Edgetech PPM1, Operator'S Manual, Page: 6 / 29

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3.0 INTRODUCTION
3.1 GENERAL DESCRIPTION
The PPM1 is a trace moisture analyzer designed specifically for monitoring low
levels of moisture, and reading them out in units of dew point and parts-per-
million. This trace moisture analyzer utilizes an electro-chemical (P
2
O
5
) sensor in
combination with a proprietary semi-permeable diffusion membrane. The
principal of
operation applies the Faraday’s Law of Electrolysis. It is a
fundamental measurement of the moisture present. The PPM1 is ideally used
in relatively clean, dry, inert gas applications. It is provided in a wall-mount
configuration. See Figure 3-2.
The PPM2 is a portable, battery operated version of the PPM1. It is provided in a
carrying case, with two gel-cell rechargeable batteries built in. See Figure 3-3.
3.2 TECHNICAL DESCRIPTION
3.2.1 THE PPM1/PPM2 SYSTEM
The gas passing through the phosphorous pentoxide cell results in a small
electrical current proportional to the quantity of water molecules (moisture) in the
sample. The small electrical current from the cell is conditioned so that it directly
corresponds to the concentration of moisture in the measured gas. This linear
output signal is available to actuate the programmable Alarm Relays, and also
provides information to the Digital Display and Analog Outputs. A
microprocessor controls all data processing and input and output functions of the
Instrument, ensuring a high accuracy measurement of moisture. Two separate
humidity parameters may be monitored and displayed.
3.2.2 THE PHOSPHOROUS PENTOXIDE (P
2
O
5
) MOISTURE SENSOR
The Sensor utilizes an electrochemical (phosphorous pentoxide - P
2
O
5
- coated)
sensor in combination with a proprietary semipermeable diffusion membrane.
This membrane allows the water vapor molecules to pass from the dry gas
stream into the sensor cavity at a rate that is a function of the partial pressure of
the water across the membrane. It also allows the hydrogen and oxygen
molecules, the byproducts of the electrolysis, to diffuse back into the gas stream.
The membrane “meters” the gas flowing into the sensor cavity, ELIMINATING
THE NEED FOR PRESSURE REGULATION AND CONSTANT FLOW, which
are required in most other electrolytic hygrometers.
The phosphorous pentoxide moisture sensor, also called an electrolytic cell, has
the ability to measure trace quantities of water vapor in a dry gas. Operation is
based on the fact that if a small dc voltage is applied, water vapor molecules can
be electrolyzed into their elements of hydrogen and oxygen. To electrolyze each
water molecule, it is known that two electrons are required. Therefore, the