how the pressure trace trigger works

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mtngun
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how the pressure trace trigger works

Postby mtngun » Sun Feb 10, 2008 6:58 pm

Someone asked how the PT trigger algorithm works. Here is Burt Mitchell's answer:

I regularly run with a trigger level of 1 or 0. I don't seem to have the bolt closing problem that many folks do. I also do my testing way out in the boonies. There aren't any electrical lines within a couple of miles. One time when I was testing a plane started circling around. He must have been talking on his radio. I had to turn up my trigger level until he left the area. Either that or he had a very noisy electrical system.

What I was trying to say about the gage placed at the muzzle was that all I got was a trace that went high and flat lined. Since I was trying to use the signal for timing, I had the barrel dimensions screwy so that I would see a very fast rise time signal. Since the event happened out at about the 1.5 mSec point it never had time to drop off again before the trace was complete.

Since the trace was being started by another PT module, triggering wasn't an issue.

Triggering is tricky. Especially when you are dealing with a waveform that isn't exactly the same from one shot to the next.

The way PT triggers is like this.

There is a value called the "Trigger Level". It's actually an indicator of microstrain, but I don't try to quantify it. It's just a relative value.

When you tell PT to Shoot, the unit starts looking at the values out of the A to D converter. When it has seen 50 readings that are equal to, or greater than, the trigger level, it decides that a real event is happening. If even 1 of the readings is less than the trigger level it starts counting all over again. In other words, it needs to see 50 consecutive readings greater than or equal to the trigger level.

When it starts to capture it uses those 50 readings already taken as the first half millisecond of the trace and then builds the rest of the trace starting from that point.

OK. If you have a pressure buildup that is kind of slow (relatively speaking), it can look like the traces you sent. For traces 3 and 4 the pressure built up to the trigger level long enough for the unit to start capturing. Then the pressures leveled off for awhile before building up again. This could simply be due to barrel heating. Secondary spikes are _very_ sensitive to temperature.

FYI. You can sometimes see this "delay" affect with a strain gage placed over the chamber. To me it indicates a load with a problem. It starts to build pressure, then holds steady for a bit before the big pressure rise really starts. If one shot in your shot string is delayed due to a pressure rise delay, it won't act the same as the other shots. If nothing else it gives more time for the shooter to move and point the bullet somewhere else.

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