Light flux drives photosynthesis, so its measurement in water gives important information about the depth of the euphotic zone and the potential distribution and importance of autotrophic production. The secchi disk yields a measure of water clarity termed "secchi depth", but the quantum sensor is more versatile and gives accurate measurements under any conditions.

A quantum sensor measures the moles of photons of photosynthetically active radiation (PAR) arriving at the sensor. There are two sensor types: flat and spherical. The flat sensor measures downwelling light only and the spherical sensor measures light as a phytoplankton cell would see it. Either sensor type can be connected to the LI-1000 DataLogger via an underwater cable for field measurements.

The LI-1000 is a remarkable device. It can take readings using two quantum sensors simultaneously, and can be hard-wired through the 37 pin interface port to accept input from up to six additional sensors of user choice. The LI-1000 has 32 K of memory that can store readings from 8 channels for sampling intervals and durations chosen by the user. Data can then be downloaded from the LI-1000 via the 25 pin serial port to a computer using ProComm or other communications software. Data can then be plotted, analyzed, etc.

The LI-1000 is a rugged machine, but there are some things to watch. It is water resistant, but NOT waterproof. If you drop it in the water, it will die. The sensors and cable are waterproof. The logger must also be kept out of direct sunlight or it will overheat. Finally, it is powered by 6 alkaline D cells, so battery status must be considered, although battery life is very long with this instrument for most uses.

The LI-1000 can be set up to take readings in two alternate modes: instantaneous and log. In instantaneous mode readings can be taken from the display screen or stored in the logger's memory with a keystroke. This is excellent for taking a few, quick measurements in different places. Log mode allows light flux to be measured and averaged or integrated over preset periods of time at preset intervals. Log mode requires more programming, but is much more useful for studies of daily light flux, etc.

Instantaneous Mode

Connect a sensor to an underwater cable by pushing the two-pronged connector of the sensor itself into the cable, aligning the marks on the cable and sensor. If the marks are opposite you will get negative (but accurate readings). The cable can then be connected to the logger at one of the bayonet mount terminals on the top of the logger. The terminal to the left is for "Channel 1" and the one on the right is for "Channel 2".

Turn on the LI-1000 by pushing the "FCT ON" button firmly. Before you take readings you MUST configure the channels in which you intend to take readings. Once started you must complete the configuration procedure before doing anything else; the logger will not let you do otherwise.

You do configurations by channel, starting with Channel 1. Push the CFG button and the display will read "MODE IS INST", meaning that you have selected instantaneous mode. Press ENTER to accept (or up or down arrow to toggle to LOG). You will then see a message saying "ch1 is OFF" (or LIGHT or GEN or THERM, using the up or down arrows to toggle among them); toggle to LIGHT and press ENTER. The next display says "range = A". This means the machine's sensitivity range will be automatically adjusted for different light intensities. Press ENTER to accept, and don't mess with other choices. The next display is "mult = xxx.x" where the number is a calibration coefficient. Each sensor has a small plastic card listing its factory-calibrated coefficient. This number differs for measurements of light flux in air or water. Choose the proper one and enter the number, including the minus sign first, from the keypad, then press ENTER. The next display is "label=XX" where XX is simply 2 characters chosen to denote the particular channel or tye of measurement you are doing. This does not matter much, but the 2 characters you choose will be displayed in your data readouts. Note that letters must be selected by using the up or down arrow then mashing the appropriate button, e.g., up arrow-9 gets you "Q". Press ENTER to accept whichever characters you choose. The next display is "ave=X" where X is some number. You may enter any number; this denotes the period in seconds over which the logger will take a moving average of light flux. This feature allows the LI-1000 to average short scale fluctuations in light intensity, such as from moving clouds, but requires that you wait to get an accurate reading. I usually use 10. Press ENTER to accept the number you choose. This completes your configuration of channel 1. You may repeat this process for channel 2 if you are using 2 sensors (and likewise for additional sensors hard-wired in on the pin connectors), or simply toggle each channel OFF and press ENTER until you have gone past channels 8 and M (the latter used for math functions). The next display will be 1X and a decimal number, which gives readings in microeinsteins (=micromoles of photons) m-2 sec-1. You have now completed configuration and can use the quantum sensor(s) and logger as you wish. It can be turned off by pushing OFF; it also will turn itself off after a preset time with no key activity to save battery life.

Measurements of light flux can now be taken by reading the display after a suitable time has elapsed (bearing in mind the averaging time) and/or by hitting ENTER whenever you want to store a reading in memory. The machine will store the reading for each channel along with a time/date stamp. The only trick is that you have to remember where you were when each reading was taken.

Use of the memory to store data requires that time and date be set before you begin (also important for use of log mode). To do this, push TIME. The display will read a preset date. If it is incorrect, simply enter two numbers for year (YY), two for month (MM) and two for day (DD), then press ENTER. If you err, push the back arrow to delete the wrong character and enter the right one. Push ENTER to accept the date. The next display will give military time in hours and minutes. If this is not right, enter the correct numbers as above, then push ENTER, which will return you to the main display.

Log Mode

Use of log mode requires you to know a few more things before you begin and also depends on setting your start time correctly. Failure to do this correctly will yield a 32K memory with NO data in it.

First, turn the logger on as above. Sensors need not be connected until you are ready to deploy, but you will need to know their calibration numbers if you have not already programmed them in during an earlier configuration.

Next, check to see if the memory is empty. Use your head if it is not! Check memory status by mashing the FCT ON key again after turning the logger on, then use the down arrow to toggle to "FCT:Mem Status". Now press ENTER and the display will say "MEMORY STATUS" and then "32K XX% unused". If XX is less than 99%, you will then clear it IF THAT IS OKAY TO DO. Consider other users! You can download data from another user or use of the machine as instructed below. Then, to clear, first return to "FCT:Mem Status" by pushing ENTER, then push the down arrow to toggle to "FCT:Clear RAM" and push ENTER. Toggle "NO" to "YES" with the down arrow and push ENTER. Memory is now clear, all data gone forever. You may use the machine with partly used memory, but when you dump your data you will get the other data, too. If you do a long set of readings or use a lot of memory or a lot of memory is already used, you run the risk of overloading and bumping the oldest data from the memory. Thought must be given!

Next, you MUST configure the machine to log as you wish. Begin by mashing CFG, toggling to "LOG" mode, and pressing ENTER. Then configure each channel you intend to use by toggling with the arrow key to your choice and pressing ENTER when you have what you want. You must complete gthe configuration (run through all the channels) before you can do anything else. If you goof, you must redo the configuration from start to finish, but you can mash CHAN to toggle from one channel to the next until you reach the one you want to correct.

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