A silly (?) question to you chemists out there

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A silly (?) question to you chemists out there

Post by richardh08 »

While experimenting with some crackle-like compositions I've come across something that, to me, doesn't appear to make physical sense. I have two separate bits of data that, individually, appear to be reasonable, but lead to results that differ from each other by a factor of around two orders of magnitude. I can only think that I must have made a mistake somewhere along the line, but I can't see what it is.

To avoid influencing anyone into perhaps making the same mistake, I'll not say more about it until I have an answer to the following chemistry question.

How much energy would you expect to be released by the combustion of 1 gram of a stoichiometric mix of copper oxide and aluminium?

The answer ought to be reasonably straightforward, but I'm desperately hoping I've made a conceptual error and come up with an answer that is about 100 times too small.
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Re: A silly (?) question to you chemists out there

Post by Lloyd »

We need to know two more things to do that, Richard:

1) what you call "stoichiometric" for this combination. I assume 2Al-3CuO (molar weights), but want to be sure.
2) what you expect the combustion products to be. Theories vary on what that reaction yields. (!)

Tnx.
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Re: A silly (?) question to you chemists out there

Post by richardh08 »

Fair points, Lloyd.

I'm assuming that 'stoichiometric' relates to 3CuO + 2Al -> 3Cu + Al2O3, which comes out pretty close to 80% CuO, 20 %Al.

But I suspect that the finer details, even with a different mix of reaction products, don't really matter with respect to the problem I have. Any answer that is in the right ballpark will tell me if it's this calculation where I've lost, or gained, a factor of 100!
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Re: A silly (?) question to you chemists out there

Post by Lloyd »

I'm SURE not the guy to give you a definitive answer. But from the NIST tables, using both enthalpies of formation and enthalpies of fusion (for the metals), I get a net +1192kJ total for that reaction.

That's with 3 moles of CuO and 2 moles of Al. I didn't run the calculation out to 1 gram.

(but) carrying that further, the combination would be 238.64g of CuO + 53.96g Al... total = 292.6g..

So 1192kJ/292.6 = 4.074kJ/1 gram
???

<shrug>
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Re: A silly (?) question to you chemists out there

Post by richardh08 »

Aaaah - that's pretty close to what I calculated it as. I really hoped I'd made a stupid mistake. Thanks for eliminating that one.

One more question before I explain my problem. Here's a video of a grain that didn't quite explode: https://youtu.be/c0Fk3w_Fsu8. What you first see is the prime burning and the grain goes briefly dark. Then it brightens up again for a while, before gradually fading again.

My question is, how hot do you think the grain got in that second heating stage? I'm not trying to be clever, or to trick anyone, but I would like at least one common sense estimate, opinion or guess. I have my own view on what the answer might be, but I don't want to influence anyone's answer.

Later edit: If it helps, the grain contains Bi2O3, CuO, Al (and a small amount of phenolic resin binder).
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Re: A silly (?) question to you chemists out there

Post by Lloyd »

Well, to me that's pretty simple.

The prime was insufficient to FULLY ignite the grain, but it got a few spots hot enough to begin combustion. Then the flame (slowly, in perspective) spread to the whole contents.

Why did it not explode? Well... perhaps the 'heating phase' just didn't occur fast enough... I dunno!

Based on the color emitted during the smoulder, I'd think it was less than - or about - 1500F / 815C.

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Re: A silly (?) question to you chemists out there

Post by richardh08 »

This time, I'm not bothered about whether it exploded or not, or why. The problem I have is nothing to do with that aspect. I'm sorry if that sounds a bit mysterious, but I don't want any personal bias to contaminate the issue until I've had one or two opinions. Lloyd has destroyed my hope that my mistake was a simple arithmetical error in calculating the available energy.
Puzzle.png
To be absolutely clear, I'm looking for opinions on the temperature at stages D and E in the above set of images.

I should add that I know for a fact that, following tests on the camera I used, an object at a temperature of around 400ºC or below is totally invisible.
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Re: A silly (?) question to you chemists out there

Post by Lloyd »

Richard,
I watched, and saw a reddish-yellow (orange) glow. But I may have been mistaken of that.

I used the 'metal glow vs. temperature' chart here: http://www.m4040.com/Knifemaking/PICTUR ... art-sm.gif

I could have mistaken the color... in which case, it might be as high as 2000F/1093C... but I don't think so. What I saw couldn't have been above 1800F.

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Re: A silly (?) question to you chemists out there

Post by Lloyd »

BTW... now you have me curious. How "close" was the number I got to what you calculated (just trying to see if the method I employed was sound... I'm not real good at that stuff!)

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Re: A silly (?) question to you chemists out there

Post by richardh08 »

That's hugely interesting information Lloyd. I wasn't aware of that table, but it looks to be quite sensible (even though I'm also not very good with matters relating to colour and temperature). It might end up at least partially resolving my issue, which is an incompatibility between the temperature reached and the available temperature-raising energy.

My belief WAS that the temperature was significantly higher than the one you gave, but I have long been aware that this belief was based on some very dubious assumptions, that I had no means of verifying. If I can really convince myself that the temperature is lower than I thought, my problem might go away.

I'll have to put some work in, to see if it changes things, and by how much. I can see that it will probably take a few days to get to a conclusion.

I'm pleased I asked - it just goes to show that there really is no such thing as a silly question!
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Re: A silly (?) question to you chemists out there

Post by Lloyd »

Heh! You STILL didn't answer MY question, Richard! <grin>

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Re: A silly (?) question to you chemists out there

Post by Boophoenix »

Very interesting Richard. Way above my realm of knowledge, but simplicity makes me think that you hit the verge of where the Bi203 burns. I suspect I'm a little off and there are some other variables at play though.

I have experiance in my simple testing a spot where overheating seems to ruin the reaction ( dud ) and also where introducing other variables kills the reaction or that possibly changes the required temps dramatically.
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Re: A silly (?) question to you chemists out there

Post by Lloyd »

BTW, Richard,

There are numerous such 'glow' tables available, and some give more resolution than the one I used.

It's a pretty-standard way for weldors and metal-makers to judge temperature.
(I misspell that intentionally, because a 'welder' is a machine, and the person doing it is a 'weldor')

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Re: A silly (?) question to you chemists out there

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I've now looked at a small selection of such tables and they are sufficiently inconsistent that they aren't as helpful as I would have liked. But, they do all suggest that the temperature might well be closer to the region you mention.

To answer your question, Lloyd, I thought the temperature might be around 2000K. I based that not only on the colour, but also on the fact that a significant amount of smoke/vapour is being emitted. Apart from the small amount of binder, the other ingredients and the likely reaction products all vaporise at temperatures near 2000K or above. The existence of the smoke means that my normal photographic temperature measurement doesn't work.

A quick check suggests that a temperature in the region you suggested would largely resolve the discrepancy in my data, which is a strong indication that the lower value might well be correct. However, that then throws all my other work on temperature estimates into doubt.
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Re: A silly (?) question to you chemists out there

Post by Lloyd »

OOOO!!! You're TEASING me, Richard! You STILL didn't answer my one-and-only question! (URK!)

I'm sorry... the wording of my question might have been misconstrued as applying to the combustion TEMPERATURE.

What I wanted to know was, how close were your calculations and mine on the combustion heat evolution thing?

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Re: A silly (?) question to you chemists out there

Post by richardh08 »

Sorry, Lloyd, I didn't mean to tease. Since I was looking at the potential of an order-of-magnitude error, I only made an approximate calculation. I assumed an 80:20 mix, rounded molecular weights to whole numbers and didn't take into account any energy absorbed in melting or vaporisation. I'd therefore expect my answer to be a little higher than yours, and it was: I got 4.64J/g. In terms of the accuracy of any of my real-life measurements, that's close enough to show the error lies somewhere else.

I've always known that my attempts to measure the temperature were only approximate and were significantly distorted at both the high and low end of the range. The issue came to a head when I combined them with the results from my physical model, in an attempt to see how the release of energy varied with time. The calculated total energy always turned out to be more than was actually available, and the example described in this thread is an extreme case.

On the plus side, I ended up checking my physical model much more closely than I've done before. Comparing the total loss of energy to the outside world with another back-of-the envelope calculation based on before and after temperatures gave agreement (after a quarter of a million iterations!) to within 2 or 3%, at any assumed rate of cooling. I got similar agreement for the amount of energy that was generated when I added in my simple model of the chemistry. That has greatly increased my confidence in the results that come out of the model.

The temperature measurements were always the weak link and I think that link has just got weaker!
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Re: A silly (?) question to you chemists out there

Post by Lloyd »

Richard,
Just to be clear... you wrote "I got 4.64J/g. "

Did you mean to write "kilo Joules" (kJ)?
(If not... that would be TWO [wrong! THREE... corrected after Richard noted it] orders of magnitude! <grin>)

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Re: A silly (?) question to you chemists out there

Post by richardh08 »

4.64kJ/g it is. (and that would actually have been THREE orders of magnitude <grin>)

Sorry, that WAS one of my careless mistakes!
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Re: A silly (?) question to you chemists out there

Post by Lloyd »

Ooops! Yeah, I guess we're both in that 'mode' right now... yes... THREE orders of magnitude! (duh!)

Hey... I'm not adept at doing this stuff. I'm glad that comment was even close to correct!

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Re: A silly (?) question to you chemists out there

Post by biffo »

Hi Richard am I being daft in suggesting you try to dip a thermocouple in your crackle mix however many times to get a size you like and dry and then monitor on a temp controller,are they not quick enough to react? Regards B
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Re: A silly (?) question to you chemists out there

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Something like that might be worth trying. I'm certainly going to have to find a more accurate measurement of the temperature if I want to move forwards.
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Re: A silly (?) question to you chemists out there

Post by Boophoenix »

FLIR has a sales page

http://www.flir.com/democamerasale/

Anyone have any excessively wealthy relatives and a B-day coming up or something? :lol:

Seriously though Richard impressive stuff you've been trying to accomplish. Way over my head in understanding how you're working it out, but keep at it ya might just find what you're searching for.
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Re: A silly (?) question to you chemists out there

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The more I think about that idea, Biffo, the more it seems like it might be worth giving it a go. I guess I'd have to make up my own low-heat-capacity thermocouples, and expect them to be destroyed every time I run a test.

It's certainly a much cheaper option than going down the FLIR route. Their stuff is really good, and would do everything I want and more, but have you seen their prices? :shock: They add a whole new dimension to the word 'affordable'.
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Re: A silly (?) question to you chemists out there

Post by biffo »

As you probably know it's only the junction of two pieces of dissimilar wire and some are perhaps small enough not to have a great effect on the experiment it's self.regards B
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Re: A silly (?) question to you chemists out there

Post by sambo »

I reckon you'd be able to get a decent sample rate out of it also.
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Re: A silly (?) question to you chemists out there

Post by Lloyd »

"As you probably know it's only the junction of two pieces of dissimilar wire and some are perhaps small enough not to have a great effect on the experiment it's self.regards B"
----------------
The problem - as I see it - would be the heat-sinking character of such a thermocouple. They're really simple devices to make, but they have mass, and thermal mass, specifically.

I have seen thermocouples as small as a few "mils' thick, and one of those might suffice -- say, one made from #40AWG (or finer) wire. But even then, it's thermal mass would be an issue, even if not a deterrent to trying it.

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Re: A silly (?) question to you chemists out there

Post by biffo »

Without giving this alot of thought surely any thing the egg is placed on will have an effect on the result unless it can be filmed or accessed in the air,wouldn't it?.humble regards B
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Re: A silly (?) question to you chemists out there

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I've got some 44 SWG (pretty close to 40 AWG!) wire on order, in a couple of materials with reasonably high melting points. It remains to be seen if they will make effective thermocouples and whether or not they will disrupt the effect I'm trying to measure. But it's something that has just got to be tried. If it doesn't work, I'll be no worse off than I am now - apart from a relatively small financial outlay and a chunk of my time.

In the all ground tests I've done, the grains sit on a ceramic tile with a backstop of a lump of (rusty) iron. I've not seen any indication that contact with either of them has any significant effect on the progress of the reaction.

I also think I have found an IR sensor that, while not being ideal, might do most of what I want. Unlike the FLIR kit, it only costs part of an arm and leg, so that's another possible route to follow.
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Re: A silly (?) question to you chemists out there

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My wires arrived today. An initial test indicates that I have a combination that works, at least up to bright red heat. Maximum output is about 10mV, and they are quite fragile, so it looks as if I might have to be lucky for any survive the whole ignition cycle.

All I now have to do is design the software - and any hardware I might need - to record the output, and then work out how to calibrate them!
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Re: A silly (?) question to you chemists out there

Post by Lloyd »

Richard,
If you'll only be twisting these thermocouples, but not brazing them, there are a couple of cautions that will help with accuracy, if not longevity.

First, I think that NOT brazing the union is probably wiser, in order to keep the thermal mass as low as possible. But when you do that, the thermocouple will vary in output, depending upon technique and time between 'making' and 'using'.

First, pickle the wire ends, and rinse them thoroughly in distilled water. Never touch the 'couple' ends again with bare hands.

Twist the same number of turns each time -- and by whatever means you can muster, to the same 'torque' (tightness of joint, in this case).

Then, USE the thermocouple very soon after making, lest atmospheric oxidation change the surface-resistance of the metals.

If you can braze or solder the ends, and still keep the mass low enough, that's the easier course of action, because (even though it adds another step), it holds accuracy better.

I've seen some thermocouples that were simply 'spot welded' together. That works, too.

I worked for about two years in a transformer company's metrology and product-testing laboratory. It was a 'rich' experience... and thermocouples were part of our everyday life there. I did some ceramics work later-on, and they're the meat-and-potatoes of kiln temperature measuring (as opposed to using 'slump cones').

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Re: A silly (?) question to you chemists out there

Post by richardh08 »

Thanks, Lloyd. That's very helpful information.

Working with such relatively fine wires is quite fiddly. I'm thinking it might be worth dreaming up some kind of jig to give a bit of control over the tension and twisting torque. Brazing or soldering the junction might well improve the ruggedness and reliability, provided I can keep the effective thermal mass small enough. I guess I'll have to try it both ways and see which works best.
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Re: A silly (?) question to you chemists out there

Post by Lloyd »

Richard,
The way I've read about it being done commercially was:
1) Space the wires a 'calibrated distance apart'... similarly, their 'crossing angle'.
2) Twist with a spinning head held at a fixed distance from the wire clamp, such that as the twists increase, then tension increases.
3) Twist to a 'specific tension'.
4) Braze
5) Cut to the 'calibrated' number of twisted turns for the desired junction resistance.

The Open-Circuit voltage doesn't change for any two particular alloys at a particular heat, but the ability of that junction to deliver current into a load does vary with the amount of contact area.

Also, I think it's important that not all brazing alloys or solders might affect a given junction in the same way. I've never studied it, so I don't know what those limitations might be. I think that would require some minor research to get right.

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Re: A silly (?) question to you chemists out there

Post by richardh08 »

Lloyd,
What you describe is pretty much what I had in mind. However, building something along those lines might be a bit more than I can manage!
Assuming I can construct them, I envisage connecting them to a very high impedance circuit, so very little current would be drawn.
Consistency, rather than high output, would be the prime consideration, so I'm less concerned about optimising the material used to solder the junction - unless it proves to be a serious issue.
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Re: A silly (?) question to you chemists out there

Post by Lloyd »

"...I envisage connecting them to a very high impedance circuit, so very little current would be drawn."
----------
Yes, that's appropriate, and obviously you have thought that out.

Sorry... I get a little 'didactic' when I speak of electronics, but it was my career for over 30 years, starting in the early 60s.

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Re: A silly (?) question to you chemists out there

Post by richardh08 »

Didactic is OK by me! I'd rather someone tells me a hundred things I already know, provided they also mention the one thing that I didn't know, had forgotten or thought not to be important. That applies especially to electronics which is one of the (many) areas where my knowledge is, at best, somewhat patchy.
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Re: A silly (?) question to you chemists out there

Post by Lloyd »

I had what can only be described as a 'strange' career start in electronics.

When I was 11, I became interested in it through an outfit called "The American Basic Science Club". Among the many dozens of experiments offered up as monthly kits (COD $3.95), they had an entire section on real electronics, including building a shortwave valve-based ('tubes', for Yanks) receiver from 'scratch'. That hooked me.

By the time I was 13 I had my Novice Ham Radio License using converted military-surplus gear and owned a full-up home-built stereo system. At that point, my dad got interested in what I was doing. I showed him a few things that I was doing, and he immediately signed up for what we'd now call "Continuing Adult Education". In just a few months, he was certified as a TV repairman. He and I opened a repair shop together when I was 14. And on it went.

It was a very interesting time. Not only did I have the fun of the hobby AND make money at it, I actually got to watch the whole technology evolve from valves, air-capacitors, and wire-wound resistors to what are now our modern compact 'integrated' electronics. If I'd come into the craft at any other time, I'd have missed a great part of that evolution.

We've come a LONG way in just a very short time.

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Re: A silly (?) question to you chemists out there

Post by richardh08 »

My experience was somewhat different.

I had virtually no knowledge of electronics until, in my twenties, I found myself having to design and build a digital control system for an interferometer. At the time, digital electronics was just transforming from using large ceramic-encased single-function units to DIL-packaged TTL. To cut a long story short, it worked – eventually.

Since then I’ve very occasionally dabbled in the construction of both analogue and digital circuitry. I consider myself to have a reasonable basic working knowledge, but I’m a long way from being an expert. Fortunately, the task in hand ought to be within my capability.

Construction of a usable thermocouple is proving a bit trickier than I thought. I’m aiming to use metals with as high a melting point as possible, but some of them are not suitable as they oxidise and burn out long before they melt. For the others, I’m finding it more difficult to get solder or brazing alloy to bond than I had hoped. I have a number of materials and techniques still to try, but I can see it is going to take a fair bit of time and effort.

As for the actual construction of the junction itself, it’s sufficiently fiddly to deal with such thin wires that I’ve come to the conclusion that some kind of jig is essential. After puzzling over it for a few days, I suddenly remembered that I possibly have an easy solution; somewhere in the attic is my old No. 7 Meccano set!

Right now I’m off on a hunting expedition…
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Re: A silly (?) question to you chemists out there

Post by richardh08 »

Good old Meccano! My prototype jig made a really uniform, tightly wound wire junction.

Some redesign is required: it took me over half an hour to get the wires threaded and secured in position, and about three seconds to do the twisting! I also learned that my eyes aren't as good as they used to be :(

All I really need is a bit more room to get my fingers in the right places - and perhaps an illuminated magnifier!

I also still need to experiment with whatever soldering/brazing is needed, but I'm still waiting for a few odds and ends to arrive before I can play with that.
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Re: A silly (?) question to you chemists out there

Post by Lloyd »

That's great, Richard! Old toys for new tasks!

Remember, in this next stage... Fluxes are your friends!

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Re: A silly (?) question to you chemists out there

Post by richardh08 »

Well, I now have a few thermocouples made up. They LOOK consistent in structure.

After a number of failed attempts, I've given up on trying to solder/braze the junction. With the metals I'm using (Mo & W, chosen for their high melting points) I can't get anything to stick. I even tried copper plating the junction, but that didn't work either. I suspect that's because the junctions are so small that I can't get the current density low enough to get a clean, compact coating.

So I'll have to use bare junctions, which probably means the output will be inconsistent from sample to sample, but I'm hoping that the data will be self-calibrating. If I trust my computer model, the melting point of the grain's ingredients are much more clearly indicated in the interior of the grain than they are at the surface, so I might get fairly clear reference points.

The good news is that the junctions are tiny, with a calculated weight of about 0.7mg. Provided the wires don't melt, they shouldn't significantly disturb the progress of the reaction, so I'm hopeful of getting some moderately good data. But the output is tiny - somewhere in the region of 1µV/°C, which means I'll need a decent amount of amplification.
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Re: A silly (?) question to you chemists out there

Post by Lloyd »

Well, Richard, you seem to have done well!

If you see radical variations from junction to junction, you _might_ have a try at 'inerting' your junctions (at least the microscopically-thin contact area between the twisted wires) with a dip in an 'occluding' substance, like a little - thin - clay slip, and drying them. I don't think they'd need to be 'fired'.

This shouldn't interfere with your actual contacts, but would block combustion products from attacking that union area (at least, as quickly).

On the amplifiers part -- there are so many INEXPENSIVE temperature-and-supply-compensated Op-Amps out there now-a-days, it shouldn't be too awfully hard to come up with a stable amp for low cost. Most good 'bench amplifiers', even of decades past, were pretty good standards, too.

Lloyd
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Re: A silly (?) question to you chemists out there

Post by richardh08 »

Thanks for those tips, Lloyd. I'll try coating the junction if it proves necessary. For my first attempt, I've dipped the junctions into a thin slurry of the crackle mix itself, to ensure good physical contact and to key the grains onto them. Now all I have to do is to get the grains molded in place.

...And to work out how to apply the prime, and design and build a suitable amplifier (op-amps are, indeed, the way I'm planning to go) and write the software to record the data...

Should be a piece of cake!
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Re: A silly (?) question to you chemists out there

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Molding a preformed grain onto the junction proved to be impossible (read: ridiculously fiddly and totally impractical). The only way I've found that works is repeated dipping in a slurry. The composition is weakly bound when wet, so the grains have to be left to dry for an hour or two between each dip and the next. It's tedious and time-consuming, and controlling the final grain size seems to be a matter of getting just the right consistency of the slurry. The ones I have made so far are somewhat non-uniform in size, but are possibly good enough for an initial trial. Being initially very wet, I suspect they will take longer than usual to thoroughly dry out.

I think I'll have to use the same technique to apply the prime.
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Re: A silly (?) question to you chemists out there

Post by biffo »

Sterling effort Richard I just hope you get some results that you can work with.Regards B
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Re: A silly (?) question to you chemists out there

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Mk1 circuitry and software are complete, and I spent most of today running tests with a bare thermocouple junction. The behaviour is quite promising, but isn't as good as I would have hoped, so there will definitely have to be a Mk2.

I forgot about the problems caused by an op-amp's offset voltage, so that will have to be fixed, and I need to pay a bit more attention to noise reduction. Otherwise, it looks reasonably good - see the following graph, where the junction was randomly heated with a cigarette lighter flame.
Mk1Test.png
For testing, I've got three outputs, differing from each other in gain by a factor of 2. In all cases, to protect the Arduino's ADC, the output is set to saturate at just under 5V.

As you can see from the first part of the graph, there is a non-random variation in the output, with a small but noticeable peak about every 0.66 sec (frequency 1.52 Hz). I'm guessing it's due to pickup from something on the Arduino board, but I've no idea what could be generating such a low frequency signal.
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Re: A silly (?) question to you chemists out there

Post by sambo »

Hi Richard,

Are you powering this from USB or via a battery ? Have you added decoupling capacitors ? At least a 10uF and possibly a 0.1uF at a guess ?

Maybe a schematic ?

Sam.
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Re: A silly (?) question to you chemists out there

Post by richardh08 »

The Arduino is powered via USB, as it has to talk to a PC, but the amp is running off batteries.

The Mk1 circuit has no decoupling capacitors at all, which is probably the cause of a good part of the noise problem. It's only a simple-minded multi-stage non-inverting op-amp job, built on a piece of stripboard, so I doubt a schematic would clarify anything.

Initially, I had only the vaguest idea what the thermocouple's output would be, so it was more or less built as a best guess feasibility study. Given my ad hoc knowledge of analogue electronics, I was fairly certain that I'd need to go to at least one more iteration. In the circumstances, I'm quite pleased that it works as well as it does.
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Re: A silly (?) question to you chemists out there

Post by richardh08 »

I've just run a few more tests and it turns out that the noise spikes aren't at specific times, but are related to what's going on in the software on the Arduino board. The spikes actually occur once every 17 cycles of the ADC measurement loop, irrespective of the delay from one cycle to the next, and each one affects only a single measurement. Go figure.

Hopefully, some effective decoupling in the Mk2 circuit should make some improvement.
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Re: A silly (?) question to you chemists out there

Post by Lloyd »

Is your app on the Arduino programmed in the native code of the Arduino, or some interpreted language, like (say) BASIC?

It could be an artifact of the monitor loop's having gone-off to do 'garbage collection', or the like.

Hand-coded (well-done) assembly code or well-compiled C/C++/C# shouldn't manifest such idiosyncrasies.

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