In order to accurately measure very low pressures, which of the instruments listed should be used?（）

单项选择题Reduced capacity, accompanied by vibration and noise at the suction of a centrifugal pump is a result of （）.

A.cavitation
B.water hammer
C.fluid friction
D.steam knock

单项选择题The refrigerant leaves the evaporator of a refrigeration system as a low pressure （）.

A.sub-cooled liquid
B.high temperature liquid
C.oil saturated liquid
D.superheated vapor

单项选择题When operating with a negative suction head, which of the following types of pumps will require priming?（）

A.Reciprocating
B.Centrifugal
C.Rotary
D.Gear

单项选择题The discharge rate or capacity of a centrifugal pump will vary directly as the （）.

A.change in the impeller diameter
B.square of the impeller radius
C.cube of the impeller diameter
D.impeller efficiency for large changes in its size

单项选择题Which of the following best describes the thermodynamic effect on the pressure and volume relationship with regards to saturated gases?（）

A.As pressure decreases, volume increases
B.As volume increases, pressure increases
C.As volume decreases, pressure decreases
D.As pressure increases, volume increases