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III. Calibration

A. Temperature
The temperature sensor is typically accurate to within 2șC before calibration. To improve accuracy, you will need an extension cable between the circuit board and the sensor as well as an ice-water slurry.

Place the sensor in the slurry and measure its output voltage using a voltmeter. Use the measured value in the following correction factor:

K=2.73/measured_value

Multiply your raw A/D readings by this correction factor to improve the precision of your temperature measurements.

B. Pressure
Your pressure sensor will require calibration and there are two possible methods to do it. The first method requires that you monitor your local weather channel for a couple of weeks while the second method necessitates some specialized equipment.

If you choose the first method, create a table and label the first column as Pressure Sensor and the second column as actual. Next, for about two weeks, monitor and record your local weather channel for the actual pressure. At the same time, measure and record the voltage across the pressure sensor. You will need to connect the Emitter and Collector of Q1 together so that power is routed to the sensor.

Enter your data into Excel and use a linear interpolation function to derive the equation:

Actual=K*measured+offset.

Use the following steps for the second method:

1. Acquire some flexible tubing, a vacuum pump (available at Pep Boys), and a manometer or vacuum gauge.

2. Again, short the emitter and collector of Q1 to provide power to the pressure sensor.
3. Record the initial voltage across the sensor as V0
4. Get the local barometric pressure (in inches of mercury – typically around 30) and record as P0
5. Connect the vacuum gauge and vacuum pump to the sensor using the following diagram:
Vacuum Gauge for Wireless Weather Station

6. Squeeze the pump until the gauge indicates a vacuum of 1” of mercury.
7. Record the voltage across the sensor as V1.
8. Perform the following calculations:

Calculation 1

Calculation 2

Using the above terms, barometric pressure can be calculated from the sensor voltage as:

Pressure = m * Voltage + b

C. Humidity
I found the humidity sensor to be pretty accurate and to not require calibration. However, be sure to follow the construction tips in this article to insure good performance.

IV. Weather Monitor Application Program

To take full advantage of the Remote Weather Monitor, I wrote a simple program in Visual Basic to collect the data from the base station, perform the calibration corrections and data conversions, and display it on the screen. Buttons are provided to display the basic data using a small window or to plot the data for the last 24 hours. See Figure 8 for a screen shot of the monitor window.


V. Closing

An engineer’s work is never complete – there’s always room for improvement. A couple of things could be done to further embellish this design. First and foremost, the protocol between the remote and base stations could be modified to support multiple remotes. This would allow monitoring the climate, inside, outside, in the attic, etc.

The second improvement would be the addition of an anemometer. If time had permitted, I would have designed one based on ultrasonic transducers. This would have allowed measurements of wind speed and direction and, when combined with temperature, wind chill.

But, I’ll get to those projects later. According to my data, it’s too nice of a day to be inside.

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