Nansen bottle

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A Nansen bottle is a device for obtaining samples of water at a specific depth. It was designed in 1894 by Fridtjof Nansen and further developed by Shale Niskin in 1966.

Description[edit]

Sailor holding a Nansen bottle

The bottle, more precisely a metal or plastic cylinder, is lowered on a cable into the ocean, and when it has reached the required depth, a brass weight called a "messenger" is dropped down the cable. When the weight reaches the bottle, the impact tips the bottle upside down and trips a spring-loaded valve at the end, trapping the water sample inside. The bottle and sample are then retrieved by hauling in the cable.[1]

A second messenger can be arranged to be released by the inverting mechanism, and slide down the cable until it reaches another Nansen bottle. By fixing a sequence of bottles and messengers at intervals along the cable, a series of samples at increasing depth can be taken from a single action.[1]

The sea temperature at the water sampling depth is recorded by means of a reversing thermometer fixed to the Nansen bottle. This is a mercury thermometer with a constriction in its capillary tube which, when the thermometer is inverted, causes the thread to break and trap the mercury, fixing the temperature reading.[1] Since water pressure at depth will compress the thermometer walls and affect the indicated temperature, the thermometer is protected by a rigid enclosure. A non-protected thermometer is paired with the protected one, and comparison of the two temperature readings allows both temperature and pressure at the sampling point to be determined.

Niskin bottle[edit]

Niskin bottle about to be deployed

The Niskin bottle is an improvement on the Nansen bottle patented by Shale Niskin in March 1966. Instead of a metal bottle sealed at one end, the 'bottle' is a tube, usually plastic to minimize contamination of the sample, and open to the water at both ends. Each end is equipped with a cap which is either spring-loaded or tensioned by an elastic rope. The action of the messenger weight is to trip both caps shut and seal the tube.[1] A reversing thermometer may also be carried on a frame fixed to the Niskin bottle. Since there is no rotation of the bottle to fix the temperature measurement, the thermometer has a separate spring-loaded rotating mechanism of its own tripped by the messenger weight.

A modern variation of the Niskin bottle uses actuated valves that may be either preset to trip at a specific depth detected by a pressure switch, or remotely controlled to do so via an electrical signal sent from the surface. This arrangement conveniently allows for a large number of Niskin bottles to be mounted together in a circular frame termed a rosette.[2] As many as 36 bottles may be mounted on a single rosette. Thermistor temperature sensors are more commonly employed on Niskin bottle rosettes due to their higher accuracy compared to mercury thermometers.

References[edit]

  1. ^ a b c d Tomczak, Matthias (16 March 2000). "Oceanographic instrumentation Lecture 13". mt-oceanography.info. Retrieved 7 April 2018.
  2. ^ Pinet, Paul R. (2012). Essential Invitation to Oceanography. Colgate University. p. 98. ISBN 9781449686437.

External links[edit]