Air Speed sensor tube length

ArduPlane
1

Does the digital airspeed sensor tube length makes any accuracy issues if its too long?

There is no documentation on the max length allowed. I need to use long tubes unless not allowed so doing some due diligence here.

2

in theory long tubes/hoses should not make a difference unless they are crimped here and there or they pass by areas of varying temperatures. Why?

Firstly, the vehicle velocity is calculated based on the pressure difference between the 2 tubes in which the air flow is practically none / fluids inside are close to static. One side measures the head on stagnation pressure in front of the vehicle, and the other side measures the ambient air pressure. Stagnation pressure is ambient + dynamic pressure which is purely due to the vehicle velocity (1/2 rho v^2)…

So the length of the tubes would not impose any viscous flow and pressure losses. However, if the tubes are extremely long there might be some latency for the pressures to adjust and reach the sensors. I am not sure if this would be noticeable.

Secondly, if the tubes pass around hot zones like batteries, ESCs and motors, the air inside may locally heat up. Going back to high school chemistry, PV=nRT. So higher local temperatures may raise the pressure in one of the tubes and mess up the delta measured.

Before questioning the length and channeling of the tubes, have you positioned the pitot tube at a location where the air flow would hit the tube head on? Have you run pre-flight calibrations?

I think there are 4 items to check based on the above thoughts…

2 Likes
3

Hi,

Thanks for detailed explanation and good reminder of high school science :slight_smile:

I have used these sensors before and yes the air flow is optimal.

You brought up an interesting thought of lipo batteries heating up. Something to consider.

Thanks again…

4

In general it’s better to avoid too long tubes and place the sensor close the probe. However, if that forces you to have >50cm I2C cables, then, yes, a compromise is in order.

1 Like
5

So I2C cable can not be longer then 50cm? I wasn’t aware of that.

I think is directly proportional to kzh used.

6

“the maximum bus length of an I2C link is about 1 meter at 100 Kbaud, or 10 meters at 10 Kbaud. Unshielded cable typically has much less capacitance, but should only be used within an otherwise shielded enclosure.”