The periodic measurement of carbon monoxide (CO) in the exhaust of such equipment is mandated by several states and by federal regulations (MSHA) in order to assess engine operating conditions and compliance with engine exhaust emission limits. Both Pennsylvania and West Virginia state regulations limit the tailpipe CO concentration of mining equipment used in underground coal mines to 100 parts per million (ppm) at any time under a normal engine operating temperature range.
The tailpipe field measurement of CO on a unit of diesel-powered mining equipment can be performed with several commercial devices. Most of these devices consist of a sampling probe, a sample conditioner, an analytical unit and a pump to draw the sample. For underground mining applications, the analytical unit is usually an electrochemical cell. The choice of the electrochemical cell is particularly important because the cell has to provide reliable measurement in the range of interest. Generally, the function of the sample conditioner is to remove water vapor, diesel particulate matter (DPM) and other compounds from the sample that might affect the measurement.
Currently, two commercially available instruments are being used to measure tailpipe CO concentrations from diesel-powered mining equipment in West Virginia coal mines. One of these instruments is generally used as an ambient monitoring device, and questions have been raised concerning its accuracy when this instrument is used to measure tailpipe CO concentrations. The subject of this informational report is the comparison of these two field instruments with a reference laboratory analyzer when challenged with the same sample extracted from a diesel engine exhaust tailpipe.
Overview of Instruments
The two CO measurement instruments compared in this study were: an ECOM-J2KN gas and smoke particle analyzer and an iTX multigas analyzer. The ECOM-J2KN is commonly used to sample exhaust gas in maintenance shops and for exhaust stack gas sampling. The iTX multigas analyzer is a portable, handheld instrument, which is typically used by miners as an ambient atmospheric monitor. Both instruments are equipped with electrochemical cells to measure the concentration of the pollutants of interest. The analyzer used as a reference for the measurement of CO in the same tailpipe was a CAI 602 nondispersive infrared (NDIR) analyzer and is assumed to have greater precision and accuracy than the electrochemical cell sensors. A heated line, with the temperature maintained at 180°C, was used to draw the samples to the NDIR analyzer. Water vapor and DPM were removed from the sample by an M&C gas conditioner located ahead of the analyzer. The NDIR was calibrated less than a week before the comparison test was performed (using a 10-point calibration procedure).
For the purpose of this test, all analyzers were employed simultaneously to measure the carbon monoxide concentration in the tailpipe of a Mercedes-Benz 904 Tier 2 diesel engine. The engine was located in the NIOSH Diesel Engine Emission Laboratory at the NIOSH Office of Mine Safety and Health Research Laboratory in Pittsburgh, Pa. (Figure 1). The measurements were conducted at the outlet of a diesel oxidation catalyst (by Clean Air Catalyst) installed in the exhaust system of the diesel engine.
The comparison protocol was designed to represent field measurement procedures used for both instruments. Sampling protocols were agreed upon by NIOSH, West Virginia Office of Miners’ Health, Safety and Training, and both of the analyzer manufacturers prior to the laboratory comparison. Representatives of the two analyzer manufacturers were present at the testing and were in charge of the use of their respective analyzers.
The ECOM-J2KN was turned on and calibrated according to manufacturer’s procedures. A heated sampling line, which was allowed to warm up for a minimum of 20 minutes before any samples were taken, was used with the ECOM to ensure that samples would not be affected by moisture. Instrument carbon monoxide (CO), nitric oxide (NO) and nitrogen dioxide (NO2) fresh air values were documented and were at zero for this test. ECOM samples were obtained by removing the sampling port plug from the exhaust pipe and inserting the ECOM sampling probe to a position approximately 1 inch from the opposite wall of the exhaust pipe. CO, NO and NO2 values were recorded when a stable CO value was observed.
The iTX multigas monitor was turned on and was zeroed according to manufacturer’s recommendations. The iTX was inserted into its sampling pump hood and approximately 18 inches of Tygon tubing (Saint-Gobain Performance Plastics) was attached to the sampling pump hood. A charcoal filter filled with 28.4 grams of activated charcoal was attached to the free end of the Tygon tubing. A stainless steel sampling probe with approximately 18 inches of Tygon tubing was then attached to the charcoal filter. When obtaining a sample, the end of the sampling probe was inserted into the center of the tailpipe. (This sampling protocol was that specified and used by West Virginia state inspectors in the field.) The iTX CO value was read and recorded after one minute while