Volume dilatation by a small tube above a huge volume. But you have to take account of your vessel dilatation.

There are several ways to measure oil densities at high temperatures, including: 1. Pycnometer Method: The pycnometer method is one of the most common methods used to measure oil density at high temperatures. In this method, a small amount of oil is placed in a pycnometer (a container with a known volume), and the weight of the pycnometer is measured before and after filling it with oil. The difference in weight gives the mass of oil, which can be used to calculate its density. 2. Oscillating U-Tube Method: The oscillating U-tube method is another technique used to measure oil density at high temperatures. In this method, a U-shaped tube filled with oil is set into oscillation using a motor-driven system. The frequency of oscillation is measured and used to calculate the density of the oil. 3. Magnetic Suspension Balance Method: The magnetic suspension balance method is a highly accurate technique for measuring oil density at high temperatures. In this method, a small amount of oil is placed in a container that is suspended by magnets in a magnetic field. The weight of the container and its contents are measured, and the density of the oil can be calculated. 4. Vibrating Tube Method: The vibrating tube method measures the natural frequency of vibration of a tube filled with oil. The frequency of vibration changes as the temperature changes, allowing for accurate measurement of density. 5. Coriolis Mass Flow Meter: Coriolis mass flow meters use the Coriolis effect to measure mass flow rate directly. They can also measure fluid density indirectly by calculating mass flow rate and volumetric flow rate simultaneously. References 1. ASTM International: ASTM D4052-19 Standard Test Method for Density, Relative Density, and API Gravity of Liquids by Digital Density Meter 2. American Petroleum Institute: API MPMS Chapter 9.1 - Standard Test Method for Density, Relative Density, or API Gravity of Crude Petroleum and Liquid Petroleum Products by Hydrometer Method 3. National Institute of Standards and Technology: NIST Technical Note 1297 - Guidelines for Evaluating and Expressing the Uncertainty of NIST Measurement Results