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D368-89 – Standard Test Method Technical Guide
📜 Scope and Significance
ASTM D368-89 (AASHTO T86) defines the hydrometer test method for measuring the specific gravity of oil-type preservatives. This method is used primarily to verify the conformance of creosote and creosote-coal tar solutions against material specifications D390 and D391. The test requires a completely liquid sample and relies on a calibrated hydrometer series.
🔬 Required Apparatus and Specifications
The standard mandates precise equipment for accuracy. The table below lists the core apparatus requirements.
| 🟦 Apparatus Item | 📏 Range / Specification |
|---|---|
| Hydrometer 125H | SG Range 1.000 to 1.050 |
| Hydrometer 126H | SG Range 1.050 to 1.100 |
| Hydrometer 127H | SG Range 1.100 to 1.150 |
| Glass Cylinder | ~300 mm long, ≥ 32 mm diameter |
| Thermometer 15F/15C | −2 to 80°C (30 to 180°F) |
⚠️ Important: Alternative hydrometers must maintain a scale length of at least 9.5 mm per 0.010 specific gravity units to qualify for use under this standard.
⚙️ Test Procedure and Calculations
Fill the cylinder with the thoroughly stirred sample to a depth that allows the hydrometer to float freely. Adjust the sample temperature to 38°C (100°F), or use the lowest fully liquid temperature if not fluid at 38°C. Insert the hydrometer and allow it to settle. Read the specific gravity at the meniscus and the temperature to the nearest 0.1°C (0.2°F).
Apply a correction by adding the specific gravity equivalent of a 1 mm scale increment to the observed reading. Then correct this value to the standard temperature of 38/15.5°C using the correction tables detailed in Test Method D347 (Table 2).
💡 Best Practice: Immersing the sample cylinder in a constant temperature bath maintained at the target test temperature greatly improves the stability and reproducibility of the reading.
❓ Frequently Asked Questions
🔍 What is the primary purpose of this test?
It is designed to determine the specific gravity of creosote and oil-type preservatives to ensure conformity with specifications D 390 and D 391.
💡 What temperature is used for testing?
The standard specifies 38°C (100°F). If the sample is not completely liquid at this temperature, the test uses the lowest temperature at which it becomes fully liquid.
⚡ What is the meniscus correction?
The standard requires adding an increment equivalent to a 1 mm reading on the hydrometer scale to the observed specific gravity value to correct for surface tension.
📌 Which thermometer is specified?
The specified instrument is an ASTM Low Softening Point Thermometer 15C (15F) with a range of −2 to 80°C (30 to 180°F), conforming to Specification E 1.