Electricity: measuring and testing – Electrolyte properties
Reexamination Certificate
2001-12-06
2003-06-10
Le, N. (Department: 2858)
Electricity: measuring and testing
Electrolyte properties
C324S441000, C324S439000, C324S448000, C324S444000, C324S1540PB
Reexamination Certificate
active
06577134
ABSTRACT:
BACKGROUND
1. Field of the Invention
This invention relates generally to apparatus for monitoring and measuring electrical and thermal properties of liquids including saline water and other conductive solutions; and more particularly to a device that can be used to measure such properties, preliminary to very accurately deriving therefrom secondary properties—such as salinity, density and the speed of sound in such liquids. Preferably the apparatus also measures water pressure, preliminary to determining depth.
2. Related Art
Temperature and electrical conductivity are among the most fundamental parameters that characterize liquids in three spatial dimensions and time. Measurement of these quantities enables derivation of various parameters including salinity, density and sound speed. These derived parameters are fundamental for many different kinds of determinations.
These range, for example, from studying stability and mixing processes in the ocean—for fundamental physical oceanographic knowledge, and for understanding the environmental and biological processes in the sea—to acoustic techniques for locating objects, or determining the direction and velocity of objects, in water. Still other determinations include sensing of disturbances in density-stratified water.
All such techniques are outside the scope of this document being well known and straightforward to apply—given usable values of salinity, density, sound speed and depth.
This latter parameter, depth, historically has been evaluated from probe drop rate. (This is true particularly of previous devices intended to be used just once and discarded, i. e. not retrieved.) Various uncontrolled factors make this approach objectionably imprecise.
In this document, devices for sensing conductivity, temperature and depth by concurrent measurements are called “CTD” sensors or simply “CTDs”. Those intended for one-time use, i. e. units that are nominally expendable, are called “XCTDs”.
Technology is available for measuring temperature and conductivity in the context of oceanographic research. Accuracy of measurements, however, in the use of such available technology, is for many purposes less than satisfactory.
There have been significant limitations in the accuracy of derived parameters such as sound speed, salinity and density. More specifically, it is accuracy in the derived parameters—primarily density, and speed of sound—which leaves much to be desired.
The present inventors have studied this problem and have gained understandings of the sources of such inaccuracy. Some of these insights are probably known to advanced artisans in related work, particularly perhaps to some scientists in oceanography.
Thus for instance it may be recognized that significant error components arise from failure to collocate the temperature and conductivity sensors. In other words, for good accuracy the sensors should be arranged so that the liquid volumes whose temperature and conductivity, respectively, are measured are a common volume—to a reasonable degree of precision.
Other related understandings, however, are believed to be novel at least in part, as seen from the perspective of a person of ordinary skill in the art of monitoring instrumentation. These insights therefore will be presented in later sections of this document which disclose the invention.
Temperature measurement—Previous work of the present inventors has provided thermistor-based temperature measurements with longterm accuracies better than 15 mdeg. The temporal resolution of the temperature in the ocean was better than 7 msec, and the temperature resolution in the microdegree range.
A simple but stable half bridge was used for the temperature sensor circuit. As will be seen, such devices are advantageously adapted for use in a novel combination CTD device.
Conductivity measurement, and combined conductivity/temperature sensors—Another limitation of measurement apparatus heretofore known is that the apparatus is in fact clearly addressed to research applications. Reasonably economical higher-volume units for robust, routine use in—for example—waterway or ocean monitoring have yet to be introduced.
The prior art of CTD sensors may be divided into two categories: expendable and nonexpendable devices. At this writing there is only one source of expendable CTDs, or XCTDs, on the market. There are several sources of nonexpendable CTDs.
Nearly all of these devices use thermistor-based temperature sensors. Thermistors are very stable and easily encapsulated—and they come in a variety of shapes, resistance ranges, and response times.
Available CTDs use various positions of the thermistor near the conductivity cell. To date there are no nonexpendable CTDs that have the thermistor placed directly into the sampling volume of the conductivity cell.
The one expendable commercial probe encloses both the conductivity cell and thermistor in a tube 20 cm (eight inches) long, thus approaching collocation but not achieving it. That device, and many of the nonexpendable CTDs as well, use electrode-type conductivity sensors.
Most of these cells, but not that of Sea-Bird Corporation, use a four-electrode design. Sea-Bird uses a three-electrode cell that is within a Wien bridge.
All of these conductivity cells are of a closed geometry. Such geometry maximizes d. c. stability at the expense of frequency response.
An inherent problem with most closed-geometry cells is that the approach does not easily lend itself to mass production. Placing or forming electrodes on the inside of a tube, by common methods, is costly and time-consuming. Handcrafting of the conductivity cell is a major cost driver of an expendable device.
Other prior conductivity cells use open-celled geometries, which the present inventors have described earlier but never commercialized. These cells were designed for doing high-frequency measurements, and not maximized for d. c. or low-frequency stability.
When expendable sensors are used in ocean-water monitoring, the cost of the discarded sensors is very high. When nonexpendable sensors are used in ocean monitoring, the sensor cost is allocated over a much greater number of studies—but an important secondary problem arises in the high cost of equipment, and also of crew and staff time, required simply for reeling the sensors back up to the surface after each probe drop.
Even taking into account the economy available through multiple reuses, today's available sensor technology is expensive when used in an expendable form. An expendable conductivity, temperature, and depth sensor (“XCTD”) costs an order of magnitude more than expendable temperature probes (XBTs) alone.
The present invention contemplates reducing the cost of XCTDs, including a depth measurement, to a level that is half the cost of a now standard XCTD, or less—perhaps approaching an order of magnitude less. One such standard unit, essentially handmade, costs $500 each in dozen lots; but expendable bathythermographs (XBTs) can cost as little as $50 per sensor. This order-of-magnitude cost difference between XBTs and XCTDs represents a major barrier to widespread use of the more desirable XCTDs.
Over the past two decades, significant progress has been made in the measurement of temperature and conductivity in the ocean. Preferred embodiments of the present invention are improvements over technology developed by the present inventors originally for high-frequency conductivity turbulence measurements.
The development of that earlier technology sensor started in the early 1980s when the present inventors were at The Johns Hopkins University Applied Physics Laboratory and introduced a four-electrode sensor—described in Farruggia, G. J., Fraser, A. B., “A Miniature Towed Conductivity Apparatus,”
Proceedings Of Oceans,
Sep. 10-12, 1984. That basic configuration underwent ongoing refinement during ensuing years, though none of those changes included the present invention.
That reusable sensor was developed to be small, robust, nonfouling, high-bandwidth conductivity measurement device, and inexpensive—though associated e
Farruggia Guy J.
Fraser Allan B.
Areté Associates
Hamdan Wasseem H.
Le N.
Lippman Peter I.
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