Fast response air temperature measurement

[John Devereux]

Hi,

I am looking for a small temperature probe with a fast response in still
air, ideally 10ms or better.

~0-100 degrees C is fine, accuracy not that critical say +/- 1
degree.

I am not having much success with google etc...

Any suggestions appreciated.

Thanks,

 

[Spehro Pefhany]

Hi,

I am looking for a small temperature probe with a fast response in still
air, ideally 10ms or better.

~0-100 degrees C is fine, accuracy not that critical say +/- 1
degree.

I am not having much success with google etc...

Any suggestions appreciated.

Thanks,

Do you really want to measure air temperature or something else?

If air, perhaps a very fine Pt RTD or thermocouple.

http://oai.dtic.mil/oai/oai?&verb=getRecord&metadataPrefix=html&identifier=ADA007284

You could also consider IR or fiber optic measurement of something
thin and low heat capacity.

http://www.gordyssensors.com/pdf/raytek_family.pdf


Best regards,
Spehro Pefhany

 

[Jon Kirwan]

I am looking for a small temperature probe with a fast response in still
air, ideally 10ms or better.

~0-100 degrees C is fine, accuracy not that critical say +/- 1
degree.

I am not having much success with google etc...

Any suggestions appreciated.

Possibly, rare earth phosphor thermometry.

Technically, if you can arrange the optics or use an appropriately
thin fiber optic cable (less than 10 micron?) you may be able to
achieve that as a physical sensor matter. I routinely do 100ms
measurements with rather thicker fibers (200 micron.) Obviously, your
source characteristics are also important in deciding just what is
good enough and what isn't. Repeatability using phosphors depends on
the instrument and method but roughly about 30mK. Accuracy can easily
be had to 500mK over that range. (There are at least four methods I
know about, each with differing specs to them.) One nice thing is
that you can literally _paint_ the surface you want to observe so that
even the fiber doesn't touch the surface and affect the temperature.
If the paint has minimal mass, it will track VERY FAST. The method is
good enough that it can be applied (and is, in fact) to IC wafers and
used to monitor their temperatures, in situ. (And you know that IC
manufacturers cannot tolerate much variation over the wafer and
certainly nothing touching it that might make a cool spot. Lamp
heating is used and the opposite side can be used to avoid changing
the emissivity of the heated side.) Usable temp ranges from maybe
-200C to maybe +500C for two or three of the methods used. Your range
is well inside this and, in fact, is just about the perfect range for
it since most everything is 'easy' there. 100 samples per second is
not impossible.

Do a search on these methods. You should see at least two companies
involved, readily. Fibers can be pretty thin. I've used 10 micron,
but I gather down to 2 micron is available.. maybe less. No real
impact on the method, so those should work. I stay active in these
areas, but not with those companies now. You may contact me
privately, if you want. I can say more, that way.

Jon

 

[Jamie]

Hi,

I am looking for a small temperature probe with a fast response in still
air, ideally 10ms or better.

~0-100 degrees C is fine, accuracy not that critical say +/- 1
degree.

I am not having much success with google etc...

Any suggestions appreciated.

Thanks,

Have you looked at Ir types?

 

[Jon Kirwan]

What's the emissivity of air?

Varies with wavelength, pressure, gas constituents (altitude affects
H20 significantly), temperature, ... well, it varies.

Not to mention that 0-100C is where most everything around is also
emitting.

Jon

 

[Spehro Pefhany]

Have you looked at Ir types?

What's the emissivity of air?


Best regards,
Spehro Pefhany

 

[John Devereux]

Do you really want to measure air temperature or something else?

Hi Spehro,

Yes, it is for actual air. The air will likely not be *completely*
still, but I don't really know what movements there will be.

If air, perhaps a very fine Pt RTD or thermocouple.

http://oai.dtic.mil/oai/oai?&verb=getRecord&metadataPrefix=html&identifier=ADA007284

That is the sort of thing I have been looking for, but all I could find
commercially (RS and Farnell etc) were bead thermocouples/thermistors
with thermal time constants in the ~1s region.

You could also consider IR or fiber optic measurement of something
thin and low heat capacity.

http://www.gordyssensors.com/pdf/raytek_family.pdf

That would be hard due to the location, I really need a probe-style
device of some sort.

Thanks,

 

[Spehro Pefhany]

Hi Spehro,

Yes, it is for actual air. The air will likely not be *completely*
still, but I don't really know what movements there will be.


That is the sort of thing I have been looking for, but all I could find
commercially (RS and Farnell etc) were bead thermocouples/thermistors
with thermal time constants in the ~1s region.


That would be hard due to the location, I really need a probe-style
device of some sort.

Thanks,

You might want to go trolling around for hot wire anemometry probes,
and run them at a low enough current that they don't heat up too much.

They can have surprisingly fast response under wind tunnel conditions,
so they might be fast enough for your application with air almost
still.


Best regards,
Spehro Pefhany

 

[Jon Kirwan]

Thermal conduction is amazingly slow, but does speed up quadratically as
you reduce the size of the object. If it's thermal-mass limited, it
speeds up more slowly since the conduction goes as surface area but mass
goes as volume, so the time constant goes as the length.

I'm there. I just failed to notice this was about measuring 'air'
when I posted. I've never done this, but temperature dependent
fluorescent properties are probably found in some gases and aerosols.
The oxygen in the air may quench it, though. Air is a tough target
for those rates.

I suppose a good thing is that it is still air. If it had a velocity,
there'd be a need a way to calibrate out that effect on most sensors
that were not moving with it at the same speed and direction.

The thermal conductivity of air is very small, only about 0.025 W/m/k,
and its volume heat capacity is also very small, so still air is a very
unpromising heat transfer medium. Doing the actual calculations and
seeing how very slow and inaccurate any solid-material heat sensor is in
still air is very educational, and not at all difficult.

Yes, I gathered that just after I posted. My mistake. It may yet be
possible in the situation. But we just don't know enough details (or
I don't, at the moment of writing this.)

Jon

 

[Jon Kirwan]

<snip about this being an 'air' measurement>

I'm there. I just failed to notice this was about measuring 'air'
when I posted. I've never done this, but temperature dependent
fluorescent properties are probably found in some gases and aerosols.
The oxygen in the air may quench it, though. Air is a tough target
for those rates.

I suppose a good thing is that it is still air. If it had a velocity,
there'd be a need a way to calibrate out that effect on most sensors
that were not moving with it at the same speed and direction.


Yes, I gathered that just after I posted. My mistake. It may yet be
possible in the situation. But we just don't know enough details (or
I don't, at the moment of writing this.)

If it wasn't clear, your point about heat transfer in still air and
mine regarding net velocity against the sensor provide a dilemma. To
get better transfer, circulate lots of air over the sensor implying
relative velocities; but to get accuracy, don't do that as the
velocity cannot be discerned vs the temperature by some sensor which
is affected by the statistics of molecular/atomic bombardment. I
remember that this presented a problem in measuring exhaust gases,
some time back, trying to discern which part was which part in what
the sensor 'observed.'

Jon

 

[John Devereux]

But, still air doesn't have a single well-defined temperature if
there's any fast temperature change going on, UNLESS it's
a matter of adiabatic heating due to compression. So, use
a fast barometer and any old slow temperature probe.

Hi,

Yes, one of the reasons for the fast temperature change is due to a
pressure change, which I am already measuring. Other reasons are the
temperature of the enclosure and of the incoming air.

Perhaps I can use the pressure changes to "predict" the temperature
changes as I think you are suggesting. I still need a fairly fast single
temperature measurement, less than 1 second. (But better than 10ms in
this scenario).

 

[John Devereux]

Perhaps a dumb idea, but since thermistors and such have a log time
response, can you take multiple measurements and curve fit to get the
temperature faster, though with less accuracy, at least until things
settle?

Sure, good idea, that might help.

Thanks,

 

[John Devereux]

This sounds like a promising idea. I've used an acoustic resonator to
measure the He3/He4 ratio in sample gas. We had to stabilze the
temperature to get good readings.

As pointed out in later posts, even with a really small detector you
still have to get the heat from the air and into the detector.
Perhaps better to measure the air directly.

Hi,

Yes it does sound do-able. Maybe a couple of transducers facing each
other across a fixed gap, track the phase changes?

 

[John Devereux]

It probably easier to treat your test volume as a Helmholtz bottle;
the resonance frequency depends on the velocity of sound in air, which
is temperature dependent.

http://en.wikipedia.org/wiki/Speed_of_sound

You can track the resonance by monitoring the phase relationship
between the excitation and the response of your resonant volume. For a
high Q resonanace the slope is pretty steep around resonance.

Hi Bill,

I did think of that, also bouncing it off one wall. Unfortunately I
don't trust the volume to be sufficiently consistent. Although if I can
use something like that to actually *measure* the volume that would be
interesting in its own right, will look it up.

Thanks,

 

[John Devereux]

The Helmholtz resonance doesn't directly depend on the speed of
sound. It's a mass-spring resonance, not an organ pipe resonance. If
you believe the ideal gas law, the density dependence should cancel
out, leaving the temperature dependence, meaning that it will work as
a thermometer.

Unfortunately it needs a closed container, so it doesn't help the OP's
problem--he really needs to do it in the open air.

Actually it *is* a closed container, but not a very rigid or accurately
defined one. (Sorry to dribble out these scraps of information, don't
mean to!)

It needs something like a pair of piezo elements separated by a known
distance, with mounts designed to reduce the conducted acoustic wave,
and an XOR phase detector. Filter and square up the received signal,
then beat it against the transmit signal, and the PD will give you an
output voltage proportional to phase, i.e. to delay, which will go as
1/T.

That was the sort of thing I had in mind (but you have thought it
through better!)

It's tempting to use a PLL and read out the frequency, which would
give the temperature directly, but that'll need a lot more calibration
work to get rid of the phase shifts of the transducers.

Cheers

Phil Hobbs

Thanks,

 

[Spehro Pefhany]

The Helmholtz resonance doesn't directly depend on the speed of sound.
It's a mass-spring resonance, not an organ pipe resonance. If you
believe the ideal gas law, the density dependence should cancel out,
leaving the temperature dependence, meaning that it will work as a
thermometer.

Unfortunately it needs a closed container, so it doesn't help the OP's
problem--he really needs to do it in the open air. It needs something
like a pair of piezo elements separated by a known distance, with mounts
designed to reduce the conducted acoustic wave, and an XOR phase
detector. Filter and square up the received signal, then beat it
against the transmit signal, and the PD will give you an output voltage
proportional to phase, i.e. to delay, which will go as 1/T.

It's tempting to use a PLL and read out the frequency, which would give
the temperature directly, but that'll need a lot more calibration work
to get rid of the phase shifts of the transducers.

Cheers

Phil Hobbs

I would look at using a single self-resonant piezo bender element
mounted in a Helmholtz cavity with a small opening to the atmosphere,
then just read the frequency off with a counter. This could be really,
really inexpensive, in fact it should work using an ordinary piezo
beeper.

 

[John Devereux]

[...]

"Whoops.  1/sqrt(T)."

That sounds better.  So at 300K a one degree temperature change will
only give you a one part per thousand phase shift? (Did I do that
right?)  If you want 10 ms ~ 100 Hz, you need a drive freq of 100
kHz?

OK don't quote me on the above.  I'm probably missing something.

Opps, cancel that, The sqaure root will only give you a factor of 1/2
in reduced sensitivity. So 1/600 change in frequency for a one degree
change at 300K. And the frequency number is only true if you wanted
to measure frequency by counting the number of cycles for 10ms.

Ah, I see where you got that 100kHz from now! I was assuming looking at
the phase all along - there will probably be less than a wavelength
between Tx and Rx anyway.

I think the PSD idea of Phil H. will let you meausure changes in times
of order one period if your signal is strong enough. If you have to
signal average....

Should be pretty strong with the transducers facing each other. Gap
would really need to be <~10mm anyway for the application.

Say John you mentioned some container. How big is it? If it's small
enough you might be able to lock on to one of the acoustic modes.

It's A few litres, but a bit variable and floppy unfortunately.

 

[Rich the Newsgroup Wacko]

Whatever sensor you go with, you can decrease the system's response time
if you can arrange to move the air across it with a fan.

Other than some exotic methods (acoustics, etc), you are actually
measuring the temperature of a sensor that has thermal mass and must
gain/lose heat to reach equilibrium with the surrounding air. The smaller
the temp difference from equilibrium, the slower that natural convection
will assist in this transfer.

Get a small (CPU cooler?) fan.

This thread should be titled, "Fast air response temperature" measurement,
i.e., "FART" meaasurement. ;-P

Cheers!
Rich

 

[Steve]

Omage sells some non-insultated thermocouples that are 36 gauge wire
fro about $5 each.

 

[John Devereux]

Omage sells some non-insultated thermocouples that are 36 gauge wire
fro about $5 each.

Thanks Steve. I looked at their site again and found some that I didn't
find before.

There is a 0.025mm one with 50ms response in still air. That may be fast
enough for what I want (since there should be *some* air movement).

<http://www.omega.com/ppt/pptsc.asp?ref=IRCO_CHAL_P13R_P10R&Nav=tema02>