Field Validator; Theory Of Operation; Sensor Theory; Sample System Operation - GE DewPro MMY 245 Installation And Operation Manual

DewPro MMY245 Installation and Operation Manual

1.2.2 Field Validator

This portable unit can validate the readings of permanently installed GEI loop-powered dew point
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transmitters while they are in the loop, simply by connecting the MMY 245 to the DewPro's outlet and
monitoring the same sample. This is accomplished by monitoring the same sample as the transmitter by
connecting to its outlet. Either a single-point or two-point validation mode can be selected via the
keypad and matrix. An RS-485 connection between the portable and the transmitter provides for digital
correction of the transmitter's calibration data. The procedure takes from one to four hours and
eliminates transmitter downtime and reduces the need for sending transmitters back to the factory for
recalibration. The complete description of this feature can be found in section 4 of this manual. This
validation is not a field calibration technique since most calibrations require the standard to be four
times more accurate than the device under test. The MMY 245 does not have that accuracy advantage
and should be used to adjust transmitters in the field with this in mind.

1.3 Theory of Operation

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The DewPro MMY 245 operates as a self-contained moisture analyzer system comprised of a sensor,
electronics, power source, and sample system.
The sensor is an advanced Planar Gold/Aluminum Oxide sensor designed to respond quickly to changes
in measuring conditions. It has been used extensively in trace moisture analyzers for several years. This
trusted sensor is resistant to contamination and has proven to be a rugged and stable platform for
consistent results in measurement accuracy and reliability.
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The electronics package was designed using the basic DewPro technology developed four years prior
to the introduction of the portable configuration. With proven reliability and ruggedness and with the
addition of memory and energy management components, the MMY 245 provides the flexibility
required for many applications.
Four standard "D" cell batteries supply the power for the MMY 245. Standard alkaline batteries are
included and are recommended for replacement.
The internal sample system provides for purging the hoses after they are connected to the sample tap
and then directs the sample across the sensor.

1.3.1 Sensor Theory

The Planar Gold / Aluminum Oxide sensor is a capacitance based technique for detecting the moisture
content in gaseous samples. This sensor is manufactured using semiconductor techniques on wafers of
ceramic substrate. The sensor is manufactured by depositing a pure aluminum layer over gold contact
pads on a ceramic substrate. The aluminum is partially anodized using a proprietary process and a thin
film of permeable gold is deposited over the oxide layer. The aluminum oxide is a porous insulator and
provides a space for moisture molecules to reside at equilibrium with the sensor environment. The
permeable gold conductive layer allows moisture molecules to move freely into or out from the oxide
layer and acts as one plate of the capacitor. The aluminum layer beneath the oxide acts as the opposing
plate for the capacitor. After the sensors are created on the wafer, they are diced apart and each sensor
is then bonded to a pressure rated feed-through on a probe. The probe supports the sensor in the process
environment for which it is rated.

1.3.2 Sample System Operation

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The main component of the sample system in the DewPro MMY 245 is essentially a valve. The valve
emulates a four-way process valve during operation. In the "dry" or horizontal position, the sensor is
exposed to a very dry, -85° to –95°F, (-65° to –72°C) dew point environment created by a very strong
desiccant. This keeps the sensor dry and ready to measure. Any sensor will respond quickly to
increasing moisture and dries down much more slowly. In the "measure" or vertical position of the
valve handle, the sample stream is directed across the sensor to allow the measurement to be made.
Just as in a four-way process valve, there is a second function in operation concurrent with the "flow"
description above. When in the "dry" position, the passage for the sample connection inlet is connected
to the outlet for purging the connecting hoses. This purging operation is critical for making the
measurement as quickly as possible since a fresh sample is transferred to the immediate sensor location
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