Hach 900 MAX manual 4 Principle of Operation, Liquid Sensing, Figure, Liquid Sensor

Section 1

1.4 Principle of Operation

1.4.1 Liquid Sensing

The sampler is designed for indoor, permanent installation. All controls are located on the front panel. Capped, watertight connectors for interfacing to external devices are located along the left side of the controller.

The sampler uses a liquid sensing system to detect the absence or presence of liquid at the peristaltic pump intake. The liquid sensor (Figure 3) is located on the front of the control housing.

The liquid sensing system provides three primary benefits:

•Accurate, repeatable sample volumes

•Intake tube prerinse

•Sample retry

Figure 3 Liquid Sensor

1

2

3

1.Sensor Body

2.Sensor Cover

3.Knobs (turn to remove)

Accurate, Repeatable Sample Volumes

The liquid sensor detects the presence and velocity of the incoming sample. This information allows the sampler to automatically dispense the correct amount of liquid into the sample bottle.

The liquid sensing system allows the sampler to deliver repeatable sample volumes even with changing suction lifts. Each time the peristaltic pump pulls a sample, the microprocessor determines the time required for liquid to travel to the liquid sensor. If the suction lift increases due to a drop in level at the sample source, the time required for liquid to reach the sensor will increase. The microprocessor automatically compensates for this change by allowing the peristaltic pump to deliver sample liquid for a corresponding longer period of time. Conversely, if suction lift decreases due to an increase in level at the sample source, the time required for liquid to the sensor will decrease. Again, the microprocessor automatically compensates for this change by decreasing the sample delivery time.

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8990int.fm	Principle of Operation