# favorite temp sensors

J

#### John Devereux

Jan 1, 1970
0
John Larkin wrote:
[...]
I was pricing some 16 bit SPI DAC's and I thought 55 bucks for an 8 pin
chip was a little steep. So I can imagine what a 24 bit ADC cost...

We're paying around a buck for eight channel, 24-bit, differential
diagnostics. Delta-sigma audio stuff is really cheap.

I've thought of trying audio parts like this (for non-audio) but the
specs always look weird, as does the interface, and I end up with more
expensive "industrial" parts. Usually one is interested in what happens
at DC which audio does not care about of course.

G

#### George Herold

Jan 1, 1970
0
Hmm, if emission coefficients go as high as four then it is somehow
different from the non-ideality factor which is between 1 and 2.

They go as high as needed for diodes. For BJTs, they are
almost always very close to 1, at least with small signal
types I'm more familiar with, testing out against hand
calculuations. I think they are the same thing, as I've seen
both terms used interchangeably and I don't know of another
meaning for the term you are using.

Ic = Is*(e^(Vbe/[n*k*T/q]) - 1)

With 'n' being your ideality thing. Elsewhere, that's just
called the emission coefficient. You can see it called
'ideality' here, under the "Shockley diode equation"

http://en.wikipedia.org/wiki/Diode#Shockley_diode_equation

But if you read that paragraph, you will also see it called
what I call it, as well. So I really do think we are talking
about the exact same thing. If not, you need to show me the
equation you are talking about and the factor in it.

The emission coefficient isn't just about recombination, by
the way. It hides several things under a single umbrella,
including carrier generation.

And as if that wasn't enough, things are dramatically
different at different currents, as well. At low currents,
you have recombination of carriers AT THE SURFACE, which must
be accounted for. You have recombination of carriers in the
emitter base space charge layer. And you have the formation
of emitter base surface channels. Each of these (only
ideally, though again) do vary with Vbe and with different
tau's and added to the other shockley equation. At higher
currents the injection of minority carriers into the base
region starts to become significant, relative to the majority
carrier concentration. Since space charge neutrality is
maintained, the total majority carrier concentration
increases by the same amount. The non-ideality factor for
this is usually taken to be 2 and is incorporated into the
EM3 model (by Webster, I believe.) This becomes another one
of those exponentials sitting as a divisor on Is.

As mentioned, Rube Goldberg would be jealous. The underlying
physics is NOT modeled, but instead just higher level
exponential math models applied until the behavior starts to
look "mostly okay." Behavior in the corners is modeled very
badly, for example, because the various exponentials don't
reflect the actual shape there very well. It's good enough
for horseshoes, though, and engineers. Not for physicists.

Grin, sure I'm all about models being close to the real physics.

Well, the spice stuff isn't very physical. If you look at the
differential equations you find that there are multiple
exponentials. The -1 term in the Shockley equation just
mentioned is there so that you calculate out a nice Ic=0 when
Vbe=0. The exponentials are only approximations of a reality
that includes MANY taus, not just one. And as you know well,
there is no single exponential that can model the sum of
multiple ones. And there are some non-exponentials that
result from the integral equations that basically no one in
the non-FAB land bothers to use. I got myself deep into this
some time ago when attempting to model the behavior of a
Hamamatsu diode against actual observation over differing die
temperatures (from -70C to about 80C) and over light flux
varying from femptoamps to microamps. The ONLY way I got
results that were close was to go back to applying dopant
concentrations, gradients, and a vague approximation (luckily
not too hard with photodiode I can examine) of the physical
dimensions. I had thought that a 1D model would be good
enough. It wasn't.

I can see why spice doesn't go there.

For the single point calibration, I measured three diodes, Vf (at
10uA) vs T.  The three diodes were picked to have high, average, and
low forward voltages at room temp.  I then found that the voltage
difference as a function of temp was mostly linear with T from 77 to
400K.  So with a standard curve*, I just measure Vf and T and do a
linear correction on all the points.
The temp points are all based on a ~$400 lakeshore calibrated diode. George H. *the standard curve is the data from the middle diode. Okay. So you had a calibrated diode to work with. I guess my question came from the fact that while the 1X/10X method provides some basic removal of the Is parameter for a given device, it does not (so far as my poor awareness permits me) provide an absolute position which probably has to come from somewhere external. Apparently, you had that. But if you had a method to develop an exact calibration down to absolute zero without any calibration, then I'd love to have learned of it. (So would a lot of people with expensive freeze points, I suppose.) Jon- Hide quoted text - - Show quoted text -- Hide quoted text - - Show quoted text -- Hide quoted text - - Show quoted text - Got it Jon, Thanks for all that! Re: absolute T calibration. The only way I know to do that is by the Johnson noise. There is some group at NIST that is trying to do that. (OK I'm sure there are a slew of other fundamental ways to get T... but the noise method can get you ~1% with out too much sweat.) George H. K #### [email protected] Jan 1, 1970 0 I don't think I need 24 bits. I will be returning to work tomorrow and need to call the person that needed this device to get more details. No matter what they claim, you're not going to get anywhere close to 24bits out of a "24 bit" audio DAC, either. They ones above are rated at something around 100dB THD + N, so that's around 17 bits. "24=bit DAC" sounds good, though. ;-) This is going to be a hand held device that needs to generate a clean and high res output analog signal from a laptop's USB port. The laptop is going to define the generated data, also the unit needs 2 outputs and 2 inputs.. This is all going to terminate to a modular plug. This will be a QC calibration tool so we must make sure we are using a good DAC. I think I'll be using a ladder dac, I see issues using DS types. It seems you've just scratched the surface of the real problem. J #### Jon Kirwan Jan 1, 1970 0 Got it Jon, Thanks for all that! Re: absolute T calibration. The only way I know to do that is by the Johnson noise. There is some group at NIST that is trying to do that. (OK I'm sure there are a slew of other fundamental ways to get T... but the noise method can get you ~1% with out too much sweat.) That's pretty fundamental. And it applies to the kTC noise in capacitors. I suspect that Johnson's success at applying Boltzmann's theory here may have given the mental analog to Shannon for his excellent paper (which is how I actually felt, finally, a better understanding of Boltzmann's work -- which wasn't nearly so easy to follow in my opinion.) I haven't read, but probably should, Johnson's work. I'd probably benefit from it. Shot noise (say, across a PN junction for example) would be another method, I suppose. There is an interesting page from NIST on Johnson noise thermometry. But sadly, it's more for those already familiar with it and wanting to know the more recent advances, than those wanting to know good Johnson noise thermometry itself can be: http://www.nist.gov/pml/div685/grp01/jnt.cfm They claim 40 microKelvin per Kelvin combined uncertainty using AC Josephson quantized voltage noise sources and using a Johnson noise thermometer, at 693 K, or about 28mK at that point. But I think their aim isn't to measure temperature, but to measure k to good accuracy. http://physicsworld.com/cws/article/news/2008/jun/04/electrical-noise-measures-boltzmann-constant http://iopscience.iop.org/1367-2630/13/7/073028 http://arxiv.org/abs/1012.4181 http://christian.j.borde.free.fr/darquie_proc_ICAP2012 (3).pdf How long do you take in using that method? I would guess from basic statistics guesswork on my part that it would "take a long time" in order that the standard deviation of the resulting noise measurements reaches a sufficient level that the measurement is well known. Uncertainty goes as sqrt(N). I imagine a good measurement takes a while. Not to mention the difficulty of separating out actual Johnson noise from other systematic noise sources in the rest of the measurement chain. And not to mention over what temperatures you might get values you can "see" in the end with much accuracy. I found these with a quick google search, too: http://web.mit.edu/8.13/www/JLExperiments/JLExp43.pdf http://www.fiziks.net/lifesciencesE/exp63.htm http://msl.irl.cri.nz/research/temperature-and-humidity/johnson-noise-thermometry There is an interesting paper called "Continuous Resistance Temperature Detector Calibration using Johnson Noise Thermometry." It points out some reasons why Johnson noise thermometry isn't widely used. (1) long cables have capacitance that alters the transmitted noise and the need to periodically measure the cable transfer function, (2) the thermal signal is small and easily contaminated by EM and microphonics, (3) a need for DSP to reject band limited noise (the shape of the Johnson noise is known a priori), (4) stability requirements for high gain, wide bandwidth. But I'm not familiar with the technique, never having been around anyone doing it. So any experience you have here would be very interesting to read about. Jon N #### Neon John Jan 1, 1970 0 RTDs are very accurate and stable and are usable over a wide temp range, things that termistors aren't. Thinfilm platinum RTDs are fairly cheap, too, much cheaper than a thermistor of similar accuracy. John, Can you recommend a product? I'm currently using a 10k thermistor mounted in a TO-220 case to measure the temperature of my heat sinks. I'd love to use an RTD but I can't imagine finding one less than the$1.50 or so we pay for the thermistors.

John
John DeArmond
http://www.neon-john.com
http://www.fluxeon.com
Tellico Plains, Occupied TN

J

#### josephkk

Jan 1, 1970
0
I don't think I need 24 bits. I will be returning to work tomorrow and
need to call the person that needed this device to get more details.

This is going to be a hand held device that needs to generate a clean
and high res output analog signal from a laptop's USB port. The laptop
is going to define the generated data, also the unit needs 2 outputs and
2 inputs.. This is all going to terminate to a modular plug.

I don't think a modular plug and jack is going to cut it for even 16 bits
for very long. For one they have rather low mating cycle ratings, like in
the 100s. 3.5 mm "stereo" jacks used as differential pair with shield per
channel would be better, but not a whole lot; still not long life. I
really just don't know of a small, good life connector with the density
you want.

B

#### Bill Sloman

Jan 1, 1970
0
I don't think I need 24 bits. I will be returning to work tomorrow and
need to call the person that needed this device to get more details.

The "24-bit" parts aren't actually 24-bit - usually closer to 20-bit - but they tend to deliver 24-bit data with the last few bits essentially random noise.

You can trade off fewer bits for higher conversion speeds, but for temperature measurement you rarely need speed,
This is going to be a hand held device that needs to generate a clean
and high res output analog signal from a laptop's USB port. The laptop
is going to define the generated data, also the unit needs 2 outputs and
2 inputs.. This is all going to terminate to a modular plug.

This will be a QC calibration tool so we must make sure we are using
a good DAC. I think I'll be using a ladder dac, I see issues using DS types.

Sigma-Delta DACs rely on a particularly sophisticated form of pulse-width modulation. Their advantage is that it's a lot easier to set up really accurate time intervals than it is to make really accurate resistor arrays.

The disadvantage is that you've got to filter the PWM modulated output to get rid of the ripple.

G

#### George Herold

Jan 1, 1970
0
That's pretty fundamental. And it applies to the kTC noise in
capacitors. I suspect that Johnson's success at applying
Boltzmann's theory here may have given the mental analog to
Shannon for his excellent paper (which is how I actually
felt, finally, a better understanding of Boltzmann's work --
which wasn't nearly so easy to follow in my opinion.) I
haven't read, but probably should, Johnson's work. I'd
probably benefit from it. Shot noise (say, across a PN
junction for example) would be another method, I suppose.

Hmm the PN diode is a bit of a pain to do shot noise with.
It's doable, but as you forward bias it, it's resistance changes and
that throws a bit of a monkey wrench into the measurment.

Doing shot noise with a photodiode is a lot easier. But you don't get
the temperature from it.
(In^2 = 2*e*Iave*bandwidth)
There is an interesting page from NIST on Johnson noise
with it and wanting to know the more recent advances, than
those wanting to know good Johnson noise thermometry itself
can be:

http://www.nist.gov/pml/div685/grp01/jnt.cfm

OK I know absolutely nothing about the QVNS part of that.

But nist is doing ~10^6 (or 10^5) type of measurements.
(there is this paper too,
arxiv.org/pdf/1101.0312)

So I'm guessing the QVNS gives them a way to calibrate the gain/
bandwidth of their signal chain.

They claim 40 microKelvin per Kelvin combined uncertainty
using AC Josephson quantized voltage noise sources and using
a Johnson noise thermometer, at 693 K, or about 28mK at that
point.

But I think their aim isn't to measure temperature, but to
measure k to good accuracy.

http://physicsworld.com/cws/article...j.borde.free.fr/darquie_proc_ICAP2012 (3).pdf

How long do you take in using that method? I would guess from
basic statistics guesswork on my part that it would "take a
long time" in order that the standard deviation of the
resulting noise measurements reaches a sufficient level that
the measurement is well known. Uncertainty goes as sqrt(N). I
imagine a good measurement takes a while. Not to mention the
difficulty of separating out actual Johnson noise from other
systematic noise sources in the rest of the measurement
chain. And not to mention over what temperatures you might
get values you can "see" in the end with much accuracy.

I found these with a quick google search, too:

http://web.mit.edu/8.13/www/JLExper...search/temperature-and-humidity/johnson-noise...

There is an interesting paper called "Continuous Resistance
Temperature Detector Calibration using Johnson Noise
Thermometry." It points out some reasons why Johnson noise
thermometry isn't widely used. (1) long cables have
capacitance that alters the transmitted noise and the need to
periodically measure the cable transfer function, (2) the
thermal signal is small and easily contaminated by EM and
microphonics, (3) a need for DSP to reject band limited noise
(the shape of the Johnson noise is known a priori), (4)
stability requirements for high gain, wide bandwidth.

But I'm not familiar with the technique, never having been
around anyone doing it. So any experience you have here would

OK, the Johnson noise part is pretty easy to understand.

Vn^2 = 4*k*T*R*bandwidth.
So you gain up a noise signal from a resistor. (with opamps and 0.1%
resistors.)
You can get the square of the voltage either by measuring a voltage
time record and doing the multiplication in software,
or with an analog multiplier.
You then need to measure (or constrain) the bandwidth.
Measuring the resistor value is pretty easy.
And then you need the amplifier noise, which I do by reducing the
input resistor to a few ohms.

Getting the bandwidth is the squishiest part.
Our 'brute force' approach is to make the bandwidth flat out to ~1
MHz,
and then use a two pole low pass at 100kHz and lower frequencies.
This throws away ~90% of the noise signal.

(I think I read somewhere that the nist group has an 11 pole low pass
that they somehow calibrate)

Re: the time to make a measurment. If the badnwidth is 100kHz, you
can think of this as a new noise measurement every 10us.
So in one second, you've made 10^5 measurments with an uncertainty of
1/sqrt(10^5) or less than 1%.

So getting to 1% is not that hard, but that's 3 degree's K at room
temp.

So the four points you listed are all relevant. The cable capacitance
is perhaps the biggest constraint.
Our original prototype had an active shield (bootstrap) that could
reduce the cable C by ~90% or so*...
but it was deemed 'a step too far' for the students to understand and
was axed.
The other thing I haven't mentioned is amplifier current noise...
which we ignore at the start by using FET opamps.

I just got done testing on of these,
http://www.teachspin.com/instruments/noise/index.shtml
I take johnson noise data for a 10k ohm resistor at room temp for a
number of bandwidths as a final test.
So here's some data.. (Have I mentioned that I love data?)

T = 295.93K
expected noise density 1.634E-16 V^2/Hz

bandwidth measured density
(Hz) (xE-16 V^2/Hz)
100k 1.648
33k 1.645
10k 1.633
3.3k 1.650 (+/- 1%)
1k 1.627 (+/- 2%)

(errors from a 3 second measurment time.)

Not any better than the transistor connected diode and 10x current
games.

George H.
(* you could gain up the active shield bootstrap a bit to get a
'better' looking spectrum,
but then the bootstrap gain knob could be subject to misuse... "how
much noise do you want"?)

J

#### Jon Kirwan

Jan 1, 1970
0
Hmm the PN diode is a bit of a pain to do shot noise with.
It's doable, but as you forward bias it, it's resistance changes and
that throws a bit of a monkey wrench into the measurment.

Doing shot noise with a photodiode is a lot easier. But you don't get
the temperature from it.
(In^2 = 2*e*Iave*bandwidth)

OK I know absolutely nothing about the QVNS part of that.

But nist is doing ~10^6 (or 10^5) type of measurements.
(there is this paper too,
arxiv.org/pdf/1101.0312)

So I'm guessing the QVNS gives them a way to calibrate the gain/
bandwidth of their signal chain.

OK, the Johnson noise part is pretty easy to understand.

Vn^2 = 4*k*T*R*bandwidth.
So you gain up a noise signal from a resistor. (with opamps and 0.1%
resistors.)
You can get the square of the voltage either by measuring a voltage
time record and doing the multiplication in software,
or with an analog multiplier.
You then need to measure (or constrain) the bandwidth.
Measuring the resistor value is pretty easy.
And then you need the amplifier noise, which I do by reducing the
input resistor to a few ohms.

Getting the bandwidth is the squishiest part.
Our 'brute force' approach is to make the bandwidth flat out to ~1
MHz,
and then use a two pole low pass at 100kHz and lower frequencies.
This throws away ~90% of the noise signal.

(I think I read somewhere that the nist group has an 11 pole low pass
that they somehow calibrate)

Re: the time to make a measurment. If the badnwidth is 100kHz, you
can think of this as a new noise measurement every 10us.
So in one second, you've made 10^5 measurments with an uncertainty of
1/sqrt(10^5) or less than 1%.

So getting to 1% is not that hard, but that's 3 degree's K at room
temp.

So the four points you listed are all relevant. The cable capacitance
is perhaps the biggest constraint.
Our original prototype had an active shield (bootstrap) that could
reduce the cable C by ~90% or so*...
but it was deemed 'a step too far' for the students to understand and
was axed.
The other thing I haven't mentioned is amplifier current noise...
which we ignore at the start by using FET opamps.

I just got done testing on of these,
http://www.teachspin.com/instruments/noise/index.shtml
I take johnson noise data for a 10k ohm resistor at room temp for a
number of bandwidths as a final test.
So here's some data.. (Have I mentioned that I love data?)

T = 295.93K
expected noise density 1.634E-16 V^2/Hz

bandwidth measured density
(Hz) (xE-16 V^2/Hz)
100k 1.648
33k 1.645
10k 1.633
3.3k 1.650 (+/- 1%)
1k 1.627 (+/- 2%)

(errors from a 3 second measurment time.)

Not any better than the transistor connected diode and 10x current
games.

George H.
(* you could gain up the active shield bootstrap a bit to get a
'better' looking spectrum,
but then the bootstrap gain knob could be subject to misuse... "how
much noise do you want"?)

Thanks, George. I'm keeping this for later, to think over.

The one question remaining to me is your statement, "Not any
better than the transistor connected diode and 10x current
games." We were discussing absolute T calibration, I thought.
But the 1x/10x games are about knowing, a priori, the step
size. Different things. I must remain confused, I guess.

Anything to enlighten that would help.

Thanks again,
Jon

G

#### George Herold

Jan 1, 1970
0
Thanks, George. I'm keeping this for later, to think over.

The one question remaining to me is your statement, "Not any
better than the transistor connected diode and 10x current
games." We were discussing absolute T calibration, I thought.
But the 1x/10x games are about knowing, a priori, the step
size. Different things. I must remain confused, I guess.

Anything to enlighten that would help.

Oh nothing mysterious, both are based on some physics equation that
contains kT. With some electronics you can use either to measure kT
to ~1%.
I guess if you want to do better you can use the triple point of water
as a reference... electronics and water never seem to mix to well
though.
(Hmmm.. I could make an ice bath in my dewar and try it, could I stick
a resistor/diode in a deionized water ice bath and not have them 'wig
out'?)

OK what other 'fundamental' ways are there to measure T?
Thermocouples,
resistance of metals, (and Pt in particular)
speed of sound (in air) of an acoustic resonantor,
....others?

George H.

B

#### Bill Sloman

Jan 1, 1970
0
Oh nothing mysterious, both are based on some physics equation that
contains kT. With some electronics you can use either to measure kT
to ~1%.

I guess if you want to do better you can use the triple point of water
as a reference... electronics and water never seem to mix to well
though.

(Hmmm.. I could make an ice bath in my dewar and try it, could I stick
a resistor/diode in a deionized water ice bath and not have them 'wig
out'?)

Most of the authorities talk about a "well-stirred" ice bath. If you've gota propellor-on-a-shaft type stirrer, you can stir the contents of your Dewar flask easily enough, but most places I've been rely on Teflon coated magnets spun by an external magnetic field, and those are hard to get spinningin a Dewar flask.

The latent heat of fusion of ice is quite high, and an ice-bath in a open beaker lasts for quite a while.

<snip>

G

#### George Herold

Jan 1, 1970
0
I read somewhere that well-stirred water and crushed ice, using most
any tap water, gets within 15 mK of 0C.

I think the serious triple-point cells close a loop on electrical
conductivity.

--

John Larkin         Highland Technology, Inc

jlarkin at highlandtechnology dot comhttp://www.highlandtechnology.com

Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom laser drivers and controllers
Photonics and fiberoptic TTL data links
VME thermocouple, LVDT, synchro   acquisition and simulation- Hide quoted text -

- Show quoted text -

Oh I was more worried about the electrical conductivity (or something
else?) of the water. The last time I tried sticking resistors and
diodes in tap water it didn't work, I'm not sure why.

George H.

G

#### George Herold

Jan 1, 1970
0
Most of the authorities talk about a "well-stirred" ice bath. If you've got a propellor-on-a-shaft type stirrer, you can stir the contents of your Dewar flask easily enough, but most places I've been rely on Teflon coated magnets spun by an external magnetic field, and those are hard to get spinning in a Dewar flask.

The latent heat of fusion of ice is quite high, and an ice-bath in a openbeaker lasts for quite a while.

<snip>

My probe is 'open to the elements', in direct contact with the water.

George H.

B

#### Bill Sloman

Jan 1, 1970
0
Oh I was more worried about the electrical conductivity (or something
else?) of the water. The last time I tried sticking resistors and
diodes in tap water it didn't work, I'm not sure why.

The last tap water I measure had a conductivity of 300 usiemens/cm, which is high enough to be signifcant.

De-ionised water is a couple of orders of magnitude better, but atmosphericCO2 dissolves in it to give you more ions than you'd expect from a pH of 7..

I don't know how much the dissolved salts in tap-water lower the melting point.

http://www.isotech.co.uk/files/document_library_file-50.pdf

includes a discussion of more subtle influences, such as atmospheric pressure and isotope levels.

B

#### Bill Sloman

Jan 1, 1970
0
52:03 -0700 (PDT), George Herold
On Mon, 15 Apr 2013 06:21:27 -0700 (PDT), George Herold

The last tap water I measure had a conductivity of 300 usiemens/cm, whichis high enough to be significant.

De-ionised water is a couple of orders of magnitude better, but atmospheric CO2 dissolves in it to give you more ions than you'd expect from a pH of7.

I don't know how much the dissolved salts in tap-water lower the melting point.

It looks as if freezing point depression in water is about -1.86K per mole.

Tap water seems to have anything from 0.001 to 0.003 moles of dissolved salts - mostly magnesium and calcium carbonates - which would depress the freezing point by between 2 and 5mK.

G

#### George Herold

Jan 1, 1970
0
The last tap water I measure had a conductivity of 300 usiemens/cm, whichis high enough to be signifcant.

De-ionised water is a couple of orders of magnitude better, but atmospheric CO2 dissolves in it to give you more ions than you'd expect from a pH of7.

I don't know how much the dissolved salts in tap-water lower the melting point.

http://www.isotech.co.uk/files/document_library_file-50.pdf

includes a discussion of more subtle influences, such as atmospheric pressure and isotope levels.

Yeah I was thinking that maybe if I poured some liquid nitrogen into a
dewar with DI water in the bottom, that I could make a DI ice/water
bath. Hmm maybe just supermarket distilled water would work.

George H.

J

#### josephkk

Jan 1, 1970
0
Oh nothing mysterious, both are based on some physics equation that
contains kT. With some electronics you can use either to measure kT
to ~1%.
I guess if you want to do better you can use the triple point of water
as a reference... electronics and water never seem to mix to well
though.
(Hmmm.. I could make an ice bath in my dewar and try it, could I stick
a resistor/diode in a deionized water ice bath and not have them 'wig
out'?)

OK what other 'fundamental' ways are there to measure T?
Thermocouples,
resistance of metals, (and Pt in particular)
speed of sound (in air) of an acoustic resonantor,
...others?

George H.

Volume of a liquid with indication amplification by a capillary tube.
Differential expansion of two solid materials bonded together, often
amplified with a spiral construction.
Differential densities of specific gas-liquid volumes in slightly
deformable envelopes "floated" in another liquid.

Any of these jog any memories?

?-)

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