Hydrostatic Pressure and Extreme Cold: What You Need to Know

Pressure, density, temperature, volume, and level are all related whether you are measuring the pressure of liquid chemicals or the pressure of the air in your tires. However, whether in the tank or the tire, things change as the temperature fluctuates. When the temperature drops, the air molecules in a tire get closer together to stay warm. Yes, tires do get flatter in the winter as pressure and density are inversely proportional—when one rises the other falls to maintain equilibrium.  The same amount of air is present but the density increases because the air molecules are tighter together, and the pressure decreases, illuminating the idiot light on my dash.

The relationship between pressure, density, temperature, volume, and level is one users in the process industry need to understand. If liquid or gas at a chemical facility, for example, is stored outside and its level is monitored with a hydrostatic pressure instrument, the reading might be inaccurate as the weather grows colder. 

To output a level measurement from a hydrostatic pressure sensor, the density needs to be entered into the equation. Density is usually supplied as a fixed value and when it changes due to temperature fluctuations, the level output is rendered incorrect. With this cold weather, the density of the fluid increases and the actual level in the vessel drops, BUT the level output remains the same! The worst case scenario in extreme cold is that users run out of material because they do not realize that the changing density means they have more available space in a tank. That's not efficient. To maintain an accurate level measurement in extreme temperatures, users should manually adjust the density value of hydrostatic pressure devices installed on outdoor tanks.

Stay warm and stay tuned for a posting discussing how to use differential pressure to monitor density!