Archive for July, 2015
Mary found a folding saw buried under a compost heap at Vassar Farms, where it had evidently been for quite a while. It cleaned up surprisingly well:
I made a crude brass shim to stabilize the crude blade in its crudely bent metal frame; the ugly hole came from freehand punching with the rebuilt leather punch tool. Probably spent as much time doing that as they did on the whole rest of the saw: it’s not a high-quality tool.
It could be an older version of the Harbor Freight Folding Saw, minus a fancy plastic-encased joint screw. I added a dot of Loctite to discourage this one from leaping to its doom.
As with the other pruning saws in my collection, that blade scares me just looking at it. I managed to avoid slicing myself open, although I did stab a finger with a sharp brass sliver…
From Russia, probably without love, routed through Bulgaria via eBay:
They’re glass electrometer resistors from late in the Cold War:
That one presents 100 GΩ between its lead wires, which would count as open in any other circuit I’ve ever built.
The assortment arrived much richer than advertised, although I’d be even happier with a few more 10 GΩ and a few less 100 MΩ resistors. The 1000 GΩ = 1 TΩ resistor in the upper right seems absurd on the face of it, but there it sits.
I have no way to measure these, other than to build an electrometer amp and see what happens…
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)
- 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.
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.
With 10 kHz bandwidth, the pulse rise time is:
tr = 0.34 / BW = 0.34 / 10 kHz = 34 μs
The LMC6081 has a 1 V/μs slew rate that poses no limitation at all for these tiddly signals.
That’s significantly better than the stacked Darlingtons and might be Good Enough for my simple needs.
NYSDOT added a few more scab patches to Rt 44 at the top of the hill approaching Poughkeepsie:
As you can tell, that did not improve the overall suitability for bicycling or, for that matter, for driving.
For reference, this is NYS Bike Route 44, the main route from Connecticut into the Hudson Valley.
This was not the failure mode I expected:
As failures go, that one’s survivable; slightly larger epoxy dots should do the trick:
The other temple worked loose inside the brass tube and rotated freely, so I yanked it out, bashed the tip slightly flatter, and epoxied it back in place, along with overcoating the epoxy dots on the lens to forestall another failure.
This has obviously blown right by the point of absurdity, but …
Janet Drive is across Rt 44 from the patch palimpsest I’ve already described. It serves as an entrance to two strip malls and the Canterbury Gardens apartment complex and, oddly enough, turns out to be a private road owned by Canterbury.
A huge pothole is consuming the pavement in front of the mall entrance behind the Rhinebeck Bank branch:
The light gray patches mark smaller potholes filled with what appears to be Sackrete:
The potholes made turning from Rt 44 onto Janet a bit of a challenge, particularly with drivers trying to pass on the left during the turn. We now signal for and take the entire lane from Rt 44 to the mall entrance, although impatient drivers still roar around us, directly into oncoming traffic.
Because Janet is a private drive, it doesn’t quite qualify for the Tax Dollars Asleep tag, but it gives you the general idea. The road is not signed “Private Drive” and, at least in that section, is obviously used by the general public, so it’s not at all clear what repair standards apply.