API MPMS 3.1A:2005 pdf download.Manual of Petroleum Measurement Standards Chapter 3—Tank Gauging
This standard describes the following: (a) the procedures for manually gauging the liquid level of petroleum and petro- leum products in non-pressure fixed-roof, floating-roof tanks and marine tank vessels, (b) procedures for manually gauging the level of free water which may be found with the petro- leum or petroleum products, (c) methods used to verify the length of gauge tapes under field conditions and the influence of bob weights and temperature on the gauge tape length, and (d) the influences that may affect the position of gauging ref- erence point (either the datum plate or the reference gauge point). Throughout this standard the term petroleum will be used to denote petroleum, petroleum products, or the liquids normally associated with the petroleum industry. The method used to determine the volume of tank contents determined from gauge readings is not covered in this stan- dard. The determination of temperature, API gravity, and sus- pended sediment and water of the tank contents are not within the scope of this standard; however, methods used for these determinations may be found in the API Manual of Petro- leum Measurement Standards (MPMS).
3.1A.4 Outline of Method
There are two basic types of procedures used for obtaining a gauge reading—innage and outage (dip and ullage). For the innage method, the gauge reading shall be defined as the measure of the linear distance along a vertical path from the datum plate or tank bottom to the surface of the liquid being gauged. An innage gauge is a direct measurement of liquid level. For the outage method, the gauge reading shall be defined as the measure of the linear distance along a vertical path from the surface of the liquid being gauged to the tank reference gauge point. An outage gauge is an indirect mea-surement of liquid level. Figure 1 illustrates the innage and the outage methods for obtaining a gauge reading. Innage gauges are generally preferred, as these may reduce the effect of tank reference point movements. However, there are circumstances where outages will be more applicable. When the outage method is used, every effort should be made to periodically verify the tank’s refer- ence gauge height for both opening and closing conditions to ensure it has not changed. If the reference gauge height has changed, use of innage gauges is recommended.