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A dilution probe dilutes the stack gas in the probe to such a degree that the dew point of the diluted gas will be less than the lowest ambient temperature at the sampling location. This enables the CEM system to avoid the use of heat‐traced line and simplifies the gas transport system.

One of the first and most successful dilution systems uses a critical orifice coupled with an ejector pump designed into the probe body (Figure 3‐19). This probe, originally developed in the Netherlands (Bergshoeff and van Ijssel 1978), is unique in its design and construction.

This probe has also been known as the “EPM” probe named after the company that marketed it in the 1980s and 1990s.

Inside the probe, an ejector pump operates at flow rates of 1–10 l/min. A glass critical orifice (consisting of a glass tube drawn to a point, as shown in the figure) is chosen to limit the flow of sample gas to flow rates from 20 to 500 ml/min. The condition for obtaining a critical flow for the glass orifice is that the ratio of the absolute pressure at the venturi throat to the stack static pressure must be less than or equal to 0.53 (Brouwers and Verdoorn 1990). The dilution ratio, D, is calculated as follows:

(3‐2)

where

 Q1 = dilution air flow rate (l/min)

 Q2 = sample gas flow rate (l/min)

Data obtained from the gas analyzers measuring the diluted flue gas are converted into the source‐level concentrations by multiplying the analyzer response and the dilution ratio determined or adjusted at the time of initial calibration.

Dilution ratios of 50‐to‐1 to 300‐to‐1 are typical. The higher ratios are used for hot, saturated gas streams. In coupling a dilution system to an analyzer, attention must be paid to the measurement range of the analyzer. If the lowest instrument range should be 0–5 ppm, and it is required to measure a pollutant stack gas at a nominal concentration of 50 ppm of the pollutant, a 100‐to‐1 dilution ratio would provide a sample to the analyzer of 0.5 ppm. This would be at the low end of the range where the analyzer sensitivity is lowest. If the instrument noise or drift is high at this part of the scale, it could be difficult for the system to pass a relative accuracy test.

Figure 3‐19 The in‐stack EPM dilution probe.

Although the EPM dilution probe has been successfully applied, it is not the solution to all extractive sampling problems. In the cases of wet, caking, or sticky particulate matter, the probe can still become plugged even though it is pulling at a low flow rate. Dilution probes of this type have experienced difficulties when installed after wet scrubbers, where water droplets are entrained by the flue gas. If the droplets enter the probe, or water condenses in the probe from a highly saturated gas stream, the glass wool filter can become wet and the orifice can become plugged. Under normal conditions, when the probe is heated, water droplets should be vaporized and plugging should not be a problem. To avoid problems resulting from water droplets, adequate temperature control is required.

The dilution probe is sensitive to changes in stack pressures and temperatures (Jahnke and Marshall 1994; Myers 1986). In installations where the stack static pressure is highly negative (<−10 in. H₂O), the venturi vacuum may not be sufficient to overcome the pull of the stack negative pressure.

It should also be noted that air is used for dilution. This prevents the use of an oxygen analyzer in the CEM system because the contribution of stack gas oxygen to the sample would be swamped by the background 21% oxygen level of the dilution air. Most commercial oxygen analyzers are not designed to measure differences in oxygen levels at diluted values of 0.1–1%. If it is required to correct pollutant emission data for stack dilution air, a CO₂ analyzer is used in the system. When a regulation requires that pollutant concentrations be corrected to a specified oxygen concentration (such as the 6% O₂ correction specified in 40 CFR 60 Subpart BB for Kraft pulp mills (U.S. EPA 2020c)), the critical orifice can be incorporated into an assembly, which first splits the flue gas into two gas streams – one that passes through a critical orifice and is diluted and one that remains undiluted. This cannot be done when using an in‐stack dilution probe, but can be done when using external dilution systems.