On 2020-04-06 21:35, jlarkin@highlandsniptechnology.com wrote:> On Mon, 6 Apr 2020 17:43:46 -0400, Phil Hobbs > <pcdhSpamMeSenseless@electrooptical.net> wrote: > >> On 2020-04-05 13:39, jlarkin@highlandsniptechnology.com wrote: >>> On Sun, 5 Apr 2020 12:17:03 -0400, Phil Hobbs >>> <pcdhSpamMeSenseless@electrooptical.net> wrote: >>> >>>> On 2020-04-05 11:42, jlarkin@highlandsniptechnology.com wrote: >>>>> On Sun, 5 Apr 2020 10:52:29 -0400, Phil Hobbs >>>>> <pcdhSpamMeSenseless@electrooptical.net> wrote: >>>>> >>>>>> On 2020-04-05 00:09, jlarkin@highlandsniptechnology.com wrote: >>>>>>> On Sat, 4 Apr 2020 17:49:18 -0700 (PDT), pcdhobbs@gmail.com wrote: >>>>>>> >>>>>>>> Following up on blocher's sterling work,(*) >>>>>>>> >>>>>>>> Many of us use parts off-label, often very successfully. A few >>>>>>>> examples: >>>>>>>> >>>>>>>> SAV-551+ pHEMTs make very good wideband bootstraps. Their f_max is >>>>>>>> around 12 GHz, but they're amazingly stable. >>>>>>> >>>>>>> They make excellent switches too. Rds-on is never specified for RF >>>>>>> parts, but it's about 2 ohms for the 551. >>>>>>> >>>>>>>> >>>>>>>> 74HC4352s make good flying-capacitor diff amp front ends. >>>>>>>> >>>>>>>> TMUX1511s make very nice analogue lock-ins--their Coff*Ron FOM is >>>>>>>> almost in a class with relays, but 1E5 times faster. >>>>>>>> >>>>>>>> Zero-ohm jumpers have about the right resistance to stabilize LDO >>>>>>>> regulators with ceramic output caps. (It's good to be able to >>>>>>>> disconnect the supplies during bring-up, and putting the jumper >>>>>>>> between the reg and the output cap has this additional benefit. >>>>>>>> >>>>>>>> Your faves? >>>>>>> >>>>>>> Some digital bus and USB switches are cheap and are excellent, >>>>>>> super-fast analog multiplexers. >>>>>>> >>>>>>> Some chips are useful just for their ESD diodes. >>>>>>> >>>>>>> LVDS line receivers are super-fast, dirt cheap RRIO comparators. >>>>>> >>>>>> I've used them like that at your suggestion. Snazzy if you don't need >>>>>> super low offset voltage. (And if you do, every comparator slows way down.) >>>>>> >>>>>>> >>>>>>> An LED can be used as the voltage reference for an NPN current >>>>>>> source. The tempcos can be made to almost cancel. >>>>>>> >>>>>>> High value AlN resistors can be used as thermal bridges. >>>>>> >>>>>> That's interesting. I recall discussing some very expensive parts sold >>>>>> specifically for DC isolating thermal pours. >>>>>> >>>>>>> >>>>>>> Surface-mount platinum RTDs can be used in experiments, as both >>>>>>> heater and sensor, to quantify resistor heat sinking and thermal >>>>>>> transients. >>>>>>> >>>>>>> Surface-mount resistors can be last-resort fuses. >>>>>>> >>>>>>> Low-barrier schottky diodes can be used in reverse as >>>>>>> constant-current things. Tempcos are not great. The same diode can >>>>>>> charge and discharge a capacitor. >>>>>> >>>>>> Interesting. Are they reasonably repeatable unit-to-unit? >>>>> >>>>> For modest values of reasonable. It's Is, which is huge for >>>>> schottkies, hundred nA sorts of numbers. I've posted my RF detector >>>>> which is a diode and a capacitor. It's in production. >>>>> >>>>>> >>>>>>> >>>>>>> Some self-protecting SSRs can be used as electronic fuses. As can a >>>>>>> 3t regulator with the adj pin open. >>>>>>> >>>>>>> Depletion fets are nice capacitor bleeders. Ditto 3t reg as a >>>>>>> current sink. >>>>>> >>>>>> >>>>>>> >>>>>>> A bit of open-load transmission line can be a high-frequency peaker. >>>>>>> I have that option in my GHz o/e layout. >>>>>> >>>>>> Interesting. How does that work? Normally I think of O/C tlines as a >>>>>> series resonance to ground. >>>>> >>>>> Stick a drooling-rise step into one end of a transmission line and it >>>>> will overshoot and snap up the waveform at the other end. Adjust the >>>>> source impedance, or terminate a little, to trim the step response. >>>>> It's sharper than RC peaking, so compensates things like Ft rolloff or >>>>> skin effect. >>>> >>>> Okay, so not really open-circuited. I'll try it out. That could >>>> potentially have helped that single-diode sampler gizmo--its speed was >>>> limited by the rise time of the line receiver driving the pHEMT switch. >>>> That one used a capacitor plus a short, mismatched shunt stub to make >>>> the sampling pulse from a falling edge. >>> >>> The soft peaking helps when a step has a soft corner on the rise, >>> which is very common. That's why people who do fast stuff cheat and >>> measure risetime 20/80. >>> >>>> >>>>> >>>>> I had a profound, life-changing revelation recently. If you don't poke >>>>> a fast rise into a passive transmission line, you won't get a fast >>>>> reflection. I wasted all those years designing absorptive lowpass >>>>> filters. >>>> >>>> Gasp! ;) >>>> >>>>> >>>>>> >>>>>> >>>>>>> >>>>>>> Someone said never use an opamp as a comparator. They were wrong. >>>>>> >>>>>> Gotta watch for the antiparallel diodes on the inputs, though. For slow >>>>>> stuff, LM358s work great as comparators--the inputs survive going way >>>>>> above the supply. >>>>>> >>>>>>> >>>>>>> An rro opamp can also be used as a current limiter/fuse. >>>>>>> >>>>>>> You can do all sorts of things with ribbon cable. >>>>>>> >>>>>>> Some HV diodes and the c-b junctions of some transistors make >>>>>>> awesome drift step-recovery (Grehkov) diodes. >>>>>>> >>>>>>> BFT25s can be used as fA-leakage diodes. >>>>>>> >>>>>>> Dual-winding inductors, like DRQ127, can do all kinds of tricks. >>>>>> >>>>>> Annoying that they don't tell you k in the datasheet, and you can't even >>>>>> back it out from the series-connected inductance, which is quoted as >>>>>> exactly four times the parallel-connected value. >>>>> >>>>> The k's are really high. Easy to measure. >>>> >>>> That's unusual for a 'coupled inductor'. I normally expect it to be >>>> around 0.85. >>> >>> The DRQs are bifilar. Two or three 9's. >>> >>> And a lot of capacitance. They make nice autotransformer flybacks. >>> Does that have a better name? >>> >>> >>> >>>>>>> A toroidal inductor can be a liquid level sensor. >>>>>> >>>>>> That one I haven't heard about. Are you looking for the NMR signal? ;) >>>>> >>>>> A conductive liquid is a shorted turn. >>>> >>>> At sufficiently low frequency, anyway. >>>> >>>> We've started putting little Sensirion T/H sensors in a lot of things. >>>> When using TECs, it's awfully nice to be able to compute the dew point, >>>> for instance, and in outdoor applications (e.g. our fire sensors for >>>> harvesters) it's good to be able to predict when the window is liable to >>>> fog up on the inside. >>>> >>>> We use IP67+ enclosures with bags of 5A molecular sieve inside, which is >>>> super cheap and will absorb 50% of its own mass in water. Simon has had >>>> to learn a whole lot about enclosures and mechanical design generally. >>>> Turns out that you have to put an air vent on the enclosure to prevent >>>> pumping water inside due to atmospheric pressure differences. That >>>> leads to working of the O-ring seals, which wears them out. >>>> >>>> We considered using a bellows, but atmospheric pressure varies +-7% or >>>> so, which makes for a pretty big, floppy bellows. Hermetic construction >>>> is possible but very expensive, and relies on glass or ceramic insulated >>>> connectors. The glass would have had to be brazed or indium-soldered to >>>> the lid, which leads to CTE mismatch problems. >>> >>> A balloon inside? >> >> Doesn't help--you need something to reduce the pressure drop across the >> seals. An aluminized rubber diaphragm inside the box, with one side >> vented, might work. > > I meant that it would be vented to the outside. Little plastic tube or > something.Vented might be okay. Trouble is, the gas diffusion rate in elastomers is many orders of magnitude higher than in metal or glass, so it would need to be metallized. The metal film would have a lot of cracks, of course (*), but most of the surface would still be metal.> > >> >> I made a calculation that 50 grams of 5A molecular sieve would keep the >> inside dry for about 10 years. >> >> The board has a T/H sensor on it, so it can complain if the sieve gets >> saturated. >> >> We're conformal-coating the boards, and the sensors are running at zero >> bias. Doing the latter is unusual for me, but the bandwidth only needs >> to be 200 Hz or so. > > > > I guess the seive would soak up humidity and reduce the pressure > inside, so a tiny flow through the seals would introduce a little more > humidity. Wouldn't that eventually get to zero humidity and zero > pressure differential? > > I guess atmospheric changes would still pump the system slightly.Simon's the seal expert at this point. A sufficiently stiff box, with enough screws holding the lid on, and hermetic connectors, ought to be able to stay sealed pretty well. The pressure changes are nontrivial though--our box is about 3 x 5 inches, so a 7% pressure change amounts to about 15 pounds over the surface of the lid. They're also fairly slow, so it doesn't take much of a leak rate to equalize the pressure. We're using a cable gland rather than a hermetic connector, primarily for cost reasons. I suspect that enough air will flow inside the cable to manage the vent job, but we'll probably have to measure that to find out.> > Mothballs?;) If we get critters in there, we'll certainly have other problems first. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
Favourite parts with off-label uses?
Started by ●April 4, 2020
Reply by ●April 7, 20202020-04-07
Reply by ●April 7, 20202020-04-07
On Monday, April 6, 2020 at 7:32:00 PM UTC-4, Clifford Heath wrote:> On 6/4/20 5:04 am, George Herold wrote: > > On Saturday, April 4, 2020 at 8:49:22 PM UTC-4, pcdh...@gmail.com wrote: > >> Following up on blocher's sterling work,(*) > >> > >> Many of us use parts off-label, often very successfully. A few examples: > >> > >> SAV-551+ pHEMTs make very good wideband bootstraps. Their f_max is around 12 GHz, but they're amazingly stable. > >> > >> 74HC4352s make good flying-capacitor diff amp front ends. > >> > >> TMUX1511s make very nice analogue lock-ins--their Coff*Ron FOM is almost in a class with relays, but 1E5 times faster. > >> > >> Zero-ohm jumpers have about the right resistance to stabilize LDO regulators with ceramic output caps. (It's good to be able to disconnect the supplies during bring-up, and putting the jumper between the reg and the output cap has this additional benefit. > >> > >> Your faves? > > Wow nice list! > > I'd like to take a 'part' off the alt. use list. > > I use to tout the use of 20 zeners rev biased at ~10-100 uA > > as audio noise sources. When I went to replace the pack I had > > it turned out I had a 'golden string'. I had to order a bunch > > from different suppliers to find noisy batch. (And that still wasn't as > > good(noisy) as the original... but it's obvious that almost no one wants > > a noisy zener.) > > George, I'm trying to understand this. What do you need 20 zeners for? > > FWIW, I'm currently making a broadband noise source for testing filters > up to 1.5GHz. Still scratching around for the best source to put before > a string of ERA-3 MMIC amplifiers. > > The BFR93A data sheet says abs max Vbe is 2V, but it doesn't zener at > 5V. Although avalanche zeners produce much more noise, I'd rather not > boost my 5V supply (though I might need to). I wonder how much reverse > current a microwave Shottky diodes (say HSMS-286) would survive. Abs max > peak reverse voltage is 4V, so it might withstand 5V anyway... > > Any better suggestions for a device I might have in the drawer already? > > Clifford Heath.Arghh! typo. sorry a 20V zener! Run near the knee you get these big avalanche spikes, with ~1us rise/ fall times. 1 GHz noise sounds hard. What about a spectrum analyzer and tracking generator? George H.
Reply by ●April 7, 20202020-04-07
On Monday, April 6, 2020 at 5:43:54 PM UTC-4, Phil Hobbs wrote:> On 2020-04-05 13:39, jlarkin@highlandsniptechnology.com wrote: > > On Sun, 5 Apr 2020 12:17:03 -0400, Phil Hobbs > > <pcdhSpamMeSenseless@electrooptical.net> wrote: > > > >> On 2020-04-05 11:42, jlarkin@highlandsniptechnology.com wrote: > >>> On Sun, 5 Apr 2020 10:52:29 -0400, Phil Hobbs > >>> <pcdhSpamMeSenseless@electrooptical.net> wrote:<big snip>> >> We've started putting little Sensirion T/H sensors in a lot of things. > >> When using TECs, it's awfully nice to be able to compute the dew point, > >> for instance, and in outdoor applications (e.g. our fire sensors for > >> harvesters) it's good to be able to predict when the window is liable to > >> fog up on the inside. > >> > >> We use IP67+ enclosures with bags of 5A molecular sieve inside, which is > >> super cheap and will absorb 50% of its own mass in water. Simon has had > >> to learn a whole lot about enclosures and mechanical design generally. > >> Turns out that you have to put an air vent on the enclosure to prevent > >> pumping water inside due to atmospheric pressure differences. That > >> leads to working of the O-ring seals, which wears them out. > >> > >> We considered using a bellows, but atmospheric pressure varies +-7% or > >> so, which makes for a pretty big, floppy bellows. Hermetic construction > >> is possible but very expensive, and relies on glass or ceramic insulated > >> connectors. The glass would have had to be brazed or indium-soldered to > >> the lid, which leads to CTE mismatch problems. > > > > A balloon inside? > > Doesn't help--you need something to reduce the pressure drop across the > seals. An aluminized rubber diaphragm inside the box, with one side > vented, might work. > > I made a calculation that 50 grams of 5A molecular sieve would keep the > inside dry for about 10 years.Phil, I'm not sure I understand the problem. I wonder if you could say a bit more? (I do dream of someday maybe using a sealed box...) You've got a sealed box. But pressure differences can cause the seals to fail. So you put a vent in it. but now that will pump water into your box. (Because you have molecular sieve in there and it will reduce the vapor pressure of water in the box.) So you need a long vent such that the diffusion time for H2O is long.... Is that close to stating the problem correctly? Some sort of long thin tube? George H.> > The board has a T/H sensor on it, so it can complain if the sieve gets > saturated. > > We're conformal-coating the boards, and the sensors are running at zero > bias. Doing the latter is unusual for me, but the bandwidth only needs > to be 200 Hz or so. > > Cheers > > Phil Hobbs > > > -- > Dr Philip C D Hobbs > Principal Consultant > ElectroOptical Innovations LLC / Hobbs ElectroOptics > Optics, Electro-optics, Photonics, Analog Electronics > Briarcliff Manor NY 10510 > > http://electrooptical.net > http://hobbs-eo.com
Reply by ●April 7, 20202020-04-07
On Saturday, April 4, 2020 at 8:49:22 PM UTC-4, pcdh...@gmail.com wrote:> Following up on blocher's sterling work,(*) > > Many of us use parts off-label, often very successfully. A few examples: > > SAV-551+ pHEMTs make very good wideband bootstraps. Their f_max is around 12 GHz, but they're amazingly stable. > > 74HC4352s make good flying-capacitor diff amp front ends. > > TMUX1511s make very nice analogue lock-ins--their Coff*Ron FOM is almost in a class with relays, but 1E5 times faster. > > Zero-ohm jumpers have about the right resistance to stabilize LDO regulators with ceramic output caps. (It's good to be able to disconnect the supplies during bring-up, and putting the jumper between the reg and the output cap has this additional benefit. > > Your faves? > > Cheers > > Phil Hobbs > (*) who may be bulegoge's good twin, given the similarity of their emails ;) >all the same. I do not try to be anonymous, I just try to make myself anonymous to the first level of google search.> -- > Dr Philip C D Hobbs > Principal Consultant > ElectroOptical Innovations LLC / Hobbs ElectroOptics > Optics, Electro-optics, Photonics, Analog Electronics > Briarcliff Manor NY 10510 > > http://electrooptical.net > http://hobbs-eo.com
Reply by ●April 7, 20202020-04-07
On Monday, April 6, 2020 at 2:09:59 PM UTC-4, dagmarg...@yahoo.com wrote:> On Saturday, April 4, 2020 at 8:49:22 PM UTC-4, pcdh...@gmail.com wrote: > > Following up on blocher's sterling work,(*) > > > > Many of us use parts off-label, often very successfully. A few examples: > > > > SAV-551+ pHEMTs make very good wideband bootstraps. Their f_max is around 12 GHz, but they're amazingly stable. > > > > 74HC4352s make good flying-capacitor diff amp front ends. > > > > TMUX1511s make very nice analogue lock-ins--their Coff*Ron FOM is almost in a class with relays, but 1E5 times faster. > > > > Zero-ohm jumpers have about the right resistance to stabilize LDO regulators with ceramic output caps. (It's good to be able to disconnect the supplies during bring-up, and putting the jumper between the reg and the output cap has this additional benefit. > > > > Your faves? > > > > Cheers > > > > Phil Hobbs > > (*) who may be bulegoge's good twin, given the similarity of their emails ;) > > o CMOS gates for power supplies, precision voltage switching, > class-C r.f. power amps. > > o Various semiconductor junctions as varicaps. > > o I'm still searching for a way to use crappy ceramic caps as > varactors. Tuning a WWVB loop antenna might finally be that chance.Hi James, I spent part of the weekend trying to think about crappy ceramic caps as part of a parametric oscillator. (After re-reading Pippard's "Physics of Vibration" the oscillator seems like an easier first step.) But I have no idea how to couple two different signals into/ through the capacitor. Well maybe a bridge. Do you have any clever ideas? George H.> > o LEDs as detectors. > > Cheers, > James Arthur
Reply by ●April 7, 20202020-04-07
On Tue, 7 Apr 2020 06:03:15 -0700 (PDT), George Herold <ggherold@gmail.com> wrote:>On Monday, April 6, 2020 at 7:32:00 PM UTC-4, Clifford Heath wrote: >> On 6/4/20 5:04 am, George Herold wrote: >> > On Saturday, April 4, 2020 at 8:49:22 PM UTC-4, pcdh...@gmail.com wrote: >> >> Following up on blocher's sterling work,(*) >> >> >> >> Many of us use parts off-label, often very successfully. A few examples: >> >> >> >> SAV-551+ pHEMTs make very good wideband bootstraps. Their f_max is around 12 GHz, but they're amazingly stable. >> >> >> >> 74HC4352s make good flying-capacitor diff amp front ends. >> >> >> >> TMUX1511s make very nice analogue lock-ins--their Coff*Ron FOM is almost in a class with relays, but 1E5 times faster. >> >> >> >> Zero-ohm jumpers have about the right resistance to stabilize LDO regulators with ceramic output caps. (It's good to be able to disconnect the supplies during bring-up, and putting the jumper between the reg and the output cap has this additional benefit. >> >> >> >> Your faves? >> > Wow nice list! >> > I'd like to take a 'part' off the alt. use list. >> > I use to tout the use of 20 zeners rev biased at ~10-100 uA >> > as audio noise sources. When I went to replace the pack I had >> > it turned out I had a 'golden string'. I had to order a bunch >> > from different suppliers to find noisy batch. (And that still wasn't as >> > good(noisy) as the original... but it's obvious that almost no one wants >> > a noisy zener.) >> >> George, I'm trying to understand this. What do you need 20 zeners for? >> >> FWIW, I'm currently making a broadband noise source for testing filters >> up to 1.5GHz. Still scratching around for the best source to put before >> a string of ERA-3 MMIC amplifiers. >> >> The BFR93A data sheet says abs max Vbe is 2V, but it doesn't zener at >> 5V. Although avalanche zeners produce much more noise, I'd rather not >> boost my 5V supply (though I might need to). I wonder how much reverse >> current a microwave Shottky diodes (say HSMS-286) would survive. Abs max >> peak reverse voltage is 4V, so it might withstand 5V anyway... >> >> Any better suggestions for a device I might have in the drawer already? >> >> Clifford Heath. > >Arghh! typo. sorry a 20V zener! Run near the knee you get these big >avalanche spikes, with ~1us rise/ fall times. > >1 GHz noise sounds hard. What about a spectrum analyzer and tracking >generator? > > >George H.NoiseCom sells noise zeners that go to 110 GHz. https://www.noisecom.com/products/components/nc100-200-300-400-series-chips-and-diodes You get spikes at low zener current. As current increases, it approaches symmetric gaussian noise. Here is my extensive scientific research on zener noise: https://www.dropbox.com/s/z6ozl646hfnbb2q/Zener_Noise.pdf?dl=0 But to characterize a filter, a sweep generator sounds better than noise. Noise is too, well, noisy. -- John Larkin Highland Technology, Inc Science teaches us to doubt. Claude Bernard
Reply by ●April 7, 20202020-04-07
On Tuesday, April 7, 2020 at 6:03:27 AM UTC-7, George Herold wrote:> 1 GHz noise sounds hard. What about a spectrum analyzer and tracking > generator?The ones I've seen are waveguides around a gas-filled discharge tube. Recombination noise just couples to the ports, like holding a conch shell to your ear.
Reply by ●April 7, 20202020-04-07
On Tuesday, April 7, 2020 at 2:23:52 PM UTC-4, whit3rd wrote:> On Tuesday, April 7, 2020 at 6:03:27 AM UTC-7, George Herold wrote: > > > 1 GHz noise sounds hard. What about a spectrum analyzer and tracking > > generator? > > The ones I've seen are waveguides around a gas-filled discharge tube. > Recombination noise just couples to the ports, like holding a conch shell to your ear.Rigol has a ~1 GHz SA with TG option. ~$3k? Maybe a used one? Or a boat anchor. George H.
Reply by ●April 7, 20202020-04-07
On 2020-04-07 09:15, George Herold wrote:> On Monday, April 6, 2020 at 5:43:54 PM UTC-4, Phil Hobbs wrote: >> On 2020-04-05 13:39, jlarkin@highlandsniptechnology.com wrote: >>> On Sun, 5 Apr 2020 12:17:03 -0400, Phil Hobbs >>> <pcdhSpamMeSenseless@electrooptical.net> wrote: >>> >>>> On 2020-04-05 11:42, jlarkin@highlandsniptechnology.com wrote: >>>>> On Sun, 5 Apr 2020 10:52:29 -0400, Phil Hobbs >>>>> <pcdhSpamMeSenseless@electrooptical.net> wrote: > <big snip> > >>>> We've started putting little Sensirion T/H sensors in a lot of things. >>>> When using TECs, it's awfully nice to be able to compute the dew point, >>>> for instance, and in outdoor applications (e.g. our fire sensors for >>>> harvesters) it's good to be able to predict when the window is liable to >>>> fog up on the inside. >>>> >>>> We use IP67+ enclosures with bags of 5A molecular sieve inside, which is >>>> super cheap and will absorb 50% of its own mass in water. Simon has had >>>> to learn a whole lot about enclosures and mechanical design generally. >>>> Turns out that you have to put an air vent on the enclosure to prevent >>>> pumping water inside due to atmospheric pressure differences. That >>>> leads to working of the O-ring seals, which wears them out. >>>> >>>> We considered using a bellows, but atmospheric pressure varies +-7% or >>>> so, which makes for a pretty big, floppy bellows. Hermetic construction >>>> is possible but very expensive, and relies on glass or ceramic insulated >>>> connectors. The glass would have had to be brazed or indium-soldered to >>>> the lid, which leads to CTE mismatch problems. >>> >>> A balloon inside? >> >> Doesn't help--you need something to reduce the pressure drop across the >> seals. An aluminized rubber diaphragm inside the box, with one side >> vented, might work. >> >> I made a calculation that 50 grams of 5A molecular sieve would keep the >> inside dry for about 10 years. > > Phil, I'm not sure I understand the problem. I wonder if you could say > a bit more? > (I do dream of someday maybe using a sealed box...) > You've got a sealed box. > But pressure differences can cause the seals to fail. > So you put a vent in it. > but now that will pump water into your box. > (Because you have molecular sieve in there and it will > reduce the vapor pressure of water in the box.) > So you need a long vent such that the diffusion time for > H2O is long.... > > Is that close to stating the problem correctly? > > Some sort of long thin tube? >Air pressure varies by about +-7% (~1 psi) over time scales of a day or two. Our enclosures have a volume of about 300 ml, and will be mounted on top of a harvester. It will be painted a famous dark green colour, so its albedo will be low. Thus on sunny days, it will experience temperature fluctuations of as much as 70C over time scales of hours. That will lead to fluctuations on the order of +-2 psi in a sealed enclosure. This isn't a whole lot of force, but there are all sorts of white papers from Parker and other companies that say that pumping liquid water past O-rings is a significant problem in outdoor equipment. One reason is that the pressure variations basically centre on zero, and O-ring seals really really like a single sign of pressure difference, which causes the O-rings to stay seated. In a vented 300 ml enclosure with 50g of 5A molecular sieve, we can get about 24000 cycles of 20% air exchange at a dew point of 20C before the sieve saturates, so that's many years. Liquid getting in is much worse. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
Reply by ●April 7, 20202020-04-07
"Phil Hobbs" wrote in message news:bc1dc43e-b9a1-7e91-15dc-5f851bc46c7b@electrooptical.net...> >On 2020-04-07 09:15, George Herold wrote: >> On Monday, April 6, 2020 at 5:43:54 PM UTC-4, Phil Hobbs wrote: >>> On 2020-04-05 13:39, jlarkin@highlandsniptechnology.com wrote: >>>> On Sun, 5 Apr 2020 12:17:03 -0400, Phil Hobbs >>>> <pcdhSpamMeSenseless@electrooptical.net> wrote: >>>> >>>>> On 2020-04-05 11:42, jlarkin@highlandsniptechnology.com wrote: >>>>>> On Sun, 5 Apr 2020 10:52:29 -0400, Phil Hobbs >>>>>> <pcdhSpamMeSenseless@electrooptical.net> wrote: >> <big snip> >> >>>>> We've started putting little Sensirion T/H sensors in a lot of things. >>>>> When using TECs, it's awfully nice to be able to compute the dew >>>>> point, >>>>> for instance, and in outdoor applications (e.g. our fire sensors for >>>>> harvesters) it's good to be able to predict when the window is liable >>>>> to >>>>> fog up on the inside. >>>>> >>>>> We use IP67+ enclosures with bags of 5A molecular sieve inside, which >>>>> is >>>>> super cheap and will absorb 50% of its own mass in water. Simon has >>>>> had >>>>> to learn a whole lot about enclosures and mechanical design generally. >>>>> Turns out that you have to put an air vent on the enclosure to prevent >>>>> pumping water inside due to atmospheric pressure differences. That >>>>> leads to working of the O-ring seals, which wears them out. >>>>> >>>>> We considered using a bellows, but atmospheric pressure varies +-7% or >>>>> so, which makes for a pretty big, floppy bellows. Hermetic >>>>> construction >>>>> is possible but very expensive, and relies on glass or ceramic >>>>> insulated >>>>> connectors. The glass would have had to be brazed or indium-soldered >>>>> to >>>>> the lid, which leads to CTE mismatch problems. >>>> >>>> A balloon inside? >>> >>> Doesn't help--you need something to reduce the pressure drop across the >>> seals. An aluminized rubber diaphragm inside the box, with one side >>> vented, might work. >>> >>> I made a calculation that 50 grams of 5A molecular sieve would keep the >>> inside dry for about 10 years. >> >> Phil, I'm not sure I understand the problem. I wonder if you could say >> a bit more? >> (I do dream of someday maybe using a sealed box...) >> You've got a sealed box. >> But pressure differences can cause the seals to fail. >> So you put a vent in it. >> but now that will pump water into your box. >> (Because you have molecular sieve in there and it will >> reduce the vapor pressure of water in the box.) >> So you need a long vent such that the diffusion time for >> H2O is long.... >> >> Is that close to stating the problem correctly? >> >> Some sort of long thin tube? >> > >Air pressure varies by about +-7% (~1 psi) over time scales of a day or >two. Our enclosures have a volume of about 300 ml, and will be mounted on >top of a harvester. It will be painted a famous dark green colour, so its >albedo will be low. Thus on sunny days, it will experience temperature >fluctuations of as much as 70C over time scales of hours. That will lead to >fluctuations on the order of +-2 psi in a sealed enclosure. > >This isn't a whole lot of force, but there are all sorts of white papers >from Parker and other companies that say that pumping liquid water past >O-rings is a significant problem in outdoor equipment. One reason is that >the pressure variations basically centre on zero, and O-ring seals really >really like a single sign of pressure difference, which causes the O-rings >to stay seated. > >In a vented 300 ml enclosure with 50g of 5A molecular sieve, we can get >about 24000 cycles of 20% air exchange at a dew point of 20C before the >sieve saturates, so that's many years. > >Liquid getting in is much worse. > >Cheers > >Phil HobbsRube Goldberg (or someone bored and tired of staring at the walls :-)) would suggest a spring-loaded piston with a solenoid release. Compress spring and "arm" solenoid release, seal in box with oring seals and hermetic wire feedthroughs, trigger solenoid so piston releases and raises the pressure in the box, and voila, happy orings :-). If you have to open the box for service, just rearm and fire the piston again once the box is resealed. -- Regards, Carl Ijames
