Wednesday, November 28, 2018

The one device you can't do without for serious water sampling

I am of course talking about the depth sampler. If you want to collect a water sample at a given depth, you simply need this standard piece of equipment. Depending on the length of your rope, it can easily go down to 100 feet and beyond.

You can suspend this guy from a surface craft (e.g. kayak, motor boat) or a bridge overlooking the water body of interest. Once you have reached the desired depth as indicated on the marking on the rope, you release the messenger (a metal weight) which rushes down along the rope towards the sampling bottle. Upon hitting the bottle's "trigger", the 2 ends snap shut like a fish trap to enclose your water sample within the bottle.

Purchase Considerations

  1. Alpha (aka van Dorn) vs. beta (Kemmerer). Alpha samplers are not recommended for chemical analysis (trace metals and organics) of the water sample due to possible cross contamination (mercury, phosphorus) from the materials in its construction (seals & closure tube). For chemical analysis, one is advised to use a beta sampler.
  2. PVC vs. acrylic. Whether alpha or beta, the bottle body can be either made of PVC or acrylic. PVC is opaque but durable while acrylic is clear and less durable. If you need to see the contents within the sampling bottle, by all means, go for acrylic.
  3. Horizontal vs. vertical. A horizonal sampler means the sampling bottle descends into the water parallel to the bed. Water enters from both sides of the bottle. This is useful for sampling at the thermocline, other stratification levels or just above the bottom sediment. A vertical sampler on the other hand descends with its body perpendicular to the bed. Water flow is not restricted during descent, making it desirable for collecting plankton and suspended sediments.
Figure: Lowering the depth sampler into the water body of interest. In this case, we were using the alpha, horizontal, acrylic version.

Figure: After retrieving the depth sampler, the water sample was drained into a suitable container.

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