Hi. My name is Mark Bailey. I'm an application engineer here at Agilent Technologies and
today we're actually in the standards lab here in Loveland, Colorado. With me today is
Stan Pond. Stan is the owner of Pond Engineering. One of his specialties is making accurate
measurements and actually creating instruments that will render triple points. Triple point
of water. Triple point of other materials.
Yes, precision thermometry.
Precision thermometry. OK. And one of the first questions you might have – I did – was
"Wait a minute. Triple point of water. I sort of remember hearing something about that in
a high school physics class or some place." So what is triple point of water?
OK. The triple of point of water is the point at which we have solid, liquid, and vapor
phases in thermal equilibrium. The international temperature scale of 1990 is founded on triple
point of water as the base for the interpolation instrument, which is the standards-grade platinum
resistance thermometer, or SPRT.
And we actually have an SPRT here today. We're going to use that at some point. But as I
remember from physics, that was – wasn't that the definition of 0°C?
It's actually 0.01 because there's a pressure difference between one standard atmosphere,
the ice point, and there's also differences associated with the dissolved gases in the
water that make up the total difference of 0.01 C, the difference between zero and the
triple point of water.
So you actually make instruments that are able to create that triple point of water, right?
That's correct. Triple point.
Why don't you tell us a little bit about that? Why don't you show us what that is and how
that works? OK. Let's see. I need to make this big. There we go.
As I mentioned before, the ITS 90 establishes the triple point of water as the base point
for all interpolation up and down the scale. What's illustrated here are the various sub-ranges
using different fixed points within the sub-range, with the triple point of water as the base
for all of those sub-ranges.
The fixed points themselves are constructed using an envelope so that the outside pressure,
local atmospheric pressure is not a factor in the realization of the fixed point.
In essence, we utilize a pure material inside an envelope with the intent being to realize
a plateau where if we freeze the material and then add energy to it, we bring the temperature
up to the melting point of that material or where the solid material begins to melt.
Then any additional heat input does not produce any sensible temperature increase until we
get to the point where we've completely melted all the material inside the cell. Then there's
a temperature increase that's sensed by the SPRT.
OK. So what that means is you basically have a temperature that you can hold – you realize
a plateau. It stays at that same temperature over time for a certain amount of time?
Correct. But you can't hold it forever.
You can't hold it forever but there's a significant period of time varying from a few hours to
many tens of hours.
Really? So tell me about triple point.
The triple point itself is illustrated by this diagram where we've got solid, liquid
and vapor phases of high-purity water that produces a pressure temperature diagram.
One temperature at the triple point where all three phases co-exist at one pressure.
But the point that we're interested in because I'm interested in primary thermometry is the
temperature point. By definition it's .01°C. That temperature then is measured using an
SPRT or standards-grade platinum resistance thermometer, utilizing a four-wire connection
to a high-purity platinum element suspended in such a way that it's relatively strain free.
So what's the typical value of an SPRT? The resistance value?
The resistance value for most SPRTs that are used in reasonable temperature ranges is 25.5 ohms.
There are high-temperature SPRTs that are down in the 2.5 ohm and even 0.25 ohm.
Wow. So to measure that you got to be pretty careful.
You got to be.
What kind of concerns do you have in trying to make a measurement on a 25 ohm resistor?
Primarily looking to maintain a four-wire connection in performing the resistance measurements
and the other primary consideration is self-heating of the resistive element. Because to measure any
resistance we have to pass a current through the element. By the act of passing a current
through that element we dissipate some heat in the resistive element. That heat's got
to go somewhere.
So basically where there's current and where there's resistance, there's power. There's heat.
Correct.
It's like your toaster, right? Push the thing down and it gets red hot. You don't want that
to happen here.
Yep. We're not putting quite that much current through it but the effect is the same. Yes.
So this illustration is of a thermometer down inside a triple point of water cell. The power
being dissipated by the resisted element is illustrated by the purple arrows. That power
produces a temperature gradient from the theoretical definition of the triple point of water.
The phase equilibrium between the liquid and solid phases in this case and the sensing element itself.
That temperature gradient can range from a few milli-kelvin up to several tens
of milli-kelvin or higher depending on what the excitation current is. So that's
the reason you need to pay attention to setting up the measuring instrument carefully
to pay attention to self-heating.
So I think I got the theory down a little bit. Triple of point of water is I've got ice,
I've got water, and I've got what you call vapor. We might call it air or something like that.
Not air.
Not air. OK. Vapor. And the three of these co-exist at the same time, right?
Correct.
And that is defined as 0.01°C centigrade.
Correct.
OK. So you sort of talked about how you might create that and how you measure it.
Let's do it for real, OK?
OK.
So we got with us today a third visitor who is Stan's – this is what? The K51?
Correct.
Tell me what that does.
K51 is a thermo-electrically cooled core that in this case has cooled the cell down to
approximately 8°C below zero.
So the water is less degrees than?
The water is less than freezing. In fact you can see a little ice on the outside of this
envelope kind of attesting to the fact that, yes, it's below zero. It's forming a little
bit of frost there and dew, condensation on the outside. So the liquid water in this cell
is cooled to approximately eight degrees below zero. If we agitate that cell by shaking the
top of it they will form a nucleation site. And one crystal will start and then those
crystals will propagate down the length of the cell.
So can we see that?
So I'll hold the bottom of the cell and we'll agitate it and it will form ice crystals down
the length of the cell.
Cool. Look at that.
Now the water inside this cell now has warmed up from approximately eight degrees below
zero to 0.01. The ice has formed which will now act as a buffer maintaining that 0.01°C
for a long period of time. In this case, approximately eight hours inside the maintenance system.
OK. So we've got ice. We've got water.
Yep.
And we've got air.
We got – not air.
We got vapor. OK. Good. All right, so we put it back in.
So that's how we'll go back in the maintenance system. We'll change the set point from -8
up to a value that will maintain it for approximately eight hours. And then we can use that instrument
then to make the actual measurements.
OK. Well let's try it. So while you're setting the set point I'm going to grab this SPRT.
And I got to be careful. This thing, by the way where you can buy cheap little resistive devices
to make temperature measurements for a few bucks, this thing is what, Stan? Couple thousand?
A couple, $3,000.
Oops.
Yeah, you don't want to do that. The thing is it's a little bit tricky too is the instrument
may be a couple, $3,000 but you can spend up to $5,000 just having it calibrated.
OK. And this one by the way has been through the ringer a little bit. You can tell if we
look at it that it's gotten hot and other things. We also haven't calibrated this in
a quite a while. So it'll be interesting to see how close to real we are. I don't doubt
that you are truly at 0.01.
And the uncertainty on this instrument is approximately a half milli-kelvin. The uncertainty
on that instrument, the combination of the two is somewhere between five and seven milli-kelvin.
So you've got a good ratio between the standard and the unit being tested and calibrated.
So I'm actually using a 34420A. This is Agilent's nanovolt micro-ohm meter. And we're going
to do – we're doing resistance measurements. So we're actually on the 100 ohm range and
we put it in the lower power mode because of what you talked about. We don't want to
heat up the element. We're also doing four-wire ohms obviously. And we have some other things
in here like offset compensation and we're doing some other magic trying to make the
best measurement possible. So can I do the honors?
Absolutely.
OK. I'll try not to break anything. I slide this down into here. Eventually it'll stop.
Oh, I found the bottom. OK. So we've waited a few minutes. Things have reached an equilibrium
where they've equilibrated is the term that's used. And as I look at it I don't seem to
be at 0.01°C. So something's going on. I've got about a 30 milli-degree Celsius offset.
So what I'm going to do is go in and actually measure using a low-power, four-wire ohms
technique and see what the resistance really is at this point with my SPRT. So to do that
I'm going to go to four-wire ohms and it measures 25.2734. OK.
So I'm going to go in and see what I've got set up actually under the RTD. What did I
set it up as? And it looks like it's set up at 25.2704. OK. So I want to come across.
Take that digit. Bump it by three. That was the measured value. If I enter that, now I'm
going to go back to temperature. Oh, and look at that. It's 0.01°C with a one or two bouncing
around on the least significant digit. So it seems to work.
OK. So we've gone through quite a few things today. We've talked about what the heck triple
point of water is. Right?
Correct.
Stan's explained that to us. He's explained how to realize a triple point using the techniques
that he's got built into his product. Right?
Correct.
And, finally, we've shown how to use that to calibrate a system consisting of an SPRT and a
measurement device to be able to accurately measure.
Correct. And the uncertainty of the triple point of water in this situation is approximately
a half milli-kelvin and the ratio between that uncertainty and the uncertainty of the
instrument and SPRT combination at about a 10:1 ratio.
So hopefully this has been interesting to you and entertaining and we look forward
to seeing you next time.