Low compressor head pressure in a refrigeration system can be caused by（）.

When a refrigeration compressor has developed a high head pressure as a result of a refrigerant overcharge, you should （）.

The output volume of a positive fixed displacement pump can be changed only by （）.

The control mode in which the final control element is moved from one of two flexed positions to the other is known as （）.

Pitting in the suction areas of a centrifugal pump bronze impeller is usually caused by （）.

Thermostatic expansion valves can be adjusted for （）.

A salinity indicator is used to determine the （）.

The rate of heat transfer from a hot region to a cold region is affected most by the （）.

An excessive charge of refrigerant in an air-cooled refrigeration system can cause （）.

The major source of chemical contaminants in hydraulic fluid is （）.