Instructor's Manual - Water Sampling
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.
Instructions for sampling with a Van Dorn Bottle:
NOTE: Van Dorn bottles are devices that are attached to the winch line by the deckhand with assistance from participants.
- The deckhand will ask one of the people assigned to assist him/her to get the right hand Van Dorn bottle for the bottom sample from the rack. The deckhand will demonstrate how the water sampler works.
- One person, wearing a Personal Flotation Device (life jacket), will stand on the "hero platform" and assist the deckhand in attaching the Van Dorn bottle to the winch line. A safety line is temporarily attached to the bottle to prevent accidental loss of the bottle should it be dropped while attaching it to the wire. The safety line is then removed and the deckhand will release the brake on the winch, lowering the bottle into the water.
- The assigned person will then get the remaining Van Dorn bottle for the top sample from the rack and it will also be attached to the line. Typically the lower bottle is lowered to a depth 1.0 to 1.5 meters above the bottom and the upper bottle is 1.0 to 1.5 meters below the surface.
- When the deckhand gives the signal, the assigned person drops the messenger down the wire that triggers the closing of the bottles.
- The deckhand will use the winch to retrieve the bottles and remove them from the line. The assigned person will transport each bottle to the storage rack and immediately record the temperature of the water.
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.
Instructions for sampling with a Kemmerer Water Sampler:
- Obtain the sampler and check the knot at the bottom of the sampler for tightness and size. The knot should be sufficiently large so that it will not pull through the central tube of the sampler.
- To prepare the sampler for making a cast, cock the sampler by pulling the trip head into the trip plate. Holding the top and bottom stoppers and giving a short, hard pull to the bottom stopper does this.
- Tie the free end of the line to the railing of the vessel. This is done to prevent accidental dropping of the sampler should the person operating the sampler let go of the line.
- Lower the sampler to the desired depth. When the sampler is at the desired depth, attach a messenger to the line. This is best done with the messenger held over the deck until it is securely attached to the line.
- Release the messenger. It will slide down the line to the sampler where it will trigger the stoppers and they will close the ends of the tube. The stoppers seal by their own weight.
- Retrieve the sampler. Untie the line from the railing and carry the sampler to the main cabin where the sample of water can be analyzed.
- Dry off the Kemmerer water sampler and return it to the storage location. Always store the sampler in the open position (stoppers not in the tube).
Page last modified March 20, 2012