FAQ 1 Barometer Readings | FAQ 2 Barometric Trend | FAQ 3 Barometer Changes and the Weather |
FAQ 4 Altimeter and Thermometer Accuracy | FAQ5 Long-term Adjustment



Vertech FAQ 1: Barometer Readings


Why doesn't the barometer move?
There are two kinds of barometers; FIXED LOCATION and MOVING LOCATION. Fixed location barometers can only be used at a fixed elevation, while moving barometers are designed to be used at varying elevations. The Vertech, like aircraft altimeters, contains a moving location barometer. A moving location barometer shows a barometer reading and an altitude reading. The altitude reading will change whenever there is a change in elevation OR a change in the weather, but the barometer reading does not move until the user makes an adjustment.

Pilots adjust their altimeters by physically changing their barometer readings according to weather reports they receive from the ground. This keeps their altimeters accurate as they fly through zones where weather conditions vary. Vertech users can also adjust their altitude reading by changing the barometer based on local weather reports, but most will adjust their barometer reading by adjusting the altitude to a known elevations. Adjusting the altitude to a known elevation will change the barometer to its current reading. In contrast to aircraft altimeters and the Vertech, wall-mounted barometers are designed to be used at a FIXED ELEVATION, and DO move with every change of pressure. When someone buys one of these fixed location barometers, they need to adjust it to the correct sea level barometer when they get home from the store. After the initial adjustment, the barometer reading will go up and down as the weather changes, and will show roughly the correct sea level barometer reading.

If the Vertech barometer moved with pressure changes, it would be useless. The barometer would move up and down with weather changes AND with elevation changes, and it would not be possible to obtain the actual sea level barometric pressure at different elevations.

How can I get the current sea level barometer reading?
To obtain the current sea level barometer reading, you must adjust the altitude in the Barometer Adjust function to a known elevation. Performing this adjustment will give you the new sea level barometer. The barometer reading will not change until the next adjustment is made. When at a known elevation, go to the Weather Station function and press Adjust to reach the Barometer Adjust function. The barometer and altitude readings will blink, indicating that they can now be changed with the Option button. If the altitude reading already matches the known elevation, this means that the sea level barometer has not changed since the last adjustment. If it does not match, the sea level barometer has changed, and adjusting the altitude reading will give you the new sea level barometer.

Skiers can track the barometer during the day if they know the elevation at a certain place on the mountain, and this will usually be at the base lodge. At the beginning of the day, they can adjust the altitude (in the Barometer Adjust function) to obtain the sea level barometer reading. Later in the day, they can again check and adjust the altitude at the lodge to get the current sea level barometer. A skier can also track weather changes with the Barometric Trend function described in the next section which does not require that they know the elevation at the lodge.

A backpacker also gets the sea level barometer by adjusting the altitude at known elevations. During a day's hike, a backpacker updates the barometer reading by adjusting the altitude (in the Barometer Adjust function) at a known elevation. Backpackers with topographic maps can locate their position on the map. Backpackers without topo maps will be able to update their sea level barometer reading when they pass landmarks of known elevations like lakes or mountain passes that are marked on general maps.

People who are interested in tracking the sea level barometer from day to day must start by finding the correct elevations for their home and work. Now, they can always find the sea level barometer whenever they are at home or work, simply by adjusting the altitude reading in the Barometer Adjust function to match their known elevation.

The altimeter in the Altitude function doesn't match the altitude reading in the Barometer Adjust function. Why are the readings different?
The Altimeter function and Barometer function work independently. Adjusting the altitude reading in the Altitude function does not change the altitude or barometer in the Barometer Adjust function.

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Vertech FAQ 2: Barometric Trend

How does the Barometer Trend function work?
The Trend function shows movement of the sea level barometer (weather changes) since the last time it was reset. TREND READINGS ARE ONLY VALID WHEN THE USER IS AT THE SAME ELEVATION WHERE THEY LAST RESET THE TREND. A (+) symbol to the left of the Trend indicates a rising barometer, and a (-) indicates a falling barometer.

It is not necessary to know your elevation or sea level barometer to use the Trend function. Pressing the Start/Stop button resets the Trend to 0.00. If you reset the Trend at the lodge of a ski area, you can see how the barometer has changed during the day, but only when you are back at the lodge. If you look at the Trend while you are in the slopes, it will be meaningless since it will change with elevation.

If you are backpacking, you might reset the Trend when you arrive at your campsite in the afternoon. The next morning, you can see how the barometer has changed by looking at the Trend. Once you start hiking the next morning, the Trend is no longer meaningful because it will change with elevation. If you reset the Trend at your home in the morning, you can check it later in the day, at your home, to see how pressure has changed. Once you leave your home, the Trend is no longer relevant (until you return home).

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Vertech FAQ 3: Barometer Changes and the Weather

Why doesn't the barometer give actual pressure?
Barometric pressure is defined as atmospheric pressure at sea level. As you climb above sea level, the ACTUAL pressure gets lower, but the actual pressure is converted to what it would be at sea level. Actual pressure is always converted to its sea level equivalent so that weather reports are universal and consistent. If weather stations reported actual pressure, stations at high elevations would report much lower pressure than those at low elevations. Two stations only a few miles apart would give completely different reports if they were at different elevations. In order to use pressure for weather reporting and forecasting, each station must report pressure AS IF they were all at the same elevation. Imagine every weather station measuring the actual pressure. Then imagine if each stations bored a hole straight down to sea level, lowered their pressure sensing device to the bottom of the hole, and obtained readings. This is the sea level barometer.

How is barometric pressure measured?
Barometric pressure is measured in INCHES OF MERCURY Early barometers were filled with liquid Mercury that rose and fell with changes in pressure. Europeans and some marine and military weather stations use the MILLIBAR scale for measuring pressure. When the Vertech in set to measure meters instead of feet, sea level barometric pressure is converted to millibars.

29.92 inches of Mercury (I.O.M.) is the so-called "standard" sea level barometer. It is basically the average sea level barometric pressure. During the course of a year, the highest pressure your are likely to see id around 30.40, and the lowest around 29.50. This means if you never adjusted your altimeter during an entire year, you could see a total range of 900 feet. On a daily basis, altimeter will stay within about 100 feet of their initial setting in the absence of large pressure changes, but can change by as much as several hundred feet if the weather changes dramatically. Remember that pressure can sometimes change significantly without any visible change in the weather.

Low pressure (below 29.92) is associated with less stable weather patterns, and high pressure (above 29.92) is associated with more stable weather patterns. To forecast weather, it is important to know the VALUE of the sea level barometer, and also the SPEED and DIRECTION of barometric change. The pressure may be very high, say 30.30, but if the barometer is dropping rapidly, less stable weather would be predicted. Analysis of barometric is only one aspect of weather forecasting, but it is an important one. People who frequently monitor the barometer will begin to get a feel for how changing weather can be predicted based on pressure changes.

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Vertech FAQ 4: Altimeter and Thermometer Accuracy

How accurate is the Vertech Altimeter function? How does this compare to other altimeters?
The accuracy of the Vertech altimeter is the same as the accuracy of aircraft altimeters. There are two components of altimeter accuracy: FIRST, the accuracy of any altimeter depends on the USER. To get the best accuracy, an altimeter must be ADJUSTED at a known elevation regularly. The more frequently adjustments are made, the more reliable the altitude information will be. A Vertech that has been recently adjusted can be taken to different elevations and will usually be correct to within about 30 feet. A Vertech (or any altimeter) that has NOT been adjusted for more than a day may be in error by up to several hundred feet, depending on weather changes since the last adjustment.

SECOND, accuracy depends on how well an altimeter is temperature compensated. Changing temperatures do not affect pressure, but they do cause slight expansion and contraction of the parts of the altimeter that measure pressure. Each custom Vertech chip is individually "tuned" in a temperature chamber at the Avocet manufacturing facility to minimize the effect of temperature changes on altimeter/barometer readings. As a result, very large temperature changes can affect altimeter/barometer readings by as much as 70 feet/.07 I.O.M. This level of temperature compensation makes the Vertech comparable to altimeters that are much more expensive. Some uncompensated altimeters can show errors of 200 to 400 feet caused by temperature changes.


How accurate are topo maps?
The most detailed topo maps (1:24,00 scale) usually show elevation contours in 40 foot intervals. According to US Geological Survey guidelines, CONTOUR LINES PRINTED ON THESE MAPS CAN BE IN ERROR BY 20 FEET. In some cases, topo maps error can be larger than 20 feet. The biggest problem with using topo maps to try to get exact elevations is that it is difficult to precisely locate oneself on a topo map. Anyone who is experienced in using topo maps knows that you can gauge your location on the map.

Why does the thermometer read high when I wear the Vertech on my wrist?
The Vertech measures the ambient air temperature by means of a sensor inside the case. When that Vertech is worn against your skin, temperature readings will be affected significantly. Also, direct sunlight can cause the Vertech to give high temperature readings.

How do I get accurate temperature readings?
One way to assure accurate temperature readings is to use the Vertech over a jacket with the elastic Ski Band. When worn over a jacket, the accuracy depends on the thickness of the jacket. The thicker the jacket, the more accurate the temperature readings will be. If you find the temperature readings are mot accurate when the Vertech is off your wrist, you can manually adjust the temperature up or down.

To reach the Temperature Adjust, hold down the Adjust button for fifteen seconds while in the Weather Station function. Now you are in Long Term Calibration. Press the Function button and the temperature will blink. The arrow on the left side of the screen will alternate between up and down. The direction of the arrow determines the direction of your adjustment. Use the Option button to change the temperature in the direction of the arrow. Press the Adjust button when you have finished.

Why not take the temperature reading from the top of the case?
Temperature readings taken from the top of the case are no more accurate than readings from inside the case. Direct sunlight has a greater effect on temperature readings than warmth from you wrist.

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Vertech FAQ 5: Long-term Adjustment

When Should I perform the Long Term Barometer Calibration, and how do I do it?
WARNING: Do not attempt Long Term Calibration until you fully read and understand the procedure. Most Vertechs will never need Long Term Calibration. After 3 months, you can check your unit to see if Long Term Calibration in necessary. This procedure corrects for slight internal electronic drift.

  1. To determine if Long Term Calibration is necessary, compare the Vertech barometer reading to the reported sea level barometer:
  2. In the Barometer Adjust function, adjust the altitude at a known elevation, like your home. It is important that you know your exact elevation. This updates the Vertech sea level barometer readings.
  3. To get the reported sea level barometer, watch the weather report on your local TV station, or call the nearest airport to obtain the number for A.T.I.S. (Airport Terminal Information Service, sometimes called Automatic Terminal Information Service). The A.T.I.S. recording will give you current weather conditions, including "altimeter," which is the current sea level barometer It is also listed in the phone book in the US Government section under Transportation Department.
  4. If the Vertech barometer is within .05 of the reported barometer from the TV weather report or the A.T.I.S. recording, Long Term Calibration in not necessary. If the difference is greater than .05, recheck the Vertech barometer against the reported sea level barometer on different days. If the Vertech barometer reads CONSISTENTLY too high or too low by .05 or more, perform the Long Term Calibration:
  5. After adjusting the Vertech barometer at a known elevation, compare it to the local reported sea level barometer. If the Vertech reading is higher than the reported figure, it will need to be calibrated DOWN. If the Vertech reading is lower than the reported figure, it will need to be calibrated UP.

Example 1:

Reported sea level barometer:
29.87
Vertech barometer reading:
29.99
Difference: Long Term Calibration change needed:
-.12

Example 2:

Reported sea level barometer:
30.21
Vertech barometer reading:
30.14
Difference: Long Term Calibration change needed:
+.07

Long term calibration procedure
In the Weather Station function, press and hold the Adjust button for 10 seconds to enter Long Term Barometer Calibration. The number that appears in the upper display is not the sea level barometer, but is actual barometer pressure at your elevation. DO NOT change this number to match the reported sea level barometer! Use the Option button to change the displayed number UP or DOWN by the amount of the difference found in Example 1 and 2 above. In the first example above, the displayed number would be decreased by .12, and in the second example it would be increased by .07. Press Adjust to exit Long Term Calibration.

Press Adjust to enter the Barometer Adjust function. Use the Option button to adjust the displayed altitude to match your known elevation. The Vertech altitude and barometer readings are now correct.

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