How a variometer works: pressure sensor and filtering

The vario beeps and you turn on it. But the beep is the sound of a calculation inside the device, not of what the air is doing. That calculation has three layers: sensor, noise and filter. Knowing all three tells you when to trust the vario and when not to.

Layer 1: the pressure sensor

A vario does not measure altitude. A small barometric sensor measures air pressure. Pressure falls with height: near sea level, roughly 1 hectopascal every 8 metres. Modern sensors read that pressure dozens of times per second, with a precision equivalent to a few decimetres of altitude.

Climb rate is the altitude difference between two successive readings divided by the time between them. So the vario shows the rate of change of pressure; it does not measure speed itself.

That makes it sensitive to two things. First, pressure change: an approaching front, or the natural fall of pressure through the day, shifts the altitude the device shows without you moving. Second, airflow: if wind gets into the compartment holding the sensor, pressure changes locally and the vario shows false lift or sink. That is why manufacturers put the sensor in a chamber isolated from outside airflow.

Layer 2: noise

The sensor makes a small error on every reading. A few decimetres in altitude becomes large when differentiated: a 0.3-metre jump between readings, at 20 readings per second, is a false 6 m/s spike. The raw derivative is therefore unusable, a number that changes sign several times a second.

The second source of noise is you. Braking, weight-shifting, hitting turbulence all change altitude for real, but have nothing to do with the air rising. To the vario they are noise too.

Layer 3: the filter

The device smooths the raw derivative. The simplest method is an average over the last second or two; more advanced instruments use an estimation filter that also takes accelerometer data, which helps separate altitude changes caused by your manoeuvre from those caused by the air.

Filter strength is a trade-off:

• Strong filter: calm display, few false beeps, but lag. You learn you entered a thermal a second or two later; and that you left it.
• Weak filter: instant response, but every gust becomes a beep.

The "response time" or "sensitivity" setting on the instrument chooses that balance. In thermal centring, lag is expensive: by the time the beep arrives the core may already have passed you. For most pilots the middle value is right; a little faster at sites with narrow thermals, a little slower on turbulent days.

Why sound?

A visual display needs looking at, and in a thermal the place to look is outside. Sound converts climb rate into pitch and repetition rate. A well-set vario tone lets you tell "increasing" from "decreasing" without reading a number. The centring rules are built on that distinction.

The vario in the log

The value the instrument displayed is not written to the flight log. The IGC file usually gets a pressure altitude and a GPS altitude once per second. Variograph computes vertical speed from the altitude difference between those rows. That value differs from the one on the screen: coarser (one sample per second) but unfiltered (no lag). If the recording interval rises to 5 seconds, vertical speed becomes a 5-second average and short thermals vanish; see IGC data quality.

Neither on the instrument nor in the log is the measured climb the air's true climb; your wing's own sink has been subtracted from it. For what the air did you need net vario.

In short

• A vario measures pressure and computes speed.
• The raw calculation is noisy; the filter makes it usable and charges lag in return.
• The setting is a choice between lag and calm.
• Vertical speed in the log is not what the instrument showed but a recalculation from the altitude rows.