False Continuity Readings

[Scott S]

At LDRS 31 I was prepping my X-Cellerator Avbay back at the hotel the night before I planned to fly. I installed the altimeters on the sled, made all the connections and closed up the bay. I started up both altimeters, Raven 2 + ARTS2 and both altimeters indicated continuity on the apogee and main channels even though no ematches (Q2G2) were attached to the terminal blocks.

I pulled the bay apart and checked the wiring, all was good. Put in new batteries, although I didn’t know what that would do. Re assembled and had the same result. Pulled the bay apart and removed the wiring from the altimeters to the ejection charge terminal block and tested continuity with an analog Ohm meter. No continuity on any of the four pairs. Reconnected and started the altimeters, continuity on all four channels. I scrubbed the flight as it was now about midnight.

Last night I was able to investigate and decided to clean the exterior of the terminal blocks. I cleaned them with hoppes 9 cleaner and a q-tip. After I reassembled and tested, no continuity on any of the channels. The rocket only had 3 previous flights and I wrap the terminal blocks with electrical tape to protect them from the charges.

These are the terminal blocks I used. They are PCB mounted so the wire from the altimeter is soldered to the pin under the block.



I never imagined that the black powder residue would provide a path to show continuity. It also baffles me that the Ohm meter didn't show continuity but the altimeters did.

Anyone else had this problem?

Thanks for any input.

 

[hognutz63]

Yes, on my L3 certification flight with an ARTS-2. I had never flown the rocket but ground tested a couple times. There was enough residue to create a false continuity in the ARTS-2. I cleaned the terminal blocks with denatured alcohol and all was good.

Talk about some pucker factor when my electronics were not working right on my certification flight.

 

[ClayD]

At LDRS 31 I was prepping my X-Cellerator Avbay back at the hotel the night before I planned to fly. I installed the altimeters on the sled, made all the connections and closed up the bay. I started up both altimeters, Raven 2 + ARTS2 and both altimeters indicated continuity on the apogee and main channels even though no ematches (Q2G2) were attached to the terminal blocks.

I pulled the bay apart and checked the wiring, all was good. Put in new batteries, although I didn’t know what that would do. Re assembled and had the same result. Pulled the bay apart and removed the wiring from the altimeters to the ejection charge terminal block and tested continuity with an analog Ohm meter. No continuity on any of the four pairs. Reconnected and started the altimeters, continuity on all four channels. I scrubbed the flight as it was now about midnight.

Last night I was able to investigate and decided to clean the exterior of the terminal blocks. I cleaned them with hoppes 9 cleaner and a q-tip. After I reassembled and tested, no continuity on any of the channels. The rocket only had 3 previous flights and I wrap the terminal blocks with electrical tape to protect them from the charges.

These are the terminal blocks I used. They are PCB mounted so the wire from the altimeter is soldered to the pin under the block.

View attachment 93733

I never imagined that the black powder residue would provide a path to show continuity. It also baffles me that the Ohm meter didn't show continuity but the altimeters did.

Anyone else had this problem?

Thanks for any input.

Wow, I have never had that. I know what your saying can and does happen, just never had it happen. Perhaps the terminal can be held to blame a little. If the material aides in the conduction with the BP residue.

I use the MCmaster Carr Terminals. (touch safe) https://www.mcmaster.com/#terminal-blocks/=iuiw4m
mainly because i am concerned with a short across posts... But i have had these black with residue, literaly dozens of firings and never had false continuity.
I just replace my first pair actualy, because I was affraid they me be so residued I might not get a good contact.

 

[billspad]

Anyone else had this problem?

I may have had something similar. Here's the link to the thread on it. To summarize it, Raven showed continuity, ohmmeter showed continuity, charges didn't fire, data from from Raven shows power going to the terminals but no voltage drop. I used I different type of terminal strip but it did have noticeable corrosion. I eliminated all the wiring and flew the Raven in another rocket with everything connected directly to it and it worked. I suspect that there was sufficient black powder residue to conduct enough to show continuity but the connections were bad enough to not allow any current to flow.

 

[UhClem]

While the layer of gunk put down by a BP charge including sulfur compounds that form sulfuric acid when wet can be conductive, they aren't very conductive.

The problem is that most altimeters are trying to save power which means they use pretty high impedance (aka low current) continuity test circuits. As a result even fairly high resistances can trigger them. The reason your multimeter didn't show continuity is probably because that is what you were testing. Continuity testing assumes a low resistance. I dug up the manual for a random Fluke meter and it says that the continuity beeper has a turn on threshold of 25 Ohms. Try measuring resistance on the highest scale.

I have never understood the popularity of terminal blocks. They have the obvious problem of being subject to shorts unless protected and of course there is the hassle of connecting the wires in the field. Many years ago I started using Deans Micro Plugs. While I have to do a little extra prep work in soldering the wires to the connectors, I can do that at home. In the field all I do is plug it in and I am done.

I also use 4 pin Deans connectors on my altimeters. This makes them fast to install and I never swap drogue and main by accident.

 

[les]

I generally prefer to use the actual resistance measurement than the continuity "beeper" function. It can help eliminate issues and find problems.

 

[FredA]

BP residue can easily be conductive to a level to fool continuity testers.
Remember, they are trying to NOT light the ematch, so sense current is low and the system is easily fooled.

I find it good practice to put some duct tape over the terminal blocks.
This keeps the gunk off AND keeps metal bits (quicklinks) in the recovery harness from acidentally shorting across the terminal block.

 

[fyrwrxz]

I almost made reference to carbon tracking on distributor caps and rotary tuners and then *sob* realized hardly anybody would know what i was talking about...sniff....please tell me they still make Hopps No. 9 or is it up to like, No. 129 now???

 

[Scott S]

Thanks to all for the great info. May try a few of the suggestions but now atleast I know what the problem was!

 

[Coop]

I almost made reference to carbon tracking on distributor caps and rotary tuners and then *sob* realized hardly anybody would know what i was talking about...sniff....please tell me they still make Hopps No. 9 or is it up to like, No. 129 now???

HA! But no.. it's still good ol' No. 9!


Later!

--Coop