Barometer Calibration
1-Wire Indoor Sensor/Interface Kit

Download Calibration Software

There are two versions of the calibration program. One for Linux and the other for Windows. Click on the appropriate link for your operating system below:

Linux:     baromcal
Windows:     baromcal.exe

Building The Calibration Fixture

Materials Required - locate the following materials that came with the kit except as noted below.

Qty
Description
10'
1/4” O.D., 0.170” I.D. Vinyl tubing
3
fuse clips
3
#6 machine screws (not supplied)
5'
1” x 1/4” flat latice (not supplied)

Assembly

Ok
Procedure
  With a square, draw a perpendicular line across the width of the lattice approximately 3" from one end.
  Mount two of the fuse clips, equally spaced, on this line. The screws should be attached on the line. The open ends of the clips should align with length of the latice. See the pictorial for details.
  Measure exactly 40" from the perpendicular line drawn above and draw a second perpendicular line on the latice.
  Mount the third fuse clip centered on this line. As with the upper clips, the open ends should be alinged with the length of the lattice.
  If this has been done correctly, you should measure exactly 40" from the top of the top clips to the top of the bottom clip.
  You may wish to drill a suitably sized hole at the top of the fixture so it can be hung during the calibration procedure.

Complete fixture (annotated):

The Procedure Explained

The first recorded use of the mercury barometer was in 1642 by a gentleman named Torricelli. With this type of barometer, the height of mercury in a glass column closed at the top is connected to a small vessel of mercury that is open to the atmosphere at the bottom. As the atmospheric pressure increases, it pushes down on the mercury in the vessel, forcing it higher in the column. When atmospheric pressure drops, less pressure is exerted on the mercury and the height of the column falls. The measurement has traditionally been made in inches. The calibration method used with this kit is based on this principal, although mercury is not used.

A fact that is little known is that the same principal can be applied to water. Water, however, is considerably lighter than mercury. Therefore the height of the column to apply the same amount of pressure against the atmosphere must also be greater. Knowing this, and the conversion factors between mercury and water densities, we can achieve calibration by varying the height of a column of water and noting the difference in pressure.

The software calibration program will guide you through the steps to get the correct gain and offset for your particular installation. After adjusting an initial offset, the program will prompt you to change the height of one side of the calibration tubes and to adjust the gain potentiometer on the interface board as required. Once the gain has been set, you will then make a final adjustment of the offset which is used to compensate for altitude. The latter adjustment will require access to the current barometric pressure from the nearest NWS office or airport.

Preparing For Calibration

Ok
Procedure
  Place the mounted interface board at a location where you can easily adjust the gain and calibration potentiometers and is at least 6” higher than the two top clips in the calibration fixture. While all steps are important this one is especially so. Placing the board higher than the calibration fixture will prevent water from running back into the sensor if something goes amiss.
  Using whatever means that works for you, fill the tubing with water. Note that bubbles will form in the tube. Tap the tubing to remove the bubbles. This will take some time and patience. When you are finished with this step, water should be even with the tops of both tubes and there should be no bubbles in the water. The last point is very important.
  Clip one end of the tubing in one of top clip on the fixture so that approximately 2 feet of tubing extends from the top. Form a loop that extends about 2/3 the distance between the top and bottom clips of the fixture. Push the tubing in the remaining top clip to hold it in place. You should have a fairly long length extending from this clip to the end of the tubing. See pictorial for details.
  With the water level even with the two top clips in the calibration fixture, gently push the long free end onto the pressure transducer.
  Re-adjust the tubing so that the water level is even with the tops of both clips. This will probably require going back and forth a few times to get the levels where they should be. Take your time and be sure alignment is correct.

Calibration

During the calibration, you will be instructed to move the tubing in the clip farthest from the pressure transducer to the lower clip. This picture shows the tubing correctly placed for this step.

Ok
Procedure
  Connect a DB9 cable from your COM port to the sensor/interface board.
  Connect power to the sensor/interface board and allow it to run for 30 minutes before proceeding. This is necessary to for all the components to stabilize.
  At the command prompt, run the calibrate utility.
For Linux, the command is
./calibrate /dev/ttySn
where n is the serial port you are using. Note the leading dot before the forward slash!
 
For Windows, enter
calibrate COMn
where n is the COM port you are using.

From this point on, the calibration program will guide you through the steps to calibrate your barometer. The procedure may take up to about 30 minutes, so take your time and be patient. You will be rewarded with an accurate and stable barometer that should serve you well.

Important! If the elevation of the installed board is changed more than a few feet, the Offset setting will need to be adjusted to compensate. Just be careful not to touch the gain setting.

More Information

If you are curious about the math involved with the calibration and use of the barometer, see the Programer's Guide.


Congratulations! You have completed the assembly and initial testing of your 1-Wire® Indoor Sensor and Interface kit. Proceed to the installation.


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