Proceedings of the Air & Waste Management Association, May 1996

Accuracy considerations of portable electrochemical NOX analyzers

Dr. C. Capetanopoulos

Dr. B. Hobbs

ABSTRACT

Two key components contributing to measurement errors of electrochemical analyzers are discussed. These are the sample conditioning system and the electrochemical nitric oxide and nitrogen dioxide sensors. The problems associated with various types of conditioning systems are discussed and some experimental results are presented using analyte spiking methods. Permeation drier based systems are shown to cause the smallest loss of the analyte. Two major problems of the NO and NO2 sensors are examined. The first problem deals with the significant effect of temperature on the sensor and its associated interference rejection filter. The requirement for maintaining sensor and filter temperature below 30 deg. C is demonstrated. The second deals with the saturation and drift considerations caused by over exposure to the gas. The significance of capillary size to minimize drift for diffusion sensors is discussed. Experimental results are presented and discussed with a view to the recently published EPA CTM-022 Method.

Figures 3 and 4 exerpted from the paper and shown below, demonstrate the superior performance of the permeation drier for NO2 measurements

NO2 RESPONSE - Heated Probe & Chiller

NO2 RESPONSE - ENERAC Probe (Permeation Drier)

Note: Analyte Spiking

Region A: A mixture of NO2 and Air
Region B: A mixture of NO2 and "Wet" Stack Gas (15% H2O VAPOR)
Region C: A mixture of NO2 and Air

Figure 5 shows the significant dependence of the Nitric Oxide (NO) electrochemical sensor on ambient temperature

Figure 6 shows the significant dependence of the Nitric Oxide (NO) electrochemical sensor on the history of previous exposure to gas

Note:

• Lower Curve - Sensor not exposed to gas for 48 hours
• Upper Curve - Sensor recently exposed to 1000 PPM gas