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.

Test Methods
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 the engine was operated at high idle.

The diesel engine loading was controlled by a water-cooled 400 kW eddy current dynamometer. The following test conditions were maintained:
1.    High idle (2,200 rpm, 35 ft-lb);
2.    Rated speed with load (2,200 rpm, 60 ft-lb); and
3.    Intermediate speed, full load (1,400 rpm, 389 ft-lb), the same condition as mode 5 of the ISO 8178-8 mode test cycle for a 10-minute sampling period (a) and a 2-minute sampling period (b).

Results
The results of the study are summarized in Table 1. With the exception of test 3b, each engine operating condition was maintained for at least 10 minutes so that a sufficient amount of data could be collected from each instrument. The CO concentrations presented in Table 1 are the average value for each instrument over this time period. The standard deviation provides an indication of the scattering of the data during each test.

The West Virginia Office of Miners’ Health, Safety and Training generally uses the measurement of carbon dioxide as an indication of the desired engine operating condition during gaseous tailpipe measurement in the field. The third operating condition (3a, intermediate speed and full load) was characterized by a carbon dioxide concentration of 9%. There was concern a 10-minute sampling period during this test was too long for the iTX instrument, which can experience saturation of the electrochemical cell (and corresponding measurement error) if challenged with high CO concentrations for extended periods of time. In response to this concern, test 3a was repeated at the same operating conditions but with duration of just two minutes (test 3b). It is important to note  during this last test, a new charcoal filter was used to precondition the sample being drawn into the iTX.

The CO concentration in the exhaust varied with engine operating condition. The CO concentration was approaching 150 ppm at high idle condition, over 100 ppm at rated speed and only a few ppm at intermediate speed, full load. The activity of the diesel oxidation catalyst (DOC), which is strongly influenced by tailpipe temperature, is one of the influencing factors that affects tailpipe CO concentrations. At high idle, the tailpipe exhaust temperature is relatively low and the DOC is not active. Therefore, CO is not converted by the DOC which results in a high CO concentration. When the engine load is increased at rated speed, the temperature of the exhaust increases, as does the activity of the DOC, reducing the tailpipe CO concentration. At mode 5 of the ISO 8178 mode test cycle [3a, 3b], the DOC is fully active and the conversion of CO is almost complete.

All three analyzers compared were able to measure the variation in carbon monoxide concentration at each engine operating condition. However, the CO concentrations measured by the ECOM analyzer and iTX multigas monitor were higher than the value measured by the NDIR analyzer for each set of tests. The discrepancy in measurement value was consistently greater for the iTX multigas monitor compared to the ECOM analyzer. During the repetition of the third engine operating condition, the use of a new charcoal filter and the reduction in sampling time produced a measured CO concentration by the iTX monitor that was closer to the measured value of the NDIR analyzer.

Conclusion
The objective of the testing was to compare the response of two commercial portable monitors, the ECOM-J2KN and the iTX multigas analyzer, when used for measurement of carbon monoxide (CO) in the tailpipe of diesel-powered mining equipment. Of particular interest was the capability of the monitors to measure CO concentrations around 100 ppm in the tailpipe, which is the limit at normal engine operating temperature range in Pennsylvania and West Virginia coal mines. A diesel oxidation catalyst (DOC) is generally used to meet this limit and, for this reason, it was employed for this testing.  

The results showed both monitors were capable of measuring a very low concentration of CO at intermediate speed and full load. At this mode, the compliance status of the diesel package (engine-DOC) would have been accredited by both the monitors.

When the engine was operating at high idle condition, the concentration of CO was well above 100 ppm. This concentration is the result of a non-activated DOC. It is important to underline this operating condition does not produce temperatures within a normal engine operating range. In this case, both of the monitors were able to assess a CO concentration higher than 100 ppm.

Substantial discrepancy was detected between the CO measurements taken by the two monitors when the concentration was around 100 ppm (Engine Operating Condition 2). While the ECOM-J2KN measurement was similar to a laboratory reference measurement, the iTX multigas analyzer over-estimated the concentration of carbon monoxide in the tailpipe.

In closing, caution should be taken with the iTX multigas analyzer when measuring a concentration of carbon monoxide near the limit of 100 ppm. Also, it is important to emphasize that a water trap was not used for this testing to pre-condition the sample extracted by the iTX monitor. While the use of a water trap was not specified in the sampling protocol provided by the instrument manufacturer, all of the participants at the testing concurred its use may have improved CO measurement with this device.

Author information
Patts is a physical scientist and Cauda is a research engineer. Both are researchers at the NIOSH Pittsburgh Research Labora-tory. Patts can be reached at Tel: 412-386-6852(or E-mail: lnp2@cdc.gov).