Update Running the PAS

Running-the-PAS.md

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+# Background
+
+The PAS requires a 12 VDC supply and a USB connection to a host computer to run.
+
+The PAS data acquisition software is written in Julia.  A full waveform is acquired over the designated sampling time and an FFT is performed by the host computer in Julia.  The data are plotted to the screen for real-time viewing.  Optionally, the magnitudes for each wavelength can be saved to a text file.
+
+A separate script performs a frequency sweep and can be run prior to the start of data acquisition to determine the resonant frequency.
+
+# Turn on the PAS
+
+1. Plug the USB cable into the host computer if needed.
+1. Turn on or plug in the 12V power supply (you may also need to press the soft on/off button if using the Rigol bench supply).
+2. Ensure the microphone preamp is receiving +/-3.3 VDC using a multimeter.  If not, adjust the trimpot on the voltage regulator until the supply reads +/- 3.3 VDC.  **Do not exceed 3.6 VDC or you will destroy the microphone and the preamp board will have to be replaced!**
+
+# Open the Real-time Plots and/or Save Data
+
+1. Inspect that the `pas-daq.jl` file shows the correct resonant frequency and that the save option is set to the desired value (true or false).  This can be done with any text editor, or by issuing `nano ~/pas-daq.jl` in the Terminal.
+
+     ```julia
+     f_res = 1200 # center frequency in Hz
+     save = true    # can be true or false
+     savepath = "~/pas-data/"
+     ```
+
+2. Run the `pas-daq.jl` data acquisition script.  Open Terminal on the host computer and type the following command:
+
+     ```julia
+     julia ~/pas-daq.jl
+    ```
+
+# Perform a Frequency Sweep
+
+1. Fill the cell with a constant concentration of absorbing gas and let the signal stabilize.
+2. Set the start (`f1`) and stop (`f2`) frequencies and ensure the `pas-sweep.jl` script is set to save the data file.  This can be done with any text editor, or by issuing `nano ~/pas-sweep.jl` in the Terminal.  Note that each frequency takes approximately 2 seconds.
+
+     ```julia
+     f1 = 1190
+     f2 = 1200
+    save = true
+    ```
+
+3. Run the `pas-sweep.jl` script.  Open Terminal on the host computer and type the following command:
+
+     ```julia
+     julia ~/pas-sweep.jl
+    ```
+
+*Currently the data must be fit manually after acquisition.  Typically looking by eye in a spreadsheet program is close enough for the 3-λ system.*

WCU-PAS-Wiki.md

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-Documentation for the WCU low-cost, 3-λ photoacoustic spectrometer.