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Please e-mail LDS questions to mhead@pme.com.
Q. What does LDS measure?
LDS measures the water column temperature profile, meteorological parameters such as solar radiation and wind, and chemical parameters (DO, conductivity). The information produced by LDS helps to define the characteristics of a lake or reservoir. When coupled with CWR, these data can be used as a predictive tool for water quality management. |
Q. Why is it important to know the characteristics of a lake/reservoir?
It’s important to understand the temperature and density of the lake because it changes throughout the year (seasonal stratification). Some lakes/reservoirs become highly stratified during warmer months due to little wind and high solar radiation. This can cause a warm (less dense) layer of water to be at the surface while colder (more dense) water sinks to the bottom. The lake may be mixing completely during colder months of the year because the wind is stronger. Water density is uniform during this time. When these bodies of water are highly stratified, dissolved oxygen may begin to decrease, which can disrupt the balanced ecosystem. For more information, please visit http://faculty.gvsu.edu/videticp/stratification.htm (Grand Valley State University, Department of Geology). |
Q. Who would benefit from purchasing LDS?
Anyone who is interested in monitoring the water thermal regime. The LDS provides detailed data that is used to design and control de-stratification systems, and to hot-start numerical models for disaster predictions.
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Technical Questions
Q. The LDS temperature chain has 16 bit output that ranges from 0 at about 38 degrees C to 65535 at about 1 degree C. Can the chain be used outside this temperature range?
The lowest calibration point that we take is below 1 deg C, usually about 0.5 deg C. So the calibrated range ends here.
True, 2**16-1 = 65535 output occurs at roughly 1 degree C for all nodes. However, the 2**16-1 upper limit of 16 bit resolution is not the actual upper limit! The a/d converter will over-range 12.5% above 2**16. Due to the 16 bit transport used within the chain, the first over-range count is 0 and proceeds to 8191. Circuitry inside each node will continue to measure temperature over this range also. Each node has sensitivity that extends into the -4 deg C range and is expressed by numbers in the range 0 to 8191. Trouble is that the range 0 to 8191 also expresses temperatures in the upper 30 deg C range, so the actual temperature in this output range is ambiguous. However, I expect that you can determine if the lake is above 30 deg C or below 1 deg C from other sources, and teach your calibration software to add 2**16 when the lake is cold.
Our software automatically adds 2**16 to any number in the 0 to 8191 range if the reference temperature (from our electronic thermometer) is less than 5 deg C. You may notice numbers larger than 2**16 in the calibration files that PME sends. These have had 2**16 added to them. If you use the calibration plotting program PME provides you can also see points at values larger than 2**16. These represent real temperatures and are completely reproducible.
Note that warm temperature outputs below Nad of a few hundred (and therefore above about 38 deg C) aren't useful. There is no way to extend the warm temperature response above the calibrated range. Both the A/D and electronic circuits simply stop responding at this point. |
Q. The LDS Temperature Chain has a 16 bit output. Can more bits of resolution be provided?
Yes. At 16 bit resolution the chain can resolve 1/2 mdeg C (500 udeg C). However the noise within the sensor is much less, approximately 150 udeg C RMS. Thus additional bits of resolution can actually convey additional information. Once a chain is built its resolution is fixed. The normal resolution is 16 bits, but as many as 20 bits are available. This requires a simple circuit change prior to constructing the chain. Software changes are also needed.
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