Ed Nisley's Blog: Shop notes, electronics, firmware, machinery, 3D printing, laser cuttery, and curiosities. Contents: 100% human thinking, 0% AI slop.
Category: Science
If you measure something often enough, it becomes science
The blotches on the legend in the lower left corner show that a refilled plotter pen can accumulate a droplet of ink around its nib, which should come as no surprise. I wiped off the excess immediately after refilling each pen, let the assortment sit for a few hours to (presumably) let the new ink reach the nib, and wiped them off before inserting them in the plotter’s pen carousel. All I can say is that I used up a bunch of paper towels in the process…
A closer look at the plot shows Pretty Good If You Ask Me results:
CMYK Refilled Pens – plot detail
The two blue-ish pens have less flow than the others, resulting in dotted lines that should be continuous. As nearly as I can tell, that’s a function of how much OEM ink has solidified in the fiber nib and, most likely, the fiber rod that draws ink from the sponge reservoir inside the body.
And, of course, the colors produced by adding CMY printer ink to the surviving OEM ink aren’t found in any catalog. I’m also blithely ignoring the difference between the inks inside plotter pens intended for paper and those for overhead transparencies; at this late date, that’s defined to Not Matter.
The Dutchess Rail Trail sits atop a pipeline carrying water from the treatment plant in the City of Poughkeepsie to the GlobalFoundries (neé IBM East Fishkill) complex. For good engineering reasons, the mid-line pumping station (equipment yard visible to our left) in Page Industrial Park sits directly athwart the pipe, which forced an abrupt S-curve on a relatively steep slope into the rail trail layout.
T=0.000 s — The lead cyclist just cut in front of her companion and isn’t leaning into the turn, at which point Mary and I both realize this isn’t going to end well:
Road Rash 2015-08-15 – 131
T=0.750 s — Newton grabs control of her bike and he’s not gonna let go:
Road Rash 2015-08-15 – 176
T=1.633 s — The rear wheel locks as she passes Mary, she’s far off-center and falling to her left, the bike has gone inertial, and it’s obvious we’re about to arrive at the same place at the same time:
Road Rash 2015-08-15 – 229
T=2.100 s — Collision Alarm! I’m veering off the pavement, which is the only reason we didn’t have an offset frontal collision:
Road Rash 2015-08-15 – 257
T=2.333 s — Impact! I’m stopped and balanced on the bike, with my left foot out of the pedal cleat and heading for the ground. She’s sliding past me, pivoting around her bike’s left pedal skidding on the asphalt:
Road Rash 2015-08-15 – 271
She ended up sprawled atop her bike, facing up the slope, with the front wheel just beside the rear wheel of my bike; her foot or some part of her bike whacked my left-side underseat bag in passing, but there was no bike-on-bike collision. No injuries for her, other than perhaps a bit of road rash, but only by sheer raw good fortune.
Reviewing the video shows she lost control at the transition from the trail to the downward S-curve, a few seconds before the first picture here and about five seconds before she stopped sliding past my bike, but the problem wasn’t obvious until the scene in the first picture. Mary never had a chance to react and, with less than two seconds until the not-quite-collision, my gross-motor reaction time just barely got me out of the way.
The step change in Week 22 shows when the replacement took over. After some poking around, Amazon Prime FTW.
The square-ish pulse starting in Week 26 marks a change from 55% RH to 60%RH and back again, to see how the front panel meter compares with the low end lab-grade hygrometer in the other side of the basement near the Hobo datalogger on the water inlet; they’re all off by a bit, but well within their expected tolerances. The 5% RH height of the step suggests a good match between their incremental calibrations.
It seems dehumidifiers last a few years, no matter which Brand Name you’ve decided to trust, so there’s not much point in developing a deep emotional attachment.
For the record, the old dehumidifier sported a GE label:
As it turns out, Electrolux bought Frigidaire a while ago, then absorbed GE’s appliances in 2014, so they’re all one big happy family now.
The various names notwithstanding, a recall notice suggests Gree Electric actually makes all the dehumidifiers badged with Brand Names you might think represent something significant.
Given the ionization chamber’s tiny currents and the huge resistors required to turn them into voltages, reviewing the thermal noise I generally ignore seems in order…
The RMS noise voltage of an ordinary resistor:
vn = √ (4 kB T R Δf)
The constants:
kB – Boltzman’s Constant = 1.38×10-23 J/K
T – temperature in kelvin = 300 K (close enough)
Mashing them together:
vn = √ (16.6x10-21 R Δf)
vn = 129x10-12 √ (R Δf)
For a (generous) pulse current of 20 fA, a 10 GΩ resistor produces a mere 200 μV, so wrap a gain of 100 around the op amp to get 20 mV. An LMC6081 has a GBW just over 1 MHz, giving a 10 kHz bandwidth:
vn = 129x10-12 √ (10x109 10x103) = 1.3 mV
Which says the noise will be loud, but not deafening.
A 100 GΩ resistor increases the voltage by a factor of 10, so you can decrease the gain by a factor of ten for the same 20 mV output, which increases the bandwidth by a factor of ten, which increases the noise by a factor of … ten.
Ouch.
With the same gain of 100 (and therefore 10 kHz bandwidth) after the 100 GΩ resistor, the output increases by a factor of ten to 200 mV, but the noise increases by only √10 to 4 mV.
The LMC6081 has 22 nV/√Hz and 0.2 fA/√Hz input-referred noise, neither of which will rise above the grass from the resistor.
It’s a good thing I have a pretty deep parts stock, as one of the caps didn’t fit into its holes at all.
The Russian CI-3BG glass tube, according to the datasheet and discussion on MightyOhm, is sensitive to gamma and beta radiation, so it should serve as a simple cross-check on my ionization chamber results. It’s not clear the C8600 is applying the correct voltage to the CI-3BG tube, but it probably doesn’t make much difference; the supply is so feeble that there’s no way to actually measure the results.
A closer look at the CI-3BG suggests the active volume lies inside that spiral-wrapped section between the white insulators:
Russian CI-3BG Glass Geiger Tube – detail
In round numbers, that section is 6 mm long and 3 mm OD. Figuring the ID at 2.5 mm, that’s a volume of 30 mm3 = 0.030 cm3. That’s maybe 1/7300 of the ionization chamber volume, so, (handwaving) assuming roughly equal sensitivity, the chamber should report three orders of magnitude more pulses than this little thing.
It’s mildly sensitive to a radium-dial watch and perks up when a watch hand lines up along the spiral-wrapped volume. Given that the radium decay sequence spits out betas and no gammas, the (scaled) count may be a bit higher than the ionization chamber produces, but there are so many other imponderables that it might not matter in the least.
Feeding the output voltage into the ‘scope, with AC coupling to strip off the DC bias, produces this:
Darlington 12k load – multiple
Those cute little spikes seem to be gamma ray ionization events: they are always positive-going, there are no similar negative-going pulses, they occur irregularly at a few per second with occasional clusters, and generally seem about like random radioactive events. The picture shows a particularly busy interval; mostly, nothing happens and the baseline voltage wobbles around in a low frequency rumble.
For what it’s worth, the shielding around the circuit completely eliminates not only 60 Hz interference, but everything else, too: astonishingly good results from a fairly simple layout.
Taking a closer look at one pulse:
Darl 12k – single detail
(Vigorous handwaving begins)
The tallest spikes are typically 20 mV above the baseline, corresponding to peak output current of 20 mV / 12 kΩ = 1.5 µA and a chamber current of 1.5 µA / 100×106 = 15 fA.
They’re generally 5 ms wide, which is orders of magnitude longer than the actual ion generation time, but the area under that spike should be more-or-less proportional to the area under the actual impulse.
If you grant that and agree those pulses look mostly triangular, their integral is:
1/2 x 15 fA x 5 ms = 40 fA·ms = 40 aC
That’s “a” for “atto” =10-18 = a billionth of a billionth = hardly anything at all.
Indeed, seeing as how one coulomb contains 6.2×1018 electron charges, that pulse represents 250 ion pairs, at least assuming a zero-current baseline.
Gamma rays arrive with various energies, produce ionization trails of various lengths, and don’t necessarily traverse the entire chamber, so the pulses have various heights & widths; you can see smaller pulses sticking up out of the grass in the first scope shot. Assuming all those average out to five “big” pulses every second, the chamber collector electrode passes 200 aC/s into the transistor base → 200 aA → 0.20 fA. At 1 fA per 100 µR/h, that’s 20 µR/h of gamma background.
Working from the other end of the scale, a bit of searching shows that 1 R produces 2.08×109 ion pairs in 1 cm3 of dry air at STP. The ionization chamber dimensions give the can’s volume:
π x 4.52 x 3.5 = 220 cm3
So assuming a somewhat unreasonably large pure-gamma dose of 10 µR/h in that volume will produce:
10x10-6 x 2.08x109 x 220 = 4600x103 ion pairs/h = 1300 ion pairs/s
That’s about five “big pulses” per second, under the stack of assumptions thus far, and seems absurdly close.
An old NIST report on Calibration of X-Ray and Gamma-Ray Measuring Instruments says that 1 R/s (that’s per second, not per hour) produces a current of 300 pA/cm3 in an “ideal ionization chamber”. Scaling that down to 10 µR/h and up to the chamber volume gives an average current of 180 aA. That’s absurdly close, too.
Note bene: Because 1 C = 6.241×1018 ion pairs, 2.08×109 ion pairs is 333×10-12 C and, if you do that in one second, you get 333 pA of current from your ideal 1 cm3 ionization chamber. Those two approaches should be equally close.
(Vigorous handwaving ends)
Again, I don’t trust any of the values to within an order of magnitude and surely made a major blunder in running some of the numbers, but the results seem encouraging.
The coaxial cable’s capacitance could explain why the pulses look like triangles: the capacitance integrates a rectangular current pulse into a voltage ramp. The cable measures 200 pF and the scope input adds 13 pF, but let’s call it 200 pF across the 12 kΩ emitter resistor. Raising the voltage across that capacitance by 20 mV in 2 ms requires a current of:
200x10-12 x (20 mV / 2 ms) = 2 nA
Dividing that by 100×106 gives a chamber current pulse of 20×10-18 = 20 aA: three orders of magnitude less than the original guesstimate. That suggests the (handwaved) 15 fA chamber current, amplified by the absurd gain of two stacked Darlingtons, easily drives the cable capacitance. Something else causes the ramp.
The chamber itself has 10 pF capacitance, but it’s not clear to me how (or if) that enters into the proceedings. The entire collection of ions appears in mid-air, as if by magic, whereupon the +24 V chamber bias voltage draws them (well, the positive ones, anyway) to the transistor base without appreciable voltage change.
Perhaps the triangle represents the actual arrival of the ions: a few at first from the near side of the trail, a big bunch from the main trail, stragglers from the far side, then tapering off back to the baseline.
That’s definitely more than anyone should infer from a glitch produced by a pair of transistors…
The Smell of Molten Projects in the Morning 