How is water sampled on the vessels?
Two kinds of sampling bottles are used on the vessels: Van Dorn water sampling bottles and, less frequently, Kemmerer water samplers. The idea behind these samplers is to allow water to be collected at a known depth. The Van Dorn bottles are used for sampling at various depths and the Kemmerer water sampler is used for surface water samples.
What are Van Dorn water sampling bottles?
The Van Dorn bottles provide a means of obtaining water samples at selected depths below the surface. It consists of an open ended clear plastic cylinder that can be attached to the hydrographic wire (the steel wire wound on the winch) and lowered to any desired depth. A deckhand operates the winch. The bottles also provide a platform to which thermometers can be attached to record the temperature of the water at the location of each Van Dorn bottle.
Each end of the cylinder is fitted with a rubber cover. The Van Dorn bottle is attached to the line with the covers pulled out and twisted back and around to the side. The bottle is lowered to a pre-selected depth and left there until the thermometers attached inside come to thermal equilibrium with the water at that depth.
Two Van Dorn bottles are attached to the same line and lowered to two different pre-selected depths. A metal weight called a "messenger" is attached below the upper bottle. The water sample is taken by dropping a "messenger" down the wire. When the weight hits the catch on the upper Van Dorn bottle, the catch releases the rubber end covers. The two ends snap around and seal off the ends. The other "messenger" then travels down the winch line to the lower Van Dorn bottle causing it to be sealed.
When it is time to lower the Van Dorn bottles into the water (this is called making a "cast"), a decision is made about the depth to which to send the bottles. This decision is based upon the depth of the water at the station and the number of samples needed. Normally only two water samples (surface and bottom) are taken. In shallow areas, typically only take one water sample is taken. If a third sample is required, three Van Dorn bottles are stacked on the hydrographic cable.
When both bottles have been tripped, they are retrieved and returned to their storage rack. Water samples from each bottle can then be taken for analysis and the temperature read from the attached thermometers. You may be able to see organisms in the water samples through the clear walls of the Van Dorn bottles.
What is Kemmerer Water Sampler?
The Kemmerer Water Sampler is a device that makes it possible to obtain a sample of water from a pond, river or lake with little effort. The sampler is essentially a metal tube with stoppers on each end that can be held open when the sampler is lowered by a line to a desired depth. These same stoppers can then be triggered to close the ends of the tube when a metal cylinder called a messenger is dropped down the line holding the Kemmerer sampler. After the stoppers close the ends of the tube, the sampler is retrieved with the desired sample of water being uncontaminated by other water. This maintains the integrity of the sample.
Since only one sample of water can be obtained at a time, the process of obtaining several samples at various depths at the same sampling station is slow. The Kemmerer sampler is generally used in shallow water where only one or two sampling depths are needed. The Van Dorn bottles are used when several samples are needed from various depths at the same time. The Van Dorn bottles also allow greater depths to be explored than with the Kemmerer sampler.