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D1988-20 – Standard Test Method Technical Guide
🔬 Overview and Scope
ASTM D1988-20, developed under the jurisdiction of ASTM Committee D03 on Gaseous Fuels, establishes a standard test method for the rapid field determination of mercaptans in natural gas pipelines using length-of-stain detector tubes. Mercaptans are intentionally added to natural gas as odorants to provide a warning property, making their measurement critical for consumer safety. The method covers a total measuring range of 0.5 to 160 ppm by volume, though most field applications focus heavily on the lower end of the spectrum, specifically under 20 ppm.
Besides total mercaptans, the standard covers detector tubes for methyl mercaptan (0.5 to 100 ppm), ethyl mercaptan (0.5 to 120 ppm), and butyl mercaptan (0.5 to 30 mg/m³ or 0.1 to 8 ppm). It is the user’s responsibility to establish appropriate safety, health, and environmental practices before use, as referenced in Section 8.3 of the standard.
📌 Technical Note: Most field applications for total mercaptan detection are focused on concentrations under 20 ppm, making high-sensitivity detector tubes critical for accurate leak investigation and odorant level verification.
⚙️ Test Method and Procedure
The sample is passed through a detector tube filled with specially prepared chemicals. Any mercaptan present reacts to produce a color change, or stain. The length of the stain is directly proportional to the amount of mercaptan in the sample when exposed to a measured volume of gas. A hand-operated piston or bellows-type pump is used to draw the sample through the tube at a controlled flow rate.
The stain length is converted to parts per million (ppm) by volume using a calibration scale supplied by the manufacturer for each box of tubes. This direct-reading system is easily portable and completely suited to making rapid spot checks for mercaptans under field conditions. Users must consult manufacturer instructions for specific interference data, as hydrogen sulfide and other mercaptans are common interferences.
⚠️ Interference Caution: Hydrogen sulfide (H₂S) is a highly common interferent on mercaptan detector tubes. Many tubes incorporate a pre-cleanse layer designed to remove specific interferences up to a maximum level, but this layer has a finite capacity. Always verify the expected H₂S concentration against the tube’s stated interference limit before testing.
📊 Measurement Ranges and Interferences
Accurate measurement requires careful management of interferences, which may vary among brands due to different detection principles. Note that some detector tubes are calibrated in milligrams per cubic meter (mg/m³) rather than ppm. The conversion at standard conditions of 25 °C (77 °F) and 760 mm Hg is as follows:
mg/m³ = ppm × Molecular Weight / 24.45
| 🟦 Target Analyte | 📏 Range (ppm) | 📐 Range (mg/m³) | ⚡ Key Interferences |
|---|---|---|---|
| Total Mercaptans | 0.5 – 160 | — | H₂S, Other Mercaptans |
| Methyl Mercaptan | 0.5 – 100 | — | H₂S |
| Ethyl Mercaptan | 0.5 – 120 | — | H₂S, Methyl Mercaptan |
| Butyl Mercaptan | 0.1 – 8 | 0.5 – 30 | H₂S |
| 📐 Parameter | 🎯 Standard Value | 📌 Notes |
|---|---|---|
| Temperature | 25 °C (77 °F) | Standard condition for conversion |
| Pressure | 760 mm Hg | Standard condition for conversion |
| Molar Volume | 24.45 L/mol | Constant used in the formula |
❓ Frequently Asked Questions
🔍 What is the primary purpose of measuring mercaptans in natural gas?
Mercaptans are added as intentional odorants to provide a warning property. The odor serves to alert consumers, such as residential users, of natural gas leaks at levels that are well below the lower explosive limit.
💡 How is the sample drawn through the detector tube for analysis?
A hand-operated piston or bellows-type pump is used to draw a measured volume of the natural gas sample through the detector tube at a controlled rate of flow.
⚠️ What are common interferences when measuring total mercaptans?
Hydrogen sulfide (H₂S) is a highly common interferent. Other mercaptans can also interfere with the measurement of a specific target mercaptan. Many tubes include a pre-cleanse layer designed to mitigate these effects up to a certain limit.
📌 How is the conversion between mg/m³ and ppm performed?
The conversion formula provided in the standard for 25°C and 760 mm Hg is: mg/m³ = ppm × Molecular Weight / 24.45. Certain detector tubes are calibrated in mg/m³, making this calculation essential for consistent reporting.