Principle of Measurement of Zirconia Oxygen Analyzers

Principle of Measurement

Toray zirconia oxygen analyzers determine oxygen concentration using the conductivity of a zirconia ceramic cell. Zirconia ceramic cells only allow oxygen ions to pass through at high temperatures. With reference gas on one side and sample gas on the other, oxygen ions move from the side with the highest concentration of oxygen to that with the lowest concentration. The movement of ions generates an EMF (Electro Motive Force) which can be measured to determine the oxygen content.

Fig. 1

zirconia

zirconia

In Fig. 1, a gas with fixed oxygen concentration (normally air) is placed on the A side as the reference gas (PR), while the gas whose oxygen concentration is to be measured (PM) is placed on the B side. When this is done, ion conduction occurs from the side with higher oxygen concentration to the side with lower oxygen concentration.

The electrochemical reaction on both sides follows Formula 1, and the EMF accompanying ion conduction is given by the Nernst equation (Formula 2).

Formula 1

Formula 1

Formula 2

Theoretical Nernst equation

Since the EMF varies depending on the temperature of the zirconia sensor and the oxygen concentration of the reference gas (PR), in the actual device, the zirconia sensor is placed in a constant temperature oven, and air is generally used as the reference gas.

With Toray's zirconia sensor, a stable EMF conforming to the Nernst equation is obtained from about 500°C, as shown in the Table.

EMF

General Precautions for Use

Zirconia oxygen analyzers ordinarily operate at a high temperature close to 800°C (700°C, in the case of Toray). Therefore, please be aware that measurement may be impossible, or there may be adverse effects on sensor service life, in the following situations.

  1. Oxygen analyzers cannot be used if the set-up atmosphere is an explosion-proof zone.
  2. If the measured gas contains flammable gas (i.e. methane, alcohol, carbon monoxide), it will cause a combustion reaction, and result in measurement error.
  3. Organic silica (used in sealing compounds etc.) will cause sensor degradation. (Our company's findings indicate that this is the leading cause of degradation.)
  4. The sensor may degrade if the measured gas contains corrosive gas (i.e. fluorine-based gases, chlorine-based gases, sulfate-based gases) or poisonous substances (i.e. Si, Pb, P, Zn, Sn).
  5. If the measured gas intermittently contains a substance such as carbon monoxide with a high calorific value, the sensor will accumulate a heat history, and this will cause sensor malfunction due to problems such as platinum electrode peeling.
  6. If the measured gas contains large amounts of NOX, SOX or other corrosive gas, this will cause sensor malfunction due to problems such as platinum electrode peeling.
  7. At high temperatures, freon gas will cause an unpredictable reaction with oxygen, and this may result in measurement error.
  8. Water droplets, dust and mist will damage the sensor, shorten its service life, and cause errors.
  9. In general, these analyzers cannot be used with closed loops (circulating systems) unless they are specially designed for that purpose. The sensor may be damaged by excess pressure.

When measuring gases containing these substances, please remove those substances before supplying gas to the analyzer. For information on removal methods, please consult with the sales supervisor prior to purchase.

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