Cell; Alarm Relay - Ametek 303B User Manual

these components can release their moisture, which will be measured as if
they were in the sample stream. The reverse also is true; as sample tem-
perature falls, moisture will be absorbed into the components, lowering
the moisture readings. The only fix is to remove all absorbing/desorbing
parts from the system.

Cell

The cell converts the water-content of the sample gas passing through it
to an electrical current. The cell determines the moisture by absorbing the
gaseous water, and electrolyzing the absorbed water. This accounts for
the high degree of accuracy and specificity, since the process is virtually
unique for water and consumes at least 99.9 percent of the gaseous water
at levels as low as 1.0 ppm or as high as 2000 ppm.
The assembly is cased in epoxy resin for mechanical protection. This nor-
mally precludes damage from any cause except objects which may en-ter
through the cell ports.
If flow of a sample gas with specific moisture content is constant, the
moisture is absorbed and electrolyzed, producing a constant current.
When flow is increased, electrolyzing current increases proportionally.
Current is directly and linearly related to mass flow, and can be calculated
from Faraday's law.
When these factors are considered for water being electrolyzed by a
moisture cell, current in µA/ppm of water is given by IµA = (0.132 x flow)
where flow is the sample flow in mL/min, measured at 100 kPa (1.00 atm)
pressure and 25°C. For example, at 20 mL/min, 1 ppm of water produces
2.64 µA; 100 mL/min, 13.2 µA. The current is displayed as ppm.

Alarm Relay

The alarm relay has two contacts: normally open (NO) and normally
closed (NC). With the relay OFF, NO contacts are open and NC contacts
are closed. With the relay ON, NO contacts are closed and NC contacts
are open.
There are two relay modes controlled by jumper JP2 (figure 3-2): low-
power (pins 2 and 3) and fail-safe (pins 1 and 2). In low-power mode, the
relay coil turns ON when the moisture reading is above the alarm set-
point; the coil turns OFF with low ppm readings, such that battery power
can be conserved.
The relay works against a single setpoint setting. In the fail-safe mode, the
relay coil is ON when the moisture reading is below the alarm setpoint
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