Teledyne 3160 manual Operational Theory Micro-Fuel Cell Sensor, Sample System

Operational Theory 2

2.1.1 Micro-Fuel Cell Sensor

The Micro-Fuel Cell is a sealed plastic disposable oxygen transducer that mea- sures 1¼ inches in diameter and is ¾ inch thick. Inside of the cell are a cathode and anode immersed in 15% aqueous KOH electrolyte. At one end of the sensor is a Teflon diffusion membrane; the other end is sealed with a polyethylene membrane. At the rear of the cell is a contact plate consisting of two concentric foils. The foils mate with spring-loaded contacts in the sensor block assembly and provide the electrical connection to the rest of the analyzer.

The sensing cathode, located beneath the diffusion membrane, has a surface area of 2.48 cm2. The sample gas enters the sensor block through an inlet tube between the cathode and the sensor block cap, diffuses through the Teflon membrane, and any oxygen in the sample gas is reduced on the surface of the cathode by the following mechanism:

O	2	+ 2H O + 4e– → 4OH–	(cathode)
		2

When the oxygen is reduced at the cathode, lead is simultaneously oxidized at the anode by the following mechanism.

2Pb → 2Pb+2 + 4e– (anode)

The electrons released at the surface of the anode flow to the cathode surface via an external circuit. This current is proportional to the amount of oxygen. It is mea- sured and used to determine the oxygen concentration in the gas mixture.

The overall reaction for the fuel cell is:

2Pb + O2 → 2PbO

The output of the fuel cell is limited by the amount of oxygen in the cell at any one time, and the amount of stored anode material. In the absence of oxygen, there is no current generated.

2.1.2 Sample System

The sample system delivers gases to the Micro-Fuel Cell sensor from the ana- lyzer rear panel inlet. Depending on the mode of operation either a sample or zero gas is delivered.

The Model 3160 sample system is designed and fabricated to ensure that the oxygen concentration of the gas is not altered as it travels through the sample system.

The following list of sample system and analyzer features are among those available, but the exact configuration depends on the standard options incorporated. Paragraph 1.3 contains a list of the standard variations on the basic model.

TELEDYNE ANALYTICAL INSTRUMENTS 2-3