What is Barometric Pressure?
Barometric pressure is simply atmospheric pressure or the weight of a column of air at any point on the earth’s surface and is measured by an instrument known as a barometer.
Accurate prediction of the weather is made possible by determining the size and motion of wide spread continental air masses and is accomplished by the simultaneous measurement of barometric pressure at a number of widely spaced observation points. When these barometric pressure observations are plotted on a map and lines of equal pressure (called isobars) are drawn, areas of high and low pressure appear. These pressure systems are closely associated with the weather experienced on the ground.
High pressure results when an air mass cools and sinks and is generally associated with fair weather.
Also, high pressure can result when the changes in speed and direction of upper air winds cause the air to flow together, referred to as convergence. Convergence causes the air to descend and results in the formation of high pressure at the surface. A rising barometer reading indicates the approach of high pressure and improving weather conditions.
Low pressure results when a warm air mass rises and often results in the production of clouds and precipitation. Also, low pressure can result when the changes in speed and direction of upper air winds cause the air to flow apart, referred to as divergence. Divergence encourages the air to rise from the surface and results in the formation of low pressure. A falling barometer reading indicates the approach of low pressure and generally unsettled weather.
The standard atmosphere (symbol:atm) is a unit of pressure equal to 101325 Pa (“Pascal”) or 1013.25 millibars or hectopascals. It is equivalent to 760 mmHg (torr), 29.92 inHg and 14.696 psi.
In general, a barometer reading above 30.20 inHg would be considered high and a reading less than 29.70 inHg would be considered low.
Even though high and low pressure systems are a wide spread phenomena, a barometer reading at a single location, along with the direction of change in pressure and the current wind direction, the general weather conditions for the next 24 hours can be made with a reasonable amount of accuracy.
Digital Barometer with Optional Housing
Digital Barometer PCB
Barometer Operational Notes:
- A self-test routine which lights all indicators and display segments for approximately four seconds is performed during every power-up.
- For best performance, the digital barometer should be left on continuously, much like a digital clock. Therefore, there is no power switch. Power consumption is 175mA at 9VDC.
- Anytime the digital barometer is powered up, the Rise / Fall / Steady Indicator will remain off until the first update cycle is completed, in approximately 1 hour. The “update” LED will be flashing during this time.
- The barometer should be located indoors in a dry, protected area free of excessive dust or moisture.
Digital Barometer Circuit Board Indicators and Controls
- High / Low pressure pushbutton switch: Press and release switch to display stored high and low pressure readings. Switch will also be located on the top of the (optional) housing.
- Storm LED: A red LED that will flash if the barometric pressure drops 0.10 inHg or more in three hours. Normally off.
- System LED: Monitors system power supply. Normally on.
- Power Input: Connector for wall power transformer (9 VDC output, included with barometer). Power input also located on back of (optional) housing.
- Update LED: Indicates normal operation and that the pressure readings are being updated. Normally flashes approximately once a second.
- Rise / Fall / Steady Indicator: Used to indicate if barometric pressure is rising, falling or steady. Normally flashing. Updates once an hour.
- Four Digit LED Pressure Display: Indicates current barometric pressure in inches of mercury (inHg). Pressure readings are updated once a second.
- Zero Adjustment Potentiometer: Used to calibrate low pressure reading.
- Gain Adjustment Potentiometer: Used to calibrate high pressure reading.
Digital Barometer Installation
1.) Insert the female power connector from the wall power supply into the male connector located at the upper right corner of the digital barometer circuit board. If the barometer has the optional housing, the male power connector will be located on the lower back side of the housing. Make sure connector is firmly seated.
2.) Connect the wall transformer to a standard 120 VAC outlet. The use of an extension cord should be avoided.
3.) A self-test routine which lights all indicators and display segments for approximately four seconds is performed on power-up. Verify that all display segments, decimal points and indicator LED’s are lit during this time.
4.) After four seconds, the digital barometer will automatically exit the self test routine and briefly display the high and low pressure readings (which should be similar in value) and then begin to show the current barometric pressure.
5.) The system LED should remain on and the update LED will be flashing. The Rise / Fall / Steady Indicator will remain off for approximately one hour after power-up and then begin to indicate pressure changes.
6.) The digital barometer should be allowed to run for at least two hours before any calibration adjustments are performed.
Digital Barometer Operation
1.) The digital barometer uses a precision integrated silicon sensor and displays barometric pressure in inches of mercury (in/Hg). The five digit red LED display has a range of 28.00 to 32.00 in/Hg and has a resolution of 0.01 in/Hg. The displayed barometric pressure readings are updated approximately once a second (in cadence with the “UPDATE” LED).
2.) With careful calibration, pressure accuracy is typically +/- 0.01 in/Hg as referenced to readings provided by the local National Weather Service office. Initial calibration is required to compensate for variations in altitude at users geographic location (for consistency, barometric pressure readings are corrected for altitude). Calibration requires nothing more than a small jeweler’s type screwdriver. Once calibrated, no further action is required.
3.) The rightmost LED display is used as a pressure change indicator. When the barometric pressure is rising, the top segment will flash. If the pressure is falling, the bottom segment will flash. Both top and bottom segments flash when the pressure is steady. The pressure change indicator is updated once an hour. Sensitivity is approximately +/- 0.015 in/Hg.
4.) In addition to displaying current barometric pressure, the highest and lowest barometric readings are tracked and stored in RAM memory and can be displayed at any time via pushbutton. When the HIGH / LOW Pressure switch is pressed and released, the rightmost digit will indicate “H” when displaying the highest stored pressure value and “L” when displaying the lowest stored pressure value. The display then automatically reverts back to displaying the current barometric pressure. Since the high and low readings are stored in volatile memory, they will be reset if the barometer is powered down.
5.) There is also a red “STORM” LED that will flash if the barometric pressure drops 0.10 in/Hg or more in three hours. It will continue to flash until the next storm check update cycle – approximately three hours.
Digital Barometer Calibration
1.) Calibrating the digital barometer requires a small jeweler’s type screwdriver. The barometer should have been operating for at least two hours. Also, calibration should be performed when the barometric pressure is steady.
2.) Next, a known accurate barometric pressure reading is required as a reference standard. The best source for this pressure reading is the local National Weather Service office. This can be obtained online at http://www.weather.gov/ and then entering your location in the “Local Forecast by City” box at the top of the web page or from a NOAA weather radio that broadcasts current weather conditions for your location. Both of these sources are updated at the top of every hour. Another but somewhat less desirable source is a weather report from a local TV or radio station.
3.) Once a known accurate reading is obtained, locate the ZERO calibration potentiometer at the bottom left corner of the barometer circuit board (see artwork drawing above). If the barometer has the optional housing, there will be an access hole on the lower left side of the housing.
4.) Using a small screwdriver, slowly adjust the ZERO potentiometer until the barometer display matches the reference reading. Note: Turning the potentiometer clockwise will increase the reading and turning counter-clockwise will decrease the reading.
5.) Once calibration is performed, no further adjustment is necessary, unless the barometer is moved to a location that is at a different altitude.
NOTE: Zero calibration range (altitude compensation) = +/- 2.00 inHg (+/- 2000 ft)
CODE FOR “BIT BANGED” SPI MODULE
//"BIT-BANGED" SPI MODULE FOR MC9S08QD4
//FUNCTION TO CLEAR AND CONFIGURE DISPLAY
void spi_config (unsigned char config_value)
SPI_STROBE = 0;
for (config_data = 8; config_data; config_data--)
if (config_value & bit_7) SPI_DATA = 1;
else SPI_DATA = 0;
SPI_CLOCK = 1;
config_value = config_value << 1;
SPI_CLOCK = 0;
SPI_STROBE = 1;
//"BIT-BANGED" SPI MODULE FOR MC9S08QD4
// FUNCTION TO SEND DIGIT DATA TO DISPLAY
void spi_write (unsigned char data)
for (spi_data = 8; spi_data; spi_data--)
if (data & bit_7) SPI_DATA = 1;
else SPI_DATA = 0;
SPI_CLOCK = 1;
data = data << 1;
SPI_CLOCK = 0;
“BIT BANGED” SPI MODULE WAVEFORMS
Digital Barometer Prototype 1
This is the first prototype that used a PIC16F886 microcontroller and was programmed in assembly language. It had a EIA232 serial output that allowed logging the barometric pressure to a PC using HyperTerminal. The dip switches were used to set the log interval from every five minutes to every two hours.
Digital Barometer Prototype 2
This is the second prototype that used a FREESCALE MC9S08QG8 microcontroller.
I switched to a FREESCALE micro so I could program in C/C++ since I did not have a C complier for the PIC16F886 and was not about to pay for one.
The prototype also had an EIA232 serial output that allowed logging barometric pressure values to a PC using Hyperterminal. The log interval was selectable from every 5 minutes to every 2 hours. A Bicolor LED(s) to the right of the display was used to indicate pressure changes. RED/Falling, GREEN/Rising, YELLOW/Steady.