, replace immediatelly.
• Check the isolation valve to make sure it is fully open. If valve will
not open completely or is leaking replace immediately.
• Check alarm light. Replace light bulb every 6 months.
• Check the cable from the sensor to the humistat to make sure it is
completely seated into the sockets. Check cable for damage. If
cable is damaged replace immediately.
• Check dew point sensor for performance by using a spare sensor
and compare performance.
• Replace the PC board if a malfunction CAN NOT be resolved by
replacing the dew point sensor.