Physical Address
304 North Cardinal St.
Dorchester Center, MA 02124
API MPMS 9.1 (2012): Standard Test Method for Density, Relative Density, and API Gravity of Crude Petroleum and Liquid Petroleum Products by Hydrometer Method
A comprehensive guide to the laboratory determination of essential petroleum properties using glass hydrometers, including scope, technical protocols, and compliance with the Manual of Petroleum Measurement Standards
Scope and Applicability
API MPMS 9.1 (2012), titled Standard Test Method for Density, Relative Density, or API Gravity of Crude Petroleum and Liquid Petroleum Products by Hydrometer Method, is a key component of the American Petroleum Institute’s Manual of Petroleum Measurement Standards (MPMS). This method is technically identical to ASTM D1298 and IP 160. It provides a standardized procedure for the laboratory determination of density, relative density (specific gravity), or API gravity using a glass hydrometer.
The method is applicable to crude petroleum, petroleum products (including lubricating oils, fuel oils, and distillates), and mixtures that are liquid at the test temperature and have a Reid vapor pressure (RVP) of 1.8 bar (26 psi) or less. It covers the range typically encountered in pipeline, refinery, and terminal operations. The standard is widely used for custody transfer, inventory control, blending, and quality assurance applications where accurate knowledge of density or API gravity is required.
Tip: API MPMS 9.1 is considered the primary reference for hydrometer-based density measurements in the petroleum industry. For volatile samples with RVP above 1.8 bar, alternative methods (e.g., API MPMS 9.2 using a pressure hydrometer) are recommended to avoid loss of light ends during measurement.
Technical Requirements and Equipment
Essential Apparatus
The standard specifies precise requirements for the following equipment:
- Hydrometers — Glass instruments conforming to ASTM E100 or ISO 387, with scales graduated in kg/m³ (density), 60/60°F relative density, or °API. Each hydrometer must be calibrated at a reference temperature (usually 15°C, 20°C, or 60°F) and certified for accuracy within ±0.1 °API, ±0.1 kg/m³, or ±0.0005 relative density.
- Thermometers — Calibrated to ±0.1°C or ±0.2°F, covering the expected temperature range.
- Hydrometer cylinders — Made of clear glass or plastic, with internal diameter at least 25 mm greater than the hydrometer bulb and height sufficient to allow full immersion without contacting the bottom.
- Constant‑temperature bath — Capable of maintaining the test temperature within ±0.5°C of the desired value.
Hydrometer Scale and Precision Requirements
Table 1 summarizes the typical scale graduations and repeatability limits prescribed by API MPMS 9.1 for the three measured parameters.
| Parameter | Scale Range | Scale Division | Repeatability (same operator, same apparatus) | Reproducibility (different operators, different labs) |
|---|---|---|---|---|
| API Gravity | −1 to 101 °API | 0.1 °API | 0.1 °API | 0.2 °API |
| Density (kg/m³ at 15°C) | 600 – 1100 kg/m³ | 0.5 kg/m³ | 0.5 kg/m³ | 1.0 kg/m³ |
| Relative Density (60/60°F) | 0.600 – 1.100 | 0.0005 | 0.0005 | 0.0010 |
Important: Hydrometers must be free of visible defects (scratches, chips, uneven scale). Calibration verification with certified reference oils at least annually is mandatory for custody transfer applications.
Detailed Procedure and Correction Methodology
Sample Preparation
The sample is brought to the test temperature without losing light components. For volatile samples, a pressure‑tight sampling device is used. The sample is poured gently into the clean, dry cylinder to avoid air entrainment. Any visible air bubbles are removed by touching the surface with a clean rod or by waiting a short period.
Temperature Control and Measurement
The cylinder is placed in a constant‑temperature bath set to the test temperature (usually 15°C, 20°C, or 60°F). A calibrated thermometer is immersed in the sample. When the sample temperature stabilizes within ±0.5°C of the bath target, the hydrometer is carefully inserted and allowed to float freely. Care is taken not to wet the stem above the immersion level except for the portion being read.
Reading the Hydrometer
The standard requires reading the hydrometer after it has come to rest and the sample temperature is constant. The reading is taken at the principal surface of the liquid (the bottom of the meniscus for transparent liquids, or the top of the meniscus for opaque liquids). For °API hydrometers, the reading is taken at the top of the meniscus. The observed reading is corrected for:
- Meniscus correction — When reading at the bottom of the meniscus for transparent liquids, the error due to the meniscus is eliminated by applying the calibration correction provided by the hydrometer certificate.
- Scale correction — Adjustment to the hydrometer reading using its calibration report.
- Thermal expansion of the hydrometer glass — Correcting the hydrometer reading to the standard reference temperature using the appropriate tables (API MPMS Chapter 11.1 / ASTM D1250 / IP 200).
Calculations
After obtaining the corrected hydrometer reading at the observed temperature, the result is reconciled to the standard reference temperature (15°C, 20°C, or 60°F) using the standard density‑temperature correlations. For API gravity, the following fundamental conversion is used:
API Gravity = (141.5 / Relative Density at 60°F) − 131.5
Relative density (60/60°F) is the ratio of the density of the sample at 60°F to the density of water at the same temperature.
Best Practice: Use the official API MPMS 11.1 tables (or equivalent computer subroutines) for the final conversion of observed readings to standard conditions. Manual interpolation should be avoided if automated calculation tools are available, as they eliminate rounding errors.
Compliance Notes and Quality Assurance
Calibration and Verification
All hydrometers, thermometers, and bath controllers must be traceable to national standards (e.g., NIST). Calibration intervals should follow laboratory quality protocols or local regulations; typically annual recalibration is sufficient for routine use, but a verification check with certified reference fluids is recommended before each series of measurements, especially in custody transfer applications.
Personnel Training
Operators must be trained to read hydrometers precisely (to the nearest division) and to apply corrections correctly. Competency assessments should include a demonstration of proper filling, temperature equalization, and parallax‑free reading.
Limitations and Alternative Methods
API MPMS 9.1 is not suitable for samples that are viscoelastic, opaque, or highly volatile. In such cases, API MPMS 9.2 (pressure hydrometer method), 9.3 (vibrating densitometer), or 9.4 (constant‑rate hydrometer) should be used. The method also requires a relatively large sample volume (approx. 500 mL) and can be time‑consuming if strict temperature equilibration is necessary.
Attention: Do not use this method for liquids with a vapor pressure exceeding 1.8 bar at the test temperature. Volatile samples will lose light ends during the open‑cylinder measurement, leading to erroneously high density or low API gravity.
Record Keeping
All raw readings, corrections applied, and final results must be documented. The standard requires reporting the result to the nearest 0.1 °API, 1.0 kg/m³ (or 0.1 kg/m³ if a denser hydrometer is used), or 0.0001 relative density, together with the reference temperature and the test temperature. A statement of compliance with API MPMS 9.1 (2012) should be included in the test report.
Frequently Asked Questions (FAQ)
Q: Why is my observed hydrometer reading different from the density reported by a laboratory digital densitometer?
A: API MPMS 9.1 uses a glass hydrometer that requires manual reading and correction. Digital densitometers (API MPMS 9.3) operate on a different principle (oscillating U‑tube) and typically yield a precision of ±0.1 kg/m³ compared to ±0.5 kg/m³ for the hydrometer. The two methods may also differ if different reference temperatures or density conversion tables are applied. Always ensure both measurements are corrected to the same reference conditions before comparing.
A: API MPMS 9.1 uses a glass hydrometer that requires manual reading and correction. Digital densitometers (API MPMS 9.3) operate on a different principle (oscillating U‑tube) and typically yield a precision of ±0.1 kg/m³ compared to ±0.5 kg/m³ for the hydrometer. The two methods may also differ if different reference temperatures or density conversion tables are applied. Always ensure both measurements are corrected to the same reference conditions before comparing.
Q: Can I use a plastic hydrometer cylinder instead of glass?
A: Yes, the standard permits clear plastic cylinders provided they are chemically resistant to the sample and do not affect the measurement. However, glass is preferred because it provides better thermal uniformity and is less likely to scratch. If plastic is used, verify that it does not impart a static charge that attracts hydrometer readings.
A: Yes, the standard permits clear plastic cylinders provided they are chemically resistant to the sample and do not affect the measurement. However, glass is preferred because it provides better thermal uniformity and is less likely to scratch. If plastic is used, verify that it does not impart a static charge that attracts hydrometer readings.
Q: How do I correct the hydrometer reading when the sample temperature is not exactly the hydrometer reference temperature?
A: First, apply the hydrometer scale correction from its calibration certificate. Then, using the observed temperature, compute the density at the sample temperature. Finally, use the API MPMS Chapter 11.1 density‑temperature tables to convert this density to the standard reference temperature (e.g., 15°C or 60°F). The entire correction workflow is typically automated in modern measurement software, but a manual check is good practice for training.
A: First, apply the hydrometer scale correction from its calibration certificate. Then, using the observed temperature, compute the density at the sample temperature. Finally, use the API MPMS Chapter 11.1 density‑temperature tables to convert this density to the standard reference temperature (e.g., 15°C or 60°F). The entire correction workflow is typically automated in modern measurement software, but a manual check is good practice for training.
Q: What is the minimum sample volume required for the hydrometer method?
A: The cylinder should be large enough that the hydrometer floats without touching the walls or bottom, and the stem is fully immersed at the reading level. Typically a 500 mL cylinder is sufficient for most hydrometers. The exact minimum volume depends on the hydrometer design; refer to the hydrometer manufacturer’s guidance and the standard for specific requirements.
A: The cylinder should be large enough that the hydrometer floats without touching the walls or bottom, and the stem is fully immersed at the reading level. Typically a 500 mL cylinder is sufficient for most hydrometers. The exact minimum volume depends on the hydrometer design; refer to the hydrometer manufacturer’s guidance and the standard for specific requirements.
Published under the guidelines of the American Petroleum Institute. This article reflects the 2012 edition of API MPMS 9.1. For the latest updates and official wording, readers are advised to consult the current edition from API.