Studies have shown that bladder pumps have the widest range of accuracy for VOCs and other types of analytes across the widest range of field conditions. In a number of studies, bladder pumps generally came in very close to 100% of control samples that were taken. There are other types of devices that will work for low-flow purging and sampling. But, the key thing is that the device has to have the ability for the flow rate to be controlled within the range necessary to get the water level in the well to stabilize.