Den tirsdag den 27. september 2016 kl. 15.19.03 UTC+2 skrev krasw:
On Tuesday, 27 September 2016 11:15:02 UTC+3, Tango Whisky wrote:
Why would a mechanical vario give you more information on airmass?

That is interesting question. If it would give identical information, the varios should read exactly the same. They obviously are not, so they are either measuring TE pressure differently, or one of them is measuring same thing but poorly (which I don't agree with, in some gusting thermals my Bohli reacts more realistically than electric vario). I feel getting two "opinions" from same thing is advantage (three actually if you count inertial variometer). Heck, I would gladly install fourth variometer if that tells me something new, instead of duplicating something I already have.

I think the mechanical vario and the electrical transducer based vario should have different responses simply due to physics.

Consider a sudden small pressure drop in the TE line (like hitting very sudden lift). The mechanical vario measures the flow rate from the flask to the TE line. This flow is proportional to the pressure difference between the flask and the TE pressure. The pressure difference, and hence the vario reading, is largest right after the pressure drop and then decays exponentially back to zero. That explanation is consistent with a simple experiment done a few years ago with a 57 mm Winter vario, a syringe and a video camera. The vario reading peaked after just 0.3 s. The subsequent decay fitted perfectly with an exponential decay with a time constant of 3 s.

The electrical vario indicates the time derivative of the measured TE pressure after some low pass filtering. The response depends on the actual filtering, but the response to a sudden small pressure drop will peak after some time given by the filter type and time constant. The lab experiment with a syringe and a video camera showed that a late generation LX5000 on minimum time constant (0.5 s) would peak after 2.5 s. On the other hand, the following decay to zero was much faster than for the Winter vario.

The tests were repeated with both the Winter and the LX5000 on the same TE line in order to evaluate the influence of the mechanical vario and flask on the LX5000 response. Surprisingly, the LX5000 response to a sudden small pressure drop was essentially unchanged, and if anything it measured a bit faster.

Another test with an LX7007 showed similar results, except now the Winter on the same TE line did seem to slow the LX7007 a bit when operated on the shortest possible time constant of 0.2 s (peak after 2.0 s without Winther, peak after 2.5 s with Winther on TE line).

I prefer the electrical vario, but I know one regular WGC pilot who keeps a mechanical vario claiming that it has a faster response to sudden changes in the air mass. He doesn’t know why – and I don’t know if the considerations above are relevant at all in the real world…

-J