If all you need is a degree or two's accuracy near room temperature, almost any method will work. IC temperature sensors are generally fairly putrid--slow, inaccurate, and noisy.
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal ElectroOptical Innovations 55 Orchard Rd Briarcliff Manor NY 10510 845-480-2058 email: hobbs (atsign) electrooptical (period) net http://electrooptical.net
From the perspective of your book - temperature stabilization of electro- optical components - I agree completely that thermocouples should generally be avoided like fleas. From my perspective, with a background in power plant control, thermocouples and RTD's are the rule and thermistors the rare exception.
What really irks me about Omega is that their "Thermocouple Introduction and Theory" section is actually a "thermocouple introduction and completely bogus theory". Correct theory can be found at:
And better yet is another article which I can no longer find on the web, which I will post on ABSE with thread title "thermocouple theory article" in case anyone is interested.
Glen
I'm not too impressed with the Electronics Cooling article. The thermoelectric effect is treated as magic--the two integrals at the beginning of the article just integrate the magic along the length of the wire without explaining anything. There's some useful applications advice, but there's some pure nonsense, e.g. saying that 20 wire diameters' worth of lead length is enough to get a good measurement of gas temperature. The ratio of the wire's thermal conductance (in W/K) to its surface area goes as diameter/(length**2), so for a given accuracy, the required lead length goes as the square root of the wire diameter.
I agree that Omega isn't the best place to look for the actual physics of thermocouples, but they do have nice pictures. Anyway, that physics is more or less bottomless...you can stick with classical thermodynamics and use the grand canonical ensemble, but then you get into actual solid state physics and have to worry about things like the density of states differences in different crystal orientations, and then you get into the real quantum mechanics of disordered systems stuff. As I said, bottomless, and although I talk a good game, my actual solid state physics expertise goes about ankle deep. (I did take graduate solid state from Walt Harrison, who is the biggest wildman in all of theoretical sold state physics, but didn't pay enough attention. Same with graduate statistical mechanics.)
I'm not the worst offender, though. A lot of the explanations you hear about physics have as much merit as the Friday afternoon stock market guy 'explaining' what happened on Wall Street that week. The idea that an electret mic is a variety of capacitance mic is one example, and almost anything mentioning 'surface states' and 'traps' is another. It's not that traps and surface states don't exist, but they're very commonly used as a cloak for ignorance--understandably, since real solid state measurements are hard, and tend to involve ultrahigh vacuum.
All of that said, for instrument purposes delving into the fine details of thermocouples is putting lipstick on a pig. ;)
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal ElectroOptical Innovations 55 Orchard Rd Briarcliff Manor NY 10510 845-480-2058 email: hobbs (atsign) electrooptical (period) net http://electrooptical.net
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dHmm, that link sends me to "Microthermal imaging in the infrared" Nothing about thermal couples?
George H.
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That didn't help, but I searched the site for "thermocouple" and found it near the end of the list.
George H.
Take a look at this month's (or maybe last month's) Linux Journal.
It has an article in it about data logging and controlling a fridge from Linux using a Linux embedded device meant for an entirely different purpose.
OK, I'll bite: What's wrong with explaining electrets as permanently polarized capacitance mics? Are you saying that this is *not* the basic concept, or that it just doesn't go deep enough to do justice?
Like the various "electricity as a flowing liquid" analogies, sometimes a flawed analogy can nevertheless help get a basic concept across... as long as it's clear that it is an analogy and not an equivalence.
Best regards
Bob Masta DAQARTA v5.10 Data AcQuisition And Real-Time Analysis
If you have a surface with a nonzero net charge density on its surface (so that it produces an electric field in the air), a small current will flow due to air ions and surface water films. Therefore no object can produce an external field forever without a power source.
Electrets are just poled piezoelectrics.
Cheers
Phil Hobbs
When you
-- Dr Philip C D Hobbs Principal ElectroOptical Innovations 55 Orchard Rd Briarcliff Manor NY 10510 845-480-2058 email: hobbs (atsign) electrooptical (period) net http://electrooptical.net
On a sunny day (Sat, 04 Dec 2010 09:57:04 -0500) it happened Phil Hobbs wrote in :
A current can *only* flow if that object itself is a conductor.
Piezo mikes use the BENDING of a piezo crystal to generate voltage, An electret is in no way related to a piezo mike, if that is what you are implying.
Electret is much more like a capacitor mike, and that also goes for impedance. The old piezo mikes were not very good, maybe speech only, because the forces needed to bend the crystal., non linearity. The electret can give extremely high quality, as hardly any force is needed to move the membrane.
There were piezo pick up elements for vinyl record players, piezo mikes, piezo 'crystal' earphones, most of these later replaced by much more 'HiFi' dynamic stuff.
And 'forever' is something that modern electronics tries to avoid, for sales reasons I suppose. That said I have seem 30 year old electrets working. Considering the 100 years for most FLASH based firmware, I'd say electrets last forever.
It is like your view on LDRs, you probably have never used one.
Well, that was masta-fooly stated...
What goes up, must come down...
Spinning wheels, got to go 'round...
with ubuntu linux.
time.
Here is one way...
Another is to buy a cheap $30 Harbor Freight multimeter that has a serial or USB port on it, and then hack at the output streams from within Ubuntu, if there is no actual Linux app.
Usually, there are only windows applets for that stuff, but you could run that in a window within Ubuntu as well. If DOS applets are available, you could then simply use DOSBox, if it can see the serial or USB ports.
Resistor bolometer 2 mm sq, 2 mm behind probably a Ge window (in a little to-39 pkg) at whatever needed distance behind a half inch diameter plastic Fresnel lens with some nice read circuitry and LCD display with read and hold mode, etc., and a little laser focal point spotter.
Pretty damned good accuracy from every test I could put it through, from new batteries, all the way down to both cells being dead... the damned thing reads. (obviously the data cell is not completely dead at that point)
$20 at Harbor Freight.
Air _is_ a conductor, just not a very good one. It contains mobile ions and electrically charged dust. A positively charged surface will attract negative ions and repel positive ones, which means that a small current flows. You don't need much charge to neutralize any plausible surface charge density. Charge 10 pF to 50 volts--neutralizing it takes
14 fA for an hour, or 0.19 fA for a month. We're talking resistances of the order of 200 petaohms (300 million gigohms) to keep it charged for a month. Do the arithmetic.The only reason permanent magnets don't have the same problem is that there are no magnetic monopoles in nature. If there were a lot of those about, they'd all stick to the ends of a magnet and neutralize the external B field in just the same way that electrons and ions do for the E field in electrets.
implying.
Wrong, sorry--YCLIU. They're made of poled PVDF, which is both piezoelectric and pyroelectric. I've used it a fair amount myself--my Footprints sensors use 9-um PVDF. Piezoelectric response is a tensor quantity--for instance when you put a voltage across a normal AT-cut quartz crystal, it produces a transverse shear (i.e. the cross-section of the plate becomes rhombic in one axis). There's no bending involved. Tuning fork crystals do use bending, but for a general piezoelectric material, any coefficient of strain can produce an E field in any direction. Amorphous or polycrystalline materials like PVDF have simpler behaviour than single crystals.
needed to bend the crystal.,
move the membrane.
Sure, but that has nothing to do with the physics, which isn't at all hard--about eighth grade level, i.e. rubbing balloons on your hair or using a van de Graaf generator.
'crystal' earphones,
The old crystal and ceramic mics used Rochelle salt or PZT or barium titanate or that sort of thing. They have lots of mechanical resonances because they're stiff and heavy compared with a 9-micron PVDF film. Rochelle salt is also fragile and hygroscopic, so it takes some babying if you want decent life.
reasons I suppose.
last forever.
If they don't get hot, their internal polarization lasts many years. My
10-year-old Footprints sensors still work. But I was talking about the external field, i.e. in the air, which decays in a few minutes at most, just like when you stick a balloon to the ceiling after rubbing it on your hair. Capacitance mics work by modulating the plate separation of an air-dielectric capacitor that is held at constant voltage. No bias ==> no change in CV with plate separation ==> no current flow ==> no signal. If you rip an electret mic apart, you'll find that it is metallized on both sides, and the signal is taken between the plates.It really isn't a capacitance microphone, although its audio characteristics are somewhat similar. The plates don't move together, and there's no net field anyway, even if they did. The physics is the change in the electric polarization due to strain in the material--i.e. a poled piezoelectric.
But you demonstrate my point that the stockmarket-style explanation refuses to die. (It was all due to profit taking/program trading/unwinding short positions/uncertainty about leading economic indicators....)
;)
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal ElectroOptical Innovations 55 Orchard Rd Briarcliff Manor NY 10510 845-480-2058 email: hobbs (atsign) electrooptical (period) net http://electrooptical.net
On a sunny day (Sat, 04 Dec 2010 12:32:34 -0500) it happened Phil Hobbs wrote in :
I do not think that is correct.
Some of those things are pretty useful--microbolometers have come a really long way. Consistency is not the same as accuracy, though, and all sensors relying on radiation are (a) vulnerable to emissivity variations, and (2) slow, at least compared to an RTD or thermistor (or thermocouple).
Temperature control lives and dies by loop bandwidth, just like every other control system. Slow sensors ==> poor control. Inaccurate sensors ==>poor control. Fast, accurate sensors plus intelligent sensor placement, insulation to reduce thermal forcing, thermal grounding of leads, .... ==>good control.
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal ElectroOptical Innovations 55 Orchard Rd Briarcliff Manor NY 10510 845-480-2058 email: hobbs (atsign) electrooptical (period) net http://electrooptical.net
it does *NOT MOVE AT ALL*.
EITHER*.
course.
We're now in violent agreement, I see. I didn't mean "move in the same way", but "move closer together", i.e. the spacing doesn't change, so it can't be a capacitance mic. Clumsily put, admittedly, but the rest of the argument should have removed the ambiguity. Electret mics are not capacitance mics.
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal ElectroOptical Innovations 55 Orchard Rd Briarcliff Manor NY 10510 845-480-2058 email: hobbs (atsign) electrooptical (period) net http://electrooptical.net
IR sensors are faster, and that includes a bolometer. There is zero settling time, and emissivity will not be a factor, because just like your sensor, this would be "placed" the same every time. Accuracy is dead on, if it was calibrated right in the first place as the circuitry is usually VERY linear and very accurate if any linearization corrections are needed, they are usually hard wired in. I'll bet that it even has ambient compensation built into a single custom MCU/do-it-all chip. Things we had to engineer in with discreet components back when the finished product was $500.
Well, it was precision lab instrumentation outputs. All the Harbor Freight item has is a readout.
I'll bet that it can be relied on to plus or minus 0.3 degrees though.
They range from about 1.5 us to about half a second in response time.
It probably takes a typical TC junction a tenth of a second to settle through with a 'bead' size of about .75 to 1 mm.
with ubuntu linux.
time.
They can have long soak times, lengthening response time. If that is a factor.
IR is much faster and just as accurate and repeatable, and especially so if the usage is meant to look at the same target constantly.
Why folks shy away from IR thermometry is beyond me.
Radiation coupling is very poor at room temperature--a vacuum gap between two surfaces of unit emissivity is equivalent to the thermal conductivity of about 5 mm of air. (I once had occasion to calculate that for a sensor design.) That means that the same sensor in intimate contact with the given surface would be at least an order of magnitude faster, and probably two orders.
Optical pyrometry also doesn't work too well through insulation. ;)
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal ElectroOptical Innovations 55 Orchard Rd Briarcliff Manor NY 10510 845-480-2058 email: hobbs (atsign) electrooptical (period) net http://electrooptical.net
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