Method 4
Determination of Moisture Content in Stack Gases
40 CFR Part 60, Appendix A-3
From the method — regulatory text
Sections 1.0 and 2.0 of Method 4—Determination of Moisture Content in Stack Gases, as published at 40 CFR Part 60, Appendix A-3. Retrieved 10/07/2026. The full method is on the eCFR.
1.0 Scope and Application
1.1 Analytes.
| Analyte | CAS No. | Sensitivity |
|---|---|---|
| Water vapor (H2 O) | 7732-18-5 | N/A |
1.2 Applicability. This method is applicable for the determination of the moisture content of stack gas.
1.3 Data Quality Objectives. Adherence to the requirements of this method will enhance the quality of the data obtained from air pollutant sampling methods.
2.0 Summary of Method
2.1 A gas sample is extracted at a constant rate from the source; moisture is removed from the sample stream and determined gravimetrically.
2.2 The method contains two possible procedures: a reference method and an approximation method.
2.2.1 The reference method is used for accurate determinations of moisture content (such as are needed to calculate emission data). The approximation method, provides estimates of percent moisture to aid in setting isokinetic sampling rates prior to a pollutant emission measurement run. The approximation method described herein is only a suggested approach; alternative means for approximating the moisture content (e.g., drying tubes, wet bulb-dry bulb techniques, condensation techniques, stoichiometric calculations, previous experience, etc.) are also acceptable.
2.2.2 The reference method is often conducted simultaneously with a pollutant emission measurement run. When it is, calculation of percent isokinetic, pollutant emission rate, etc., for the run shall be based upon the results of the reference method or its equivalent. These calculations shall not be based upon the results of the approximation method, unless the approximation method is shown, to the satisfaction of the Administrator, to be capable of yielding results within one percent H2O of the reference method.
EssentialRegs notes
Our plain-English reading of Method 4. Not the method text.
What it measures
The moisture content of a stack gas, as the percent of the gas by volume that is water vapor.
How it works
A gas sample is drawn from the stack at a constant rate through a heated probe and then through a series of impingers, glass vessels in an ice bath that condense the water out of the gas. The water collected, found by weight or volume, is compared with the volume of dry gas that passed through the metering system. The moisture percentage is calculated from the two. The method also describes a faster approximate procedure that is used to set up sampling for other methods.
Equipment
A heated probe, impingers in an ice bath with silica gel in the last one to dry the gas, a leak-checked pump, a dry gas meter and temperature sensors.
When a rule cites it
Performance tests where a flow rate or concentration has to be converted between wet and dry basis. It is the moisture determination that goes with the velocity and flow methods and with isokinetic sampling.
Notes for compliance staff
Methods that already use an impinger train, such as Method 5, measure moisture as part of the test, so a separate Method 4 run is not needed for those. Method 4 supplies the moisture value that Method 2 needs for stack gas velocity and flow rate.
Cited by
Provisions in the corpus whose text cites Method 4. Subscribers land on the provision in the regulation reader; visitors see a preview of it (the GP05 sample opens in full).
40 CFR Part 60 Subpart IIII
- Table 7 to Subpart IIII of Part 60 — Requirements for Performance Tests for Stationary CI ICE With a Displacement of ≥30 Liters per Cylinder
40 CFR Part 60 Subpart JJJJ
- Table 2 to Subpart JJJJ of Part 60 — Requirements for Performance Tests
40 CFR Part 60 Subpart OOOO
40 CFR Part 60 Subpart OOOOa
40 CFR Part 60 Subpart OOOOb
40 CFR Part 60 Subpart OOOOc
40 CFR Part 63 Subpart ZZZZ
- Table 4 to Subpart ZZZZ of Part 63 — Requirements for Performance Tests
- Appendix A to Subpart ZZZZ of Part 63 — Protocol for Using an Electrochemical Analyzer to Determine Oxygen and Carbon Monoxide Concentrations From Certain Engines
49 CFR Part 192
The sections marked as regulatory text above are reproduced from the CFR. Everything under ‘EssentialRegs notes’ is our reference copy, not the method. The method as published in the CFR controls.