Factbox: How U.S. scientists calculated BP spill rate

(Reuters) - A team of government and independent experts on Thursday released its "overall best initial estimate" of the oil flowing from BP Plc's blown-out Gulf of Mexico well.

The Flow Rate Technical Group's estimate is a range from 12,000 barrels per day (504,000 gallons/1.9 million liters) to 19,000 bpd (798,000 gallons/3.02 million liters), well above BP's estimate of 5,000 bpd. The team said it used three separate techniques to arrive at the estimate. Those three measurements are:

MASS BALANCE ESTIMATE: 12,000-19,000 barrels per day

The mass balance team calculated the amount of oil on the surface of the Gulf of Mexico, using a NASA imaging spectrometer previously used to discover water on the surface of the moon.

The team also estimated that between 130,000 barrels and 270,000 barrels of oil were on the ocean surface on May 17, and a similar amount had been dispersed, skimmed or evaporated.

PLUME MODELING ESTIMATE: 12,000-25,000 bpd

The plume modeling team used video from remotely operated vehicles at the ocean floor to observe oil and natural gas escaping from the well's riser pipe. The team used advanced imaging analysis to estimate fluid velocity and flow volume.

RISER INSERTION TUBE TOOL ESTIMATE: at least 11,000 bpd

This team studied oil being collected by a siphoning device called a riser insertion tube that BP has used to collect some oil directly from one of the two leak sites. On May 25, the tool logged a collection rate of 8,000 barrels per day, the team said, as measured by a meter verified by a third party. The team estimated that at least 10 percent of the flow was not collected by the siphon, and that additional oil was escaping from a second leak site.

The Flow Rate Technical Group is comprised of federal scientists, independent experts, and academic experts. Included are members of the U.S. Geological Survey (USGS), National Oceanic and Atmospheric Administration, Energy Department, Coast Guard, Minerals Management Service, National Institute of Standards and Technology, University of California Berkeley, University of Washington, University of Texas, Purdue University, and several other academic institutions. BP is not involved except to supply raw data, USGS said.

Source: USGS statement

(Reporting by Chris Baltimore; Editing by Eric Beech)