Wednesday, January 16, 2013

Getting a sample of water

one alternative to taking water samples...

As an oceanographer, I often think about how to sample the water I’m interested in. Generally, I prefer using instruments that measure a property in place returning just an electronic data file, but sometimes, water must be taken for analysis which raises the question: how do you get water from the ocean and into a lab? The simplest solution is a bucket at the end of a rope - a technique I’ve found myself using in the past. This equipment is easy to find and easy to use. The downside is you can only get surface water this way. Getting waters from intermediate depths takes fancier gear, and it took a long time of trial and error to develop the instruments needed to collect water from these depths.

A theoretical idea…

Water sampling equipment started from an idea presented at the British Royal Society early in its history from someone who never went to sea. Robert Hook designed a box of wood to be lowered on a line. Water flow held the end valves open on the way down. When the instrument reached the desired depth, it would be hauled up, and the change in direction would close the end valves. This design ultimately evolved into our modern water sampler, however, the original wasn’t practical. The wood would swell in the water and no seal was maintained - if you hauled back the sampler and it actually contained water, there was no way to know it came from the targeted depth.

A reality check…

To reliably work, water sampling instruments made the leap from theoretical designs to functional equipment via multiple design iterations (I often wondered how much of this occurred on a deck of a ship where some poor technician was trying to make this theoretical, lab-built equipment work).

By the time of the Challenger Expedition (1872-76) a working instrument existed - a stop-cock water bottle. This sampler had spring-loaded stoppers for both ends. When it was being lowered into the ocean the stoppers would be open, allowing water to flow freely through the bottle. At the desired depth, the bottle being lowered, was pulled up slightly, allowed to fall back and then jerked to a stop - action that would close the stoppers and trap the water inside.

Several design iterations later, and still in common use, is the Niskin bottle (designed in the 1960’s). Niskin bottles are made of plastic to reduce sample contamination and the end caps have rubber washers to improve their seal. These bottles are lowered down a wire in the open position. When the bottle reaches the desired depth a metal messenger, basically a metal bead that clips onto the wire, is sent down the wire to trip the bottle closed. An added advantage is these bottles can be used in series, allowing for multiple samples to be taken at one time. They can even be arranged in a rosette for more detailed water collection schemes.

Do we need to take the water…

“To replace the laborious analysis of recovered water samples the marine scientist may employ a single sensing unit which will telemeter back to him, or record on tape, data on the temperature, salinity, conductivity, oxygen content and sound velocity of the water in which it is placed.”
                   - Historical Instruments in Oceanography by Anita McConnell, 1981

Water sampling is still necessary for many things, however, electronic sensors can be deployed for long periods of time, reporting back parameters which can provide an ongoing record at that location. Ocean networks like Venus and Neptune provide ongoing reporting that could never be obtained from water sampling alone.

As a tangent - the photo is a simple mooring I built with electronic recording instruments (salinity and temperature) that was deployed for a year, part of that under ice. Yes, it looks like a pile of rope.


  1. Great post. Though the mooring looks more like cable than rope. And shouldn't it have a bunch of marine growth on it?

    1. That is actually rope - it was poly rope left over from another project and really nasty to work with (cable would have been easier). The picture was taken before deployment, so nothing had a chance to grow on it yet.