Principles of Operation


There are four types of analyzers used in MEECO products. From the list below, choose one of MEECO's analyzers to learn more about the principles behind its operation.

Electrolytic Moisture Analyzers
Oxygen Analyzers - Zirconium Oxide
Oxygen Analyzers - Electro-Chemical Fuel Cell
Dewpoint/Humidity Analyzers

























 

Electrolytic Moisture Analyzers - Principle of Operation
(Electrolytic Cell)

All MEECO moisture analyzers utilize an electrolytic cell for moisture measurement. The electrolytic cell consists of a hollow glass tube with two electrodes spirally wound around the inside wall. The electrodes are covered with a thin, hygroscopic film of phosphorous pentoxide (P205). This delicate assembly, called the element (Figure 1) is surrounded by a protective metal body. The element is supported within the body to allow gas flow through the element. At the same time, the opening through the body around the element is sealed, so that gas can only flow through the glass tube.

Fig. 1 Element Cross Section
Figure 1
Element Cross Section

The electrode wires extend from one end of the element and pass through to the outside of the cell body. They are connected to a pair of terminals that protrude from both sides of the cell. An electrical check on the cell across the electrode terminals, prior to the application of the P205 film, will show an open circuit. The complete mechanical assembly (Figure 2) can be plumbed into a gas sampling system.

Fig. 2 Electrolytic Cell
Figure 2
Electrolytic Cell Complete Assembly

In operation, the sample gas enters the cell at a known flow rate and the P205 film absorbs all the moisture molecules present in the gas flow. A voltage, applied across the electrode terminals, electrolyzes moisture in the film. Once equilibrium is reached, the rate at which moisture molecules enter the cell will exactly match the rate at which molecules are electrolyzed. Each electrolyzed water molecule causes two electrons to be displaced from the anode to the cathode. The electrolysis current (amps) gives the electrical charge (coulombs) displaced per second. Consequently, since the elementary charge of an electron is known, the current measurement indicates the rate at which water molecules enter the cell. There is, therefore, no need for calibration versus a moisture standard. This combined with the known, total flow rate of the gas through the cell, gives a known concentration of moisture in the sample gas. For example, at 100 sccm, one ppm of moisture corresponds with a current of 13.14 microamperes. Therefore, moisture in equals current out.

Fig. 3, Principle of Operation
Figure 3
Principle of Operation


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Oxygen Analyzers - Principle of Operation
(Zirconium Oxide Sensor)

The sensor used in MEECO's Oxytek PPB Oxygen Analyzer is made of the solid state oxygen ion conducting (electrolyte) material zirconium oxide. Due to oxygen vacancies in the ceramic lattice, at temperature over 450 degrees Centigrade, oxygen ions are mobile in the solid material. It is this property that enables the measurement of oxygen in a gas of unknown composition. When two gases of differing oxygen concentrations are on opposite sides of a zirconium oxide membrane, each side with a conductive (platinum) electrode material, a DC voltage is generated that is a function of the difference in oxygen concentration. In practice, one side of the membrane is a known reference gas, typically air, and the opposite side is the unknown sample gas to be measured. In 1899 Nernst defined the relationship between the measured voltage and the unknown oxygen concentration:

E = (RT/nF)ln(P1/P2)

Where:

E is the measured voltage
R is the universal gas constant 8.314 Joules/mole °K
T is the temperature in °K
n is the number of electrons transferred
F is Faraday's constant 96,500 coul./mole
P1 is Oxygen Partial Pressure in Air Reference (20.9% in O2)
P2 is Oxygen Partial Pressure in Sample Gas

Using this relationship, the unknown oxygen concentration can be determined by simply measuring the voltage across the cell. Once the sensor is heated to a temperature of 725°C, the voltage is measured, and the oxygen concentration information is sent to the display and other outputs. The instrument is calibrated on a low level calibration gas for improved accuracy. This calibration corrects for small effects of non-ideal sensor behavior.

Due to the high operating temperature and use of platinum electrode, the presence of reducing agents such as hydrocarbons, carbon monoxide, hydrogen, etc. interferes with the measurement of oxygen. The analyzer will display lower than actual oxygen concentration in the presences of reducing gases. (For example, 2H2 plus O2 forms 2H2O and thus reduces the O2 that the analyzer reads by a ratio of 0.5 for every H2 molecule. 2CO plus O2 forms 2CO2 which also reduces O2 by 0.5 for every CO molecule. Data shows the hydrocarbons reduce O2 by approximately 0.1 for every hydrocarbon molecule.)

Caution should be taken to use the OA Series analyzers to measure inert gases only. Do not attempt to measure sulphur gases like H2S. The sulphur reacts with the platinum electrode to for platinum sulphide. This reaction degrades the electrochemical properties of the sensor.


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Oxygen Analyzers - Principle of Operation
(Electro-Chemical Fuel Cell Sensor)

The MEECO O-Boy oxygen analyzers utilize a "Fuel Cell" oxygen sensor that consists of a diffusion barrier, a sensing electrode (cathode) made of a noble metal such as gold or platinum, and a working electrode made of a base metal such as lead or zinc immersed in a basic electrolyte (such as a solution of potassium hydroxide).

Oxygen diffusing into the sensor is reduced to hydroxyl ions at the cathode:

O2 + 2H2O + 4e-     ---->     OH-

Hydoxyl ions in turn oxidize the lead (or zinc) anode:

2Pb + 4OH     ---->     2PbO + 2H2O +4e-

This yields an overall cell reaction of:

2Pb + O2     ---->     2PbO

The fuel cell oxygen sensor is a current generator. The amount of current generated is proportional to the amount of oxygen consumed (Faraday's Law). The O-Boy monitors and displays the current output of the sensor.


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Dewpoint/Humidity Analyzers - Principle of Operation
(Guarded Layer Moisture Sensor)

MEECO's guarded-layer moisture sensor is a particularly useful capacitive sensor design for dirty environments. A pair of interdigitated electrodes are deposited on an inert nonconducting ceramic substrate and covered with a moisture-sensitive thin film. The moisture in the surrounding atmosphere comes into equilibrium with that in the film, resulting in a moisture-dependent capacitance between the interdigitated electrodes. If the structure were complete at this point, a high-temperature moisture sensor would be the result. However, unless the film were very thick, it would be fouling dependent. Material adhering to the top surface of the film would alter the capacitance and, thus, the inferred moisture concentration. If the film were made thick enough to avoid this problem the response of the sensor to changes in moisture content would be unacceptably slow. To avoid these limitations, a thin, porous platinum film is deposited on the moisture-sensitive film. This structure is equivalent to two capacitors in series - a first capacitance from the first interdigitated electrode to the porous platinum film and a second capacitance from the film back to the second interdigitated electrode. Finally, a second moisture-sensitive film is deposited onto the platinum film.

The purpose of this final layer is twofold. First, the moisture-sensitive material is a mechanically strong, chemically-resistant polyimide serving to protect the platinum film. Second, a small portion of the electric field of the double capacitor extends beyond the second moisture-sensitive film. Were it not for this second layer, dirt or oil depositing on the platinum film could alter the capacitance providing a result that was dependent on sensor fouling. Thus, the second moisture-sensitive layer provides a capacitance independent of material depositing on the sensor, rendering the results independent of fouling.

The MEECO dew point transmitter has a patented sensor design (described above) which can operate over a wide temperature range. The high-performance sensor is resistant to contamination and is easily maintained. The sensor capacitance will change as the concentration of moisture in the air changes. A 1000 platinum RTD is integral to the sensor to allow temperature measurements to be made. The transmitter utilizes the moisture and temperature signals to determine the dewpoint. The probe can be installed in situ (directly in-line) and has an NIST traceable calibration.

MEECO's unique transmitter with additional cooler option allows the sensor to be used in applications with temperatures up to 1000°F where dewpoint sensors have been unable to be used before due to the harsh environment. The MEECO dew point transmitter also offers a high pressure option which allows the sensor to be exposed to process pressures up to 2000 psi. An optional calibration kit allows for calibration checks in the field, which traditional dew point transmitters are unable to utilize.

SENSOR CLEANING
The surface of the sensor may be coated with a film or with a heavy layer of contaminants. This is usually, though not always, evident by visual inspection of the sensor surface. The sensor surface is green in color and normally appears shiny, sometimes iridescent depending on lighting. While MEECO's guarded-layer moisture sensor produces a result independent of fouling, cleaning may be required to restore its rapid response. A good procedure for cleaning the sensor is as follows. Obtain three simple pump spray bottles. Fill the first with a solution of distilled water and mild detergent like Ivory soap. Fill the second with distilled water, and the third with a solution of water and isopropyl alcohol. Spray the sensor directly with the soap solution, and follow that with distilled water spray to rinse it off. Finally, a spray of alcohol will help to dry out the sensor quickly. As an alternative to the three bottles, the alcohol alone will often be adequate to clear the sensor of surface contaminants. Avoid touching the sensor surface directly as it is possible to scratch the sensor's top protective layer. If necessary, a soft camel hair brush may be used to help remove surface dirt. For convenience, the sensor may be removed from the end of the probe before cleaning. The sensor connects to the probe via four gold-plated pins. When reinstating the sensor assembly, be sure to match up the notch on the sensor assembly housing with the machined flat at the end of the probe.


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