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What is the Basic Operating Level on the 10AU for the Ammonium application?
The basic operating level for the 10AU must be set any time that you change applications.  If your other application is chlorophyll, each time you switch between chlorophyll and ammonium the basic operating level must be set again.  To set the basic operating level for ammonium, first you must obtain a standard sample of ammonium.  Using either protocol A or protocol B of the Holmes method, "A simple and precise method for measuring ammonium in marine and freshwater ecosystems" (pg. 1803), choose a standard and decide where, within the total range of the instrument, this standard represents.  For example, if you create a 5µmol/L standard and know that this represents an average concentration for the environment you are working in you will choose to set the sensitivity so that the 5µmol/L standard results in a 50% signal in the medium concentration range.  If the 5µmol/L standard represents a high concentration compared to what you expect to find in the field you may decide to set the sensitivity so the 5µmol/L standard results in a 50% signal in the high concentration range.
1.     Turn on the 10AU and allow it to warm up for at least 10 minutes.
2.     From the Home screen press ENTER.  This will take you to the main menu.
3.     Choose 2 Calibration.  This will take you to screen 2.0 (screen number is listed in the top right hand corner of the display).
4.     Choose 4 Set Concentration Range.  You should be at screen 2.4. Choose the appropriate concentration range (Low, Medium or High). Press ESC
5.     Choose 3 to get to screen 2.3 and set range control to manual.
6.     Hit the ESC button twice to go back to screen 2.0 then choose 6 to go to screen 2.6.  Follow the instructions and reset the calibration to default.
7.     Hit ESC twice to go back to the main menu, choose 3, then hit the ENT button. You should be at screen 3.2.
8.     Loosen the sensitivity lock on the front panel of the instrument by using the allen wrench that is stored next to the sensitivity adjustment knob, near the power button.
9.     Insert your standard and replace sample cap.
10.     Using the sensitivity knob slowly turn the knob to adjust the %FS to the appropriate value for your standards concentration.  A deviation of ± 5% is acceptable.  Turn the sensitivity adjustment knob a little at a time and allow the FS% to settle before making another adjustment.
11.     When the %FS reaches the designated value, tighten the sensitivity locking screw with the wrench.  Go back to screen 2.4 and set the range control to Auto.