CREATE TABLE table_21204 ("Vacuum range" TEXT, "Pressure in hPa  ( mbar ) " TEXT, "Molecules / cm 3" TEXT, "Molecules / m 3" TEXT, "Mean free path" TEXT)

What is the pressure in ultra high vacuum?
SELECT "Pressure in hPa (mbar)" FROM table_21204 WHERE "Vacuum range" = 'Ultra high vacuum'
SELECT "Pressure in hPa (mbar)" FROM "table_21204" WHERE "Vacuum range" = 'Ultra high vacuum'