Weird SLCF deployments during vacuum test

[Buckeye]

I put a Perfectflight MAWD and SLCF in a homemade vacuum chamber contraption. Christmas tree lights as deployment events. Both altimeters record the same altitude, but with a slight time shift. No biggie on that. However, during the test, I clearly noticed that the MAWD charges went off several seconds before the SLCF charges. Indeed, the data download of the SLCF shows Drogue and Main events occurring essentially on the ground at the end of my simulated flight (the MAWD does not report Drogue and Main event times.)

These MAWD and SLCF altimeters fly just fine in real life and fire the events correctly. Any idea why the vacuum ground test may confuse the SLCF (but apparently not the MAWD?)

I would contact Perfectflight directly, but recent threads show that their communications are MIA at this time.

 

[mbeels]

I would hazard a guess that it is related to a built-in mach delay. The SLCF is making sure that it was just seeing a pressure drop due to a mach transition.

 

[Buckeye]

I would hazard a guess that it is related to a built-in mach delay. The SLCF is making sure that it was just seeing a pressure drop due to a mach transition.

Ah, good point. The MAWD has no built in Mach protection, but the SLCF does.

 

[cerving]

Vacuum tests are good for making sure that the baro and deployment circuitry are working, but they're not very good for any data collecting without a thorough knowledge of exactly how the altimeter works. Just about any vacuum test is going to pull the vacuum so fast that it looks like a supersonic flight, and there isn't really a "coast" phase leading up the maximum vacuum (apogee). This can cause the deployments to appear to be very late, and it may also cause both channels to appear to fire at the same time when you release the vacuum because the vacuum may drop quickly enough that "apogee" and your main deployment "altitude" may occur within a few hundred ms. We use an automotive A/C vacuum pump and a Mason jar, with a valve to control the flow, it's better than using a vacuum cleaner or a syringe (like the old Adept rig), and we can pull over 50K worth of vacuum, but we still run into these issues when we're testing.

 

[Buckeye]

Yes, that all makes sense, cerving. Good info.

FYI, I use a jar, some tubing, and my piehole, LOL.

 

[aerostadt]

I built a crude vacuum test chamber by pulling a piston. After a short delay a see the drogue and the main chute light fire at the same time. Afterwards the altitude is not recorded and a get some kind of flashing light, which I think may be a fault code. These two altimeters had a hard landing when only the drogue chute came out. There appears to be no physical damage to the altimeters. Are the altimeters alright?

 

[aerostadt]

Vacuum tests are good for making sure that the baro and deployment circuitry are working, but they're not very good for any data collecting without a thorough knowledge of exactly how the altimeter works. Just about any vacuum test is going to pull the vacuum so fast that it looks like a supersonic flight, and there isn't really a "coast" phase leading up the maximum vacuum (apogee). This can cause the deployments to appear to be very late, and it may also cause both channels to appear to fire at the same time when you release the vacuum because the vacuum may drop quickly enough that "apogee" and your main deployment "altitude" may occur within a few hundred ms. We use an automotive A/C vacuum pump and a Mason jar, with a valve to control the flow, it's better than using a vacuum cleaner or a syringe (like the old Adept rig), and we can pull over 50K worth of vacuum, but we still run into these issues when we're testing.

I built a crude vacuum test chamber by pulling a piston. After a short delay a see the drogue and the main chute light fire at the same time. Afterwards the altitude is not recorded and a get some kind of flashing light, which I think may be a fault code. These two altimeters had a hard landing when only the drogue chute came out. There appears to be no physical damage to the altimeters. Are the altimeters alright?

 

[cerving]

I would defer that to PerfectFlite, if you have another "good" altimeter to use as a reference unit that would probably tell you a lot.

 

[manixFan]

I built a test chamber of two large syringes connected with tubing. It took a fair amount of practice to be able to draw the vacuum slowly enough to test baro only based altimeters. The graduations on the side of the syringe serve as guide as to how fast I should pull out the plunger as I count out seconds in my head. Simple and pretty easy. I can get apogees over 10,000ft. It is very hard to stay subsonic, so the trick is pulling fast until the #3 mark or so, and then pulling slowly but steady until it gets too hard to pull any further, hold for a moment, and then very slowly allow the plunger to return. Once I got the hang of it I could very reliably get good apogee and main event lights.


Tony

the plunger in the syringe that acts as the test chamber is prevented from moving by the tube around the shaft. Process: power on altimeter, slide into syringe body, insert plunger, then connect hose to 'vacuum pump' while still in startup to avoid an early trigger. The syringes are 150ml. The event lights are grain of wheat style bulbs from an auto parts store (circled in red, each has shrink wrap tubing around one lead to prevent a short):


https://www.amazon.com/DEPEPE-Conne...qid=1639270021&sprefix=150+ml+,aps,105&sr=8-8

 

[SparkyVT]

Ah, good point. The MAWD has no built in Mach protection, but the SLCF does.

MAWD has Mach protection if you enable it. See the manual. I have 295 flights with the MAWD and used the Mach inhibit many times. SLCF does the same without switches, probably done in software now.

BTW, anybody need one? I have one unit left that I'm willing to part with.

 

[aerostadt]

I built a test chamber of two large syringes connected with tubing. It took a fair amount of practice to be able to draw the vacuum slowly enough to test baro only based altimeters. The graduations on the side of the syringe serve as guide as to how fast I should pull out the plunger as I count out seconds in my head. Simple and pretty easy. I can get apogees over 10,000ft. It is very hard to stay subsonic, so the trick is pulling fast until the #3 mark or so, and then pulling slowly but steady until it gets too hard to pull any further, hold for a moment, and then very slowly allow the plunger to return. Once I got the hang of it I could very reliably get good apogee and main event lights.


Tony

the plunger in the syringe that acts as the test chamber is prevented from moving by the tube around the shaft. Process: power on altimeter, slide into syringe body, insert plunger, then connect hose to 'vacuum pump' while still in startup to avoid an early trigger. The syringes are 150ml. The event lights are grain of wheat style bulbs from an auto parts store (circled in red, each has shrink wrap tubing around one lead to prevent a short):
View attachment 493762

https://www.amazon.com/DEPEPE-Conne...qid=1639270021&sprefix=150+ml+,aps,105&sr=8-8

Thanks, Tony! I tried this today and it worked good!