The Smell of Molten Projects in the Morning
Ed Nisley's Blog: Shop notes, electronics, firmware, machinery, 3D printing, laser cuttery, and curiosities. Contents: 100% human thinking, 0% AI slop.
If you measure something often enough, it becomes science
I caught this just before it made a mess:
That container lives in the garage, where the air temperature pretty much tracks the weather.
When the air in the main compartment heats up, it pushes fluid up into the dispensing compartment. Although both caps were screwed on finger-tight, apparently the smaller cap leaks just enough that the pumped fluid can push the air out through the not-so-good seal.
Another few weeks and it’d be sitting in a puddle!
Having had the weaker of the two surviving STK batteries die 36 minutes into a ride, I tested them all:
The X axis shows W·h, rather than the usual A·h, because that seems more useful in a world of constant-power supplies.
The test current is now 1 A, rather than the previous 500 mA, to more closely match the camera’s actual load. The CBA tester doesn’t have a constant-power mode; I think that doesn’t make much practical difference.
The orange curve (STK D) is the failed battery, ending after 1.4 W·h. At an average 3.2-ish V, that’s 26 minutes, which is close enough to the actual run time, given the different current.
The red curve (STK C) is the sole STK battery of the original four from last November that actually worked.
The upper two curves come from the mostly unused Wasabi batteries (F and G), also from November. They have lost a bit of their capacity, but show the highest voltage out toward the end, so that’s good.
The black curve is the lightly used Sony OEM battery that came with the camera. Although it has about the same ultimate capacity as the other three “good” batteries, the voltage depression suggests it’ll trip out early.
The others are pretty much debris by now. I suppose they might be good for LED blinkies or some other low-voltage and low-current application, but …
So I’ll start using all four of the better batteries and see how the run times work out in actual use.
I stuck some old 12 V 7 A·h batteries in my homebrew power supply for the HP 3801A GPS Time / Frequency Standard, fired it up, put the antenna where it could see a good chunk of the sky, gave it a day to warm up / settle out, and it’s perfectly happy:
------------------------------- Receiver Status -------------------------------
SYNCHRONIZATION ............................................. [ Outputs Valid ]
SmartClock Mode ___________________________ Reference Outputs _______________
>> Locked to GPS TFOM 3 FFOM 0
Recovery 1PPS TI -38.3 ns relative to GPS
Holdover HOLD THR 1.000 us
Power-up Holdover Uncertainty ____________
Predict 366.2 us/initial 24 hrs
ACQUISITION ............................................ [ GPS 1PPS CLK Valid ]
Satellite Status __________________________ Time _____ +1 leap second pending
Tracking: 4 Not Tracking: 6 UTC 18:22:19 22 Jul 2016
PRN El Az SS PRN El Az 1PPS CLK Synchronized to UTC
3 34 104 48 * 1 36 48 ANT DLY 0 ns
17 62 308 103 6 27 220 Position ________________________
19 39 281 50 11 21 58 MODE Hold
28 80 133 64 *22 Acq .
24 12 319 LAT N 41:39:32.328
30 15 191 LON W 73:52:26.733
ELEV MASK 10 deg *attempting to track HGT +82.87 m (MSL)
HEALTH MONITOR ......................................................... [ OK ]
Self Test: OK Int Pwr: OK Oven Pwr: OK OCXO: OK EFC: OK GPS Rcv: OK
scpi >
The FFOM 0 entry says the Frequency Figure Of Merit is “within specifications” of 10-9, averaged over one day. That means the actual frequency should be within 0.010 Hz of 10 MHz.
Feeding the 10 MHz frequency reference into the (equally warmed up) HP 8591E spectrum analyzer and selecting an absurdly narrow span produces a comforting sight:
Given the horizontal resolution, that’s dead on 10 MHz.
So, yeah, that signal at 57-ish kHz really isn’t at 60.000 kHz:
Which is good to know …
The object of soldering all 40 wires in the 5 m hank of ribbon cable in series is to build a 40 turn loop antenna to receive LF radio signals like WWVB at 60 kHz. The antenna, being basically a big coil of wire, will have an inductance that depends on its layout, so putting a capacitor in parallel turns it into a resonant tank circuit. Given a particular layout (and, thus, an inductance), you can choose the capacitor to make the antenna resonant at whatever frequency you need (within reason).
With the joints soldered & reinforced with epoxy, the inductance across all 40 turns:
Back in a slightly different circular layout on the floor:
Given that inductance varies as the square of the number of turns, you’d expect a factor of four between those two inductances, but that’s not how it worked out.
Hanging the loop from a pair of screws in the floor joists to make a droopy rectangle-oid shape and driving it from a 600 Ω signal generator through a 10 kΩ resistor, it’s self-resonant at 213 kHz. Repeating that with a 470 kΩ resistor drops the resonance to 210 kHz, which isn’t different enough to notice and surely has more to do with my moving the loop while dinking with resistors.
Adding parallel capacitance (measured with an LCR meter, just to be sure) changes the resonance thusly:
Because the resonant frequency varies inversely as the square root of the capacitance, halving the resonant frequency means you’ve increased the capacitance by a factor of four. Because 250 pF halves the frequency (mostly kinda sorta close enough), the loop’s stray capacitance must be about 1/3 of that: 83 pF.
Yeah, 1/3, not 1/4: the additional capacitance adds to the stray capacitance, so it goes from 83 pF to 250 + 83 pF = 333 pF, which is four times 83 pF.
(If that sound familiar, it’s similar to the resonant snubber calculation.)
The self-resonant frequency of 213 kHz and the 83 pF stray capacitance determines the loop inductance:
L = 1/((2π · 213 kHz)^2 · 83 pF) = 6.9 mH
Pretty close to the measured value from the floor, I’d say.
To resonate the antenna at 60 kHz, the total capacitance must be:
60 kHz = 1/(2π · sqrt(6.9 mH · C)) → C = 1050 pF
Which means an additional 1050 – 83 = 970-ish pF should do the trick, which is about what you’d expect from the 64 kHz resonance with the 900 pF cap above. I paralleled pairs of caps until it resonated at 59.9 kHz.
The -3 dB points (voltage = 1/sqrt(2) down from the peak) turned out to be 58.1 and 60.1 kHz, so my kludged caps are slightly too large or, once again, I nudged the loop.
Figuring Q = (center frequency) / bandwidth = 59.1 / 2 = 30, which works out close enough to Q = X / R = 2600 / 80 = 33 to be satisfying. Using standard 26-ish AWG ribbon cable, rather than crappy 31-ish AWG eBay junk, would double the conductor area, halve the series resistance, and double the Q. Faced with that much resistance, I’m not sure better caps would make any difference.
Attaching the spectrum analyzer through a 470 Ω resistor to reduce the load:
I’d love to believe that big peak over on the left at 57.1 kHz is WWVB, but it’s not.
What’s more important: the broad hump between 56 and 62 kHz, where the increased amount of background hash suggests the antenna really is resonant, with a center frequency around 59 kHz. The -3 dB points might be 57 and 61 kHz, but at 10 dB/div with 5 dB of hash, I’d be kidding myself.
Dang, I love it when the numbers work out!
It’s faintly possible the spectrum analyzer calibration is off by 2.5 kHz at the low end of its range. The internal 300 MHz reference shows 299.999925 and it puts FM stations where they should be, but the former could be self-referential error and the latter lacks enough resolution to be comforting. I must fire up the GPS frequency reference, let it settle for a few days, see whether it produces 10.000000 MHz like it should, then try again.
The original measurements:
Riding into the Village of Wappingers Falls, there’s a lumpy patched pothole just ahead of the fairing & front wheel:
You can watch (and I can hear) the fairing flex as the front end jounces over the patch:
The hydration pack slung behind the seat also jounces and, when the reservoir bag bottoms out, the sudden pressure increase squirts water out of the bite valve, all over my face and goggles, and way out in front of the camera:
The camera runs at 60 images/second: those 28 images span all of 450 ms.
Two seconds later, the droplet stabilized into a nice round lens:
The low humidity of a lovely day evaporated the drop after another three minutes…
New hawks must somehow learn that swooping across roadways doesn’t work like swooping across lawns:
We think one of “our” new Cooper’s Hawks didn’t survive its lesson.
That’s the third dead hawk we’ve seen on recent rides; it’s been a rough few weeks for new hawks. Mary also spotted a smashed owl along one of her routes.
Yeah, they’re just birds, but …
Cropped and tweaked from a Sony HDR-AS30V helmet camera image.
Just for the record, heating four 500 g bags of silica gel at 230 °F for 12 hours overnight works exactly the way it should. Two of the bags baked down to 490 g, another was at 509 g, and the fourth had bulldog clips (rather than staples); given that they started with a measured 500 g of beads, that’s entirely good enough.
Memo to Self: don’t try to cut corners: heat the silica gel packs above water’s boiling point, let them cook overnight, don’t worry about wrecking the weird ground-cloth landscaping bags, and be done with it.
The Smell of Molten Projects in the Morning 