The instrument construction is similar to that of the gas chromatograph. The liquid chromatograph is divided into two parts: the analysis section and the control section. The analysis section is located near the process stream and is often contained in an instrument house. This section includes 1) the chromatographic oven containing the valves, columns, and detector; 2) the carrier supply; 3) an electronics compartment containing the circuitry for the detector, temperature control, valve operators, and local data handling; and 4) the sample preparation system, which may be in a separate temperaturecontrolled oven. The control section is often located in the control room and includes the programmer to control the instrument and process the data, and the data display, which may include a strip-chart recorder, a digital display, or a digital computer for further data processing. The programmer discussed here includes stream selection, which may be a physically separate unit. However, the liquid chromatograph operating conditions are different from those of the gas chromatograph. In particular, the carrier pressure is typically 1000 PSIG (7 MPa) at a flow rate of 1 ml/min, and the chromatographic cycle time is typically 3 to 20 min.There are two types of carrier pumps: air-driven and motordriven. Both are reciprocating pumps containing check valves to divert the liquid through the pump. Air-driven pumps are powered by a large-diameter air piston, which is connected to a small-diameter liquid piston. The pump recycles at the end of each stroke. The air-driven pump provides a pressure, relatively independent of flow, that is nominally some multiple of the air pressure.