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JulianEdgar · 12-09-2020, 09:37 PM

OK, BIG learning curve for me.

Firstly, I can't use the absolute readings because the measured pressures change with altitude! That is, changing in altitude by only 10 metres is a change of about 1 millibar - or no less than 100 Pascals! When you remember that the highest pressure I record on the Insight at a test speed of 80 km/h is about 300 Pascals, you can see that unless the road is dead-flat, large inaccuracies will be introduced. (And this would apply to an extent to my pressure tank reference approach - but at least there the pressure is reset before each run - and that run is unlikely to be up/down a large hill).

I spotted this altitude problem when I'd left my house with the stationary pressures all set to atmospheric (say, 955 millibars at my attitude and today's weather), but when I got to my test road they'd all changed, even when I was again stationary. I thought it must be drift in the measurements (pretty surprising when such low drift is claimed) but when I got back home, all the pressures had returned to their initial setting.

So unless I reset the atmospheric pressure reading each time I get to the (flat) test road, this approach is not best.

I then tried the differential mode, referencing against the static port of the high, front-mounted pitot tube. But something wasn't right - as soon as I connected this hose, all the other readings altered. I then realised that the hose must be blocked, and so the reference pressure increased as I pushed the hose on. These hoses are tiny (0.9mm ID) and I was using one provided as a sample by Evolution Measurement. (The other hoses I bought.) I think the sample hose was closing-up, so I went to one of my silicone hoses for the pitot as well as the pressure puck. (Only using one puck at this stage.)

This all took a few hours (and more productive than arguing with Aerohead), but now I seem to have readings that make sense. My measured stagnation pressure on the Insight is 280 Pascals, versus the 290 I've measured previously. (If anyone wants to work it out, I am at 550 metres.)

Now I think I need 16 little reels to wind the hoses on (they are a nightmare to handle) and I am not sure that I'll have enough hose so I'll try to use just enough to allow transfer from car to car. I've labelled all the pucks and I'll do the same for the hoses, otherwise it would be easy to connect them out of order.

Finally, as expected, the fast response system with no smoothing dances around a bit - plus/minus 20 Pa at 290. I have asked Evolution Measurement if they can add adjustable smoothing, but no reply on that yet.

12-09-2020, 09:37 PM	#23 (permalink)
JulianEdgar Banned Join Date: Nov 2017 Location: Australia Posts: 2,060 Thanks: 107 Thanked 1,605 Times in 1,136 Posts	OK, BIG learning curve for me. Firstly, I can't use the absolute readings because the measured pressures change with altitude! That is, changing in altitude by only 10 metres is a change of about 1 millibar - or no less than 100 Pascals! When you remember that the highest pressure I record on the Insight at a test speed of 80 km/h is about 300 Pascals, you can see that unless the road is dead-flat, large inaccuracies will be introduced. (And this would apply to an extent to my pressure tank reference approach - but at least there the pressure is reset before each run - and that run is unlikely to be up/down a large hill). I spotted this altitude problem when I'd left my house with the stationary pressures all set to atmospheric (say, 955 millibars at my attitude and today's weather), but when I got to my test road they'd all changed, even when I was again stationary. I thought it must be drift in the measurements (pretty surprising when such low drift is claimed) but when I got back home, all the pressures had returned to their initial setting. So unless I reset the atmospheric pressure reading each time I get to the (flat) test road, this approach is not best. I then tried the differential mode, referencing against the static port of the high, front-mounted pitot tube. But something wasn't right - as soon as I connected this hose, all the other readings altered. I then realised that the hose must be blocked, and so the reference pressure increased as I pushed the hose on. These hoses are tiny (0.9mm ID) and I was using one provided as a sample by Evolution Measurement. (The other hoses I bought.) I think the sample hose was closing-up, so I went to one of my silicone hoses for the pitot as well as the pressure puck. (Only using one puck at this stage.) This all took a few hours (and more productive than arguing with Aerohead), but now I seem to have readings that make sense. My measured stagnation pressure on the Insight is 280 Pascals, versus the 290 I've measured previously. (If anyone wants to work it out, I am at 550 metres.) Now I think I need 16 little reels to wind the hoses on (they are a nightmare to handle) and I am not sure that I'll have enough hose so I'll try to use just enough to allow transfer from car to car. I've labelled all the pucks and I'll do the same for the hoses, otherwise it would be easy to connect them out of order. Finally, as expected, the fast response system with no smoothing dances around a bit - plus/minus 20 Pa at 290. I have asked Evolution Measurement if they can add adjustable smoothing, but no reply on that yet.