Portable Trace Moisture Analyzer EDM-P

The Portable Trace Moisture Analyzer EDM-P utilizes a phosphorus pentoxide sensor to absorb moisture, electrolyzing water molecules into hydrogen and oxygen, and measures the electrical charge required for this electrolysis. The electrolytic hygrometer was invented in 1956 and can detect gas moisture content as low as a few parts per million, hence it is also known as a trace moisture analyzer. This instrument operates on the principle of water electrolysis, decomposing the moisture absorbed by phosphorus pentoxide into hydrogen and oxygen. According to Faraday's law, the charge required to electrolyze one gram-equivalent of water (9.01 grams) is 96,500 coulombs, which is why it is also referred to as a coulometric electrolytic hygrometer.

The measurement range of this instrument is typically 0~10~1000 ppm (dew point -76~-20°C). It is suitable for determining trace moisture in air, inert gases, hydrocarbons, and other gases that do not react with phosphorus pentoxide. It is widely used in chemical, petrochemical, electronics, metallurgical industries, and refrigeration engineering.

1. Working Principle

This instrument operates on continuous sampling based on hygroscopic electrolysis. When the gas sample flows through a specially designed electrolytic cell, the moisture is entirely absorbed by the phosphorus pentoxide acting as a desiccant, producing phosphoric acid solution:

P₂O₃+H2O→2HPO₃

When a direct current is applied to the platinum electrodes in the electrolytic cell, the phosphoric acid is electrolyzed to produce hydrogen and oxygen, regenerating phosphorus pentoxide:

4HPO₃→2H₂+O₂+2P₂O₂

During continuous measurement, the absorption and electrolysis processes occur simultaneously in the electrolytic cell. According to Faraday's law, the electrolysis current I equals:

Where I—electrolysis current (amperes);

m_w—mass of water electrolyzed (grams);

N—chemical equivalent of water;

t—electrolysis time (seconds).

If the measured gas is a mixture of water vapor (mass m_w, volume V_w) and dry gas (mass m_w, volume V_w), then:

Where M_W and M_g—molecular weights of water and dry gas

Substituting m_W into equation (3.2) gives:

At a flow rate of 100 mL/min, temperature of 25°C, and pressure of one atmosphere, the electrolysis current for 1 ppm is 13.2 microamperes. To ensure accurate instrument readings, flow rate, temperature, and pressure must remain stable, and the electrolytic cell must fully absorb the moisture in the gas stream.

2. Electrolytic Cell Construction

The electrolytic cell typically has two different structural forms: one is internally wound, and the other is externally wound, as shown in Figures 3.1 and 3.2.

Features

EDM-P can select two gas inlets, independently processing moisture content from two gas streams.

Full-color touch LCD screen.

Display can be set to show PPM (V) or Celsius (dew point).

Built-in support for 7 selectable languages.

Advanced menu operation.

Power and battery management.

USB port for data transmission.

The instrument is self-controlling.

European EP annotation.

Applications

Medical gases

Microelectronics

Chemical industry (corrosive acidic gases)

Heat treatment

Research institutes or universities

Remarks

EDM-P has been specifically designed with corrosion-resistant fittings and valves for measuring corrosive gases. All parts in contact with corrosive gases are made of PVDF material.

Performance

Lower detection limit per channel: 0.05 ppm (v)

Measurement range: 0-2000 ppm

Resolution: 0.1 ppm (v)

Sensor self-test

Analog output: 4-20 mA

Data logging: intervals from 2 seconds to 10 minutes

Battery life: approximately 5 hours when fully charged.

Dimensions: 15×25×35 cm

Weight: 6 kg (8 kg)

Measurement Unit

Valve body material: PVDF or stainless steel.

Sensor material: Pt electrodes wound on glass or ceramic substrate.

Measured gas flow rate: 20 Nl/h or 100 Nl/h

Measured gas temperature: +5…+65°C

Maximum input pressure to the cell: 200 mbar

Measurement accuracy: +/- 5% of value, minimum 0.4 ppm (v)

Response time for humid gas: <1 second. Return time, T90 <2 minutes. Reproducibility at 20 ppm (v): +/- 2%

Gas connection: 1/4 inch NPT threaded fitting

Measurement cell connection: 5-pin binder plug, sensor cable 0.5 m

Options

Flow meter, three-way valve, four-way valve, sensor can be housed in a dust-proof polypropylene case, dry gas.