CTCV: constant time; constant volume
The most common sampling mode; simple sampling performed at regular time intervals.
CVVT: constant volume; variable time
Sampling based on input from a flow meter, with sample volumes remaining constant but time intervals between samples varying according to flow. It is also possible to combine volume and time-proportional sampling to moderate extreme fluctuations and prevent samples from being taken too close together (very high flow) or too far apart (very low flow).
CTVV: constant time; variable volume
Sampling based on input from a flow meter, with time intervals between samples remaining constant but sample volumes varying according to flow. Offers representative samples if amount of flow and dirt content fluctuate widely. In some regions, this sampling mode is mandatory.
Sampling based on input from an online measuring device, e.g. pH sensor. As long as a predefined event is taking place, samples are drawn and handled according to program settings.