Three types of temperature-measuring devices are used in oceanographic work. Accurate thermometers of the standard type are employed for measuring the surface temperature when a sample of the surface water is taken with a bucket and for determining the subsurface temperatures when the water sample is taken with a thermally insulated sampling bottle (p. 354). The thermometers used for measuring temperatures at subsurface levels are of the reversing type and are generally mounted upon water-sampling bottles so that temperatures and the water sample for salinity or other chemical and physical tests are obtained at the same level. The third type are temperature-measuring instruments that give a continuous record, such as thermographs, which are employed at shore stations and on board vessels to record the temperature at some fixed level at ot near the sea surface. Many devices have been invented for
The centigrade scale is the standard for the scientific investigation of the sea. A high degree of accuracy is necessary in temperature measurements because of the relatively large effects that temperature has upon the density and other physical properties and because of the extremely small variations in temperature found at great depths. Subsurface temperatures must be accurate to within less than 0.05°C, and under certain circumstances to within 0.01°. Such accuracy can be obtained only with well-made thermometers that have been carefully calibrated and rechecked from time to time. Because of the greater variability of conditions in the surface layers the standards of accuracy there need not be quite so high.
Conventional-type thermometers for surface temperatures or for use with an insulated bottle must have an open scale that is easy to read, with divisions for every tenth of a degree. The scale should preferably be etched upon the glass of the capillary. The thermometer should be of small thermal capacity in order to attain equilibrium rapidly; it should also be checked for calibration errors at a number of points on the scale by comparison with a thermometer of known accuracy, and it should be read
Protected and unprotected reversing thermometers in set position, that is, before reversal. To the right is shown the constricted part of the capillary in set and reversed positions.
Protected Reversing Thermometers. Reversing thermometers (fig. 86) are usually mounted upon the water-sampling bottles (fig. 87), but they may be mounted in reversing frames and used independently. Reversing thermometers were first introduced by Negretti and Zambra (London) in 1874, and since that time have been improved, so that well made instruments are now accurate to within 0.01°C. On the Challenger Expedition, 1873–1876, the subsurface temperatures were measured by means of minimum thermometers, which were the most satisfactory instrument available at that time.
A reversing thermometer is essentially a double-ended thermometer. It is sent down to the required depth in the set position, and in this position it consists of a large reservoir of mercury connected by means of a fine capillary to a smaller bulb at the upper end. Just above the large reservoir the capillary is constricted and branched, with a small arm, and above this the thermometer tube is bent in a loop, from which it continues straight and terminates in the smaller bulb. The thermometer is so constructed that in the set position mercury fills the reservoir, the capillary, and part of the bulb. The amount of mercury above the constriction depends upon the temperature, and, when the thermometer is reversed, by turning through 180 degrees, the mercury column breaks at the point of constriction and runs down, filling the bulb and part of the graduated capillary, and thus indicating the temperature at reversal. The loop in the capillary, which is generally of enlarged diameter, is designed to trap any mercury that is forced past the constriction if the temperature is raised after the thermometer has been reversed. In order to correct the reading for the changes resulting from differences between the temperature at reversal and the surrounding temperature at the time of reading, a small standard-type thermometer, known as the auxiliary thermometer, is mounted alongside the reversing thermometer. The reversing thermometer and the auxiliary thermometer are enclosed in a heavy glass tube that is partially evacuated except for the portion surrounding the reservoir of the reversing thermometer, and this part is filled with mercury to serve as a thermal conductor between the surroundings and the reservoir. Besides protecting
Readings obtained by reversing thermometers must be corrected for the changes due to differences between the temperature at reversal and the temperature at which the thermometer is read, and for calibration errors. An equation developed by Schumacher is commonly used for correction:
Reversing thermometers are usually used in pairs, most commonly attached to the water-sampling bottles, but they may be mounted in special reversing frames that are either operated by messenger or that have a propeller release. The frames holding the thermometers are brass tubes which have been cut away so that the scale is visible and which are perforated around the reservoir. The ends of the tubes are fitted with coil springs or packed with sponge rubber so that the thermometers are firmly held but are not subject to strains.
Unprotected Reversing Thermometers. Reversing thermometers identical in design with those previously described but having an open protective tube are employed to determine the depths of sampling (fig. 86). Because of the difference in the compressibility of glass and mercury, thermometers subjected to pressure give a fictitious “temperature” reading that is dependent upon the temperature and the pressure. This characteristic is utilized for determining the depth of reversal. Instruments used for this purpose are so designed that the apparent temperature increase due to the hydrostatic pressure is about 0.01°C/m. An unprotected thermometer is always paired with a protected thermometer, by means of which the temperature in situ, Tw, is determined. When Tw, has been obtained by correcting the readings of the protected thermometer, the correction to be added to the reading of the unprotected thermometer, T′u, can be obtained from the equation
Special Devices. Temperatures measured by the methods mentioned above yield observations at discrete points in space and time. Subsurface observations with reversing thermometers are time consuming and the instruments and equipment are expensive. Many devices have been suggested for obtaining continuous observations at selected levels or as a function of depth. Thermographs are commonly used at shore stations and on vessels to obtain a continuous records at or near the sea surface. The thermometer bulb, usually containing mercury, is mounted on the ship's hull or in one of the intake pipes and connected to the recording mechanism by a fine capillary. The recording mechanism traces the temperature on a paper-covered revolving drum. Records of temperatures obtained by a thermograph should be checked at frequent intervals against temperatures obtained in some other way.
Various types of electrical-resistance thermometers have been designed to be lowered into the water and to give a continuous reading, but have not proved satisfactory. Spilhaus (1938, (1940) has developed an