The purpose of verifying or calibrating liquid measurement devices is to ensure the accuracy of quantities being reported. With millions of dollars at stake, fractions of an inch or tenths of a degree Fahrenheit can make quite an impact to bottom line. At its most basic, the static (as opposed to dynamic, the use of meters) quantity determination of liquid hydrocarbons is generally obtained by measuring the depth of the liquid in a storage tank and obtaining a representative temperature. Through the use of volume tables and volume expansion factors the quantity at a standard temperature can be stated.
Several factors play an important role in quantity determination. Some of the factors are product density, the presence of free water, ambient temperature, tank construction (including roof design, stilling well design, stability of the tank bottom, etc.), as well as method of creation of the tank capacity tables Additional factors come into play when measuring static liquids on board marine vessels.
Another key factor is the accuracy of the devices used to measure the volume and temperature.
The two basic main types of devices used for the static measurement of hydrocarbon liquids consist of a gauging device used to determine the depth of the liquid and temperature measuring device. Automatic or remote measurement devices are available and are verified through the use of manual equipment.
Manual gauging devices are either a hand held steel tape or portable electronic device. Electronic devices are designed for open, restricted, or closed applications. Restricted or closed systems are used in conjunction with a vapour control valve. A specially designed type of measuring stick has been designed for the use on rail cars.
Manual temperature devices are either glass stem thermometers or portable electronic thermometers. Portable electronic thermometers can be stand alone or can be built into an electronic gauging device.
There are different international standards bodies which publish the specifications for the design and verification of calibration of these devices. For the purpose of this paper we will focus on the requirements as set forth by the American Petroleum Institute (API).