An Introduction To  Paragliding Varios

Phones, Barometers And Varios

Nowadays more and more phones are equipped with a barometer also known as a pressure sensor. In the last years the built in barometers made quite some advances with respect to increased update rates, accuracy and precision. While older phones often struggled with precision and provided just a few values each second, modern phones like the Pixel 4a, Pixel 6 or Pixel 6a easily process 50 or more readings a second from their many built in sensors with decent precision. While there are more sensors, especially the inertial measurement unit (IMU), of relevance for a precise and instant vario app, the presence and type of the built in barometer is a key factor and discussed by most pilots for good reason. However, it must be noted that the mere presence of a built in barometer is not a guarantee for a working vario app. 

Disclaimer: We cannot make any recommendation for a specific phone or sensor setup to use as vario. However information on built in barometers and user feedback for using these as vario will be shared. 

Barometer - Accuracy And Precision

So what is it, that makes a great barometer for a paragliding vario? And how do phone barometers compare 🤔?

What Is Vario?

A vario in paragliding has two purposes. A vario will report the change in altitude so a pilot knows whether it is going up or down and it will indicate the current altitude itself for vertical orientation regarding for instance airspaces. The change in altitude usually is reported as climb rate in meter per second although other units can be used. The calculation of an altitude and a climb rate is based on the atmospheric pressure. This relation between altitude and pressure is expressed the barometric formula. After all a vario is a measurement device. As all measurement device their grade can be rated by their accuracy and precision which will be covered after looking at the barometric formula.

The Barometric Formula

The relation between altitude and pressure is captured in a barometric formula. Without going into details of this barometric formula the relationship can be summarized as follows: With increasing altitude the pressure will decrease and with the altitude decreasing the pressure will increase. This is quite easy to remember! Just think of less air resting on your shoulders the higher you climb and more the lower you get. 

This relationship is covered in the barometric formula which can be simplified to:

It is important to point out that this is an approximation since the true relationship is influenced by further factors such as temperature and humidity. But incorporating all this would be close to overengineering (at least for calculating a climb rate which is what varios will do and will be most important for pilots)  which is why there is a standardized approximation given by the ICAO Atmosphere.  The ICAO Standard Atmosphere for instance assumes a temperature of 15 °C and a pressure of 1013.25 hPa at mean sea level and a lapse rate of 0.65 °C per 100m. Maybe one of the key points of the barometric formula is that for calculating the (correct) mean sea level altitude the right reference pressure needs to be used. The pressure level at sea level must be known, which leads to QNH a virtual reference pressure at mean sea level that can be retrieved from METAR information of airports in the vicinity.

Accuracy Of A Barometer

Accuracy means: Measured value is close to the true value to be measured. An accurate barometer would report close to 1013.25hpa if the true pressure would be equal to 1013.25 hPa. Except for the calculation of flight levels the relevance of an accurate barometer is not crucial from a practical point of view. After all a lack of accuracy is equal to an offset in the reported value and can be fixed with adding a constant value or resetting the reference value to a new, a correcting value. Practically this is already being done when you start working with QNH, QFE or Mean Sea Level adjustments based on the GNSS. So apart from maybe Flight Levels (FL) which are always based on the same reference pressure, 100% accuracy is not as relevant as maybe thought.

Precision Of A Barometer

Precision describes the range of deviations when measuring continuously the same value. The bigger this range, the less capable is the sensor to detect small changes in pressure. Why is that? Well if this range would be 50cm wide, the barometer is practically not capable to detect a 20cm change since it is well within its own range of measurement uncertainty.  At least not based on a single reading. With that, for the calculation of a climb rate, more interesting than the accuracy of a barometer is the following question:

• What is the smallest difference in pressure or altitude that a barometer can detect?

And connected to that a question that leads directly to sampling rates and precision:

• How long will a barometer need to reliably detect that difference in pressure or altitude?

Try finding that out for your recently bought vario :-) .

Or more specific. If paragliding pilots point out that only their high end vario is capable of detecting a change in altitude of 10 cm or maybe even 5 cm. And this is what helps them to thermal. What does that actually mean?

A vario and especially the used barometer can be way off in terms of accuracy yet be extremely precise. This is not problematic since that offset can be compensated. If you know something is of by 10 meters, well take these 10 meters off!  Of course, the other way is also possible.  A barometer can be accurate but not precise, which would be much worse. This would be problematic since their is no other compensation than increasing the integration time and with that the climb rate of that vario would get more and more delayed. If a vario is not precise it will need to take a lot more samples to calculate more precise values by averaging