diy thermometer sensitivity

...

dropping

It is cute! Thanks!

If you think that is tacky, you'd laugh at what I'm using the temperature sensor for.

Michael

y

opping

Pretty cool (pun intended.)

Voltage decreases as the temperature increases. I used a 50k resistor for R2. I'm getting around 700 out of 1024 on the ADC at room temperature, and temperature dropped as I put my fingers on it. Calibration time...

Must R3 be 2x R1, or does R1 provide all the current protection I need? Si nce I choose R2/R3 = 5, would 100 ohms for R3 and 470 ohms for R2 be fine ?

Thanks again!

M

(eventually I'll need to turn on a solenoid valve for cooling water at 79 C and turn it off at 78 C).

function as a temperature sensor if you tie the base and collector together, so I did that instead of placing an order and waiting for a thermistor to arrive.

transistor below that, then tie the emitter to ground. Did that. A wire goes from the B-C-resistor junction to the ADC input on the Arduino.

ADC value is 141. Boiling water from the microwave gives me 105. Room temperature at 23 C gave me 132.

What kind of capacitor in parallel with R3?

So far after calibration I'm getting:

750 = 1 C 709 = 25.5 C 340 = 91 C

...although I do notice the numbers jump around quite a bit more. Probably need that cap.

Very nice! Thank you!

M

dropping

I'm getting around 700 out of 1024 on the ADC at room temperature, and temperature dropped as I put my fingers on it. Calibration time...

I choose R2/R3 = 5, would 100 ohms for R3 and 470 ohms for R2 be fine?

There are a few constraints.

You need R1 to be low enough to pull the collector voltage up. It's fighting the collector current and the current in R2. If R1 is too low, you'll get a lot of current in Q1 and it will self-heat. Something like 1 mA in R1 is reasonable, so it might be 2K, assuming +5 on one end and +3 on the other.

R2 and R3 should be low enough that transistor beta doesn't matter much, but you can't steal too much current from R1. Go for 250 uA in R2 maybe. With beta=100,

1 mA in the colletor, base current will be 10 uA, so that doesn't load R2||R3 much. I'm thinking 2K or 3K for R3.

You're gonna calibrate it anyhow, so a little base current error or a little self-heating don't matter.

This all assumes a fairly narrow operating temp range, which is your case.

I told you it was tacky.

--

John Larkin                  Highland Technology Inc 
www.highlandtechnology.com   jlarkin at highlandtechnology dot com    

Precision electronic instrumentation 
Picosecond-resolution Digital Delay and Pulse generators 
Custom timing and laser controllers 
Photonics and fiberoptic TTL data links 
VME  analog, thermocouple, LVDT, synchro, tachometer 
Multichannel arbitrary waveform generators

(eventually I'll need to turn on a solenoid valve for cooling water at 79 C and turn it off at 78 C).

function as a temperature sensor if you tie the base and collector together, so I did that instead of placing an order and waiting for a thermistor to arrive.

transistor below that, then tie the emitter to ground. Did that. A wire goes from the B-C-resistor junction to the ADC input on the Arduino.

ADC value is 141. Boiling water from the microwave gives me 105. Room temperature at 23 C gave me 132.

Ceramic, roughly 0.1 uF, doesn't matter much.

need that cap.

You could software filter to smooth the noise out.

Average blocks of, say, 100 samples. Or make a continuous lowpass filter,

OUT = OUT + (IN - OUT) * K (in floats)

where K is small, like 0.01. In integer math, you can do an arithmetic right-shift to approximate the multiply. >>7 is like multiplying by 1/128.

--

John Larkin                  Highland Technology Inc 
www.highlandtechnology.com   jlarkin at highlandtechnology dot com    

Precision electronic instrumentation 
Picosecond-resolution Digital Delay and Pulse generators 
Custom timing and laser controllers 
Photonics and fiberoptic TTL data links 
VME  analog, thermocouple, LVDT, synchro, tachometer 
Multichannel arbitrary waveform generators

Your guess is pretty darned close ;)

Michael

C

at

can

a e  A

he

105.

to

by 3

10

he box

)

Oh, Are you distilling ethanol from water?

ol_and_Water.png (What the bleep is an Azeotropic point?)

I assume this graph moves around with external pressure.

George H.

...

ee

ol)

Haha, I plead the Fifth. The closer I can get to the 95% EtOH-H2O azeotrop e at around 78 degrees (seems like the exact number depends on which source you read 78.2? 78.3? 78.5? I give up), the better. If I completely lose c ontrol and allow it to get as high as 95 degrees, my column simplifies to a single-stage (allowing way too much water in the distillate) and that comp letely defeats the purpose of having a column.

They never told us how important temperature control was when I was taking the Mass Transfer (eh, Diffusion Theory) class in college.

I tried manually controlling temperature with my sprinkler valve... uhoh, m y thermometer reads 85 degrees... FULL OPEN! Whoops, too much cold feed, n ow my heating element has stopped boiling and temperature plunges below 78. .. waiting for the boiler to boil again... drives ya nuts after awhile.

Thanks,

Michael

...

Oh, I didn't notice that question.

Glad you asked!

Consider a mixture of 96% alcohol and 4% water. You try to distill it. Yo u collect the cooled-down distillate vapors. The composition is... 96% alc ohol and 4% water. Distillation did nothing to separate the two.

"A well-known example of a positive azeotrope is 95.63% ethanol and 4.37% w ater (by weight).[4] Ethanol boils at 78.4 °C, water boils at 100 °C, b ut the azeotrope boils at 78.2 °C, which is lower than either of its cons tituents.[5] Indeed 78.2 °C is the minimum temperature at which any ethan ol/water solution can boil at atmospheric pressure."

You can separate the water out by passing it through a dessicating agent, l ike ANHYDROUS magnesium sulfate.

...

anol_and_Water.png

Yeah the charts are pretty. The tables from Perry's Chemical Engineering H andbook are probably more useful in this context, however.

Go down to "Equilibrium Data", Mass Fraction (g/g), and Vapour (y).

At 78.15 degrees you see the azeotrope (liquid and vapor have the exact sam e composition, 0.9558, or 95.58% ethanol). That is my target.

As temperature rises a bit you see the vapor fraction of ethanol drops. Ev en just at 79.8 degrees, less than 2 degrees above 78.15, the ethanol compo sition has dropped to 83%. So the dessicant step later on has to work that much harder to make that ethanol dry. (Trying to make a fuel here, not pl anning to drink it.) As temperature rises to 95.5 degrees, well, you basic ally just made a single-stage distiller. A beer mix will distill at around 95 degrees. It's about 5% ethanol. The vapor will be 34% ethanol or so, not quite concentrated enough to catch fire with a match.

I note that even at 78.24C on your web link that the vapour fraction has dropped 5% already. I'd almost wonder if there is a vapour sensor system you could arrange to measure in situ and use this as feedback in closed loop control.

It seems almost easier that way, because you are talking here about significant differences with only tens of milliKelvins accuracy (not precision) differences. And, as already suggested, even if you have a well calibrated temperature measurement and the sensor(s) is(are) placed optimally, you may also need atmospheric pressure and relative humidity figures in there, as well. And tables to use in controlling things. Or else you control those factors, too.

Thanks for the posts, by the way. I am not a chemist and I had only vague notions of how difficult this distillation process might be. I now have a much better appreciation for it.

Jon

I should have added that I actually have a fiber optic sensor system (uses phosphors for sensing temperature) that can work well with fibers down to 10 microns in diameter (low mass) and has a repeatable precision across instruments and sensors of about 5 milliKelvins in the temperature range you are discussing. (single sigma.) The accuracy, of course, is a matter of calibration points, accuracies, and the software and theory used to interpolate intermediary table values.

Jon

Yup, there are many possibilities. When I was taking the Thermo classes I noticed all the data were at the atmospheric boiling points, and wondered w hat the compositions of the distillate would be if you simply allowed the m aterial to evaporate at *room temperature* and collected the vapors (sittin g in an ice bath). Then when I took the Mass Transfer classes I realized i t would take a *really long time* waiting for it to all evaporate, and move d on.

Really neat stuff if you mix Electrical Engineering folks with Chemical Eng ineering folks!

Here's a pretty good overview of using distillation columns on ethanol.

Note the 14 or so stair-steps on Page 278. Each one of those steps represe nts a "stage" in the column. Or, it would be the equivalent of doing a ben ch-scale distillation 14 times. (That's why a column is preferred; to save energy.)

Michael

Thanks for the link. It was EXACTLY what I was looking for. And I had already figured out, from walking through your earlier table, that it would take maybe 10 steps to get to

95%, which I can buy as white lightening or everclear at the store. I was beginning to wonder if mass manufacturing had figured out something else that was less troublesome. Then you gave me this. So the answer is 'no, they actually do it that way and there is no shortcut.' 14 steps makes sense to me.

I've known since high school that anything more than 95% is very hard to keep around any length of time. If you so much as open the bottle much, it "sucks" water out of the air and dillutes. It also makes me understand the cost differences between USP and CP.

Thanks, again. Very informative stuff about a common subject most of us ill-appreciate.

Jon

...

You're welcome! I'm glad you have an interest in the subject!

The stuff you get at the hardware store... it might be made from fermentation, or it might be made from petroleum. If petroleum, it is probably made from hydration of ethylene gas.

Now that you mention it... there was a guy who recently invented a really cool membrane that would allow water to "evaporate" from an ethanol-water mixture, but keep the ethanol trapped in.

Not sure if it broke the 96% azeotrope. Either way, it's not like I can get my hands on that membrane, so I'm stuck trying to make ethanol from kitchen scraps. =)

Regards,

Michael

I do. My wife actually wants me to be able to provide tincture alcohol (95%) for the preparations we make for our daughter's grand mal seizure control in the case of the "apocalypse" (break down of general access to common supplies.) [No, I'm not so worried. But if it comforts her to know we can do it, then I don't mind demonstrating a capability.]

It's from the liquor store. Oregon as a state managed system. We use it for tinctures (extraction solvent.)

Crap. Even MORE to learn about!! And yes, I'd be in a similar boat as you.

Thanks again!

Jon

...

Oh... if it for human consumption, be VERY careful. Typical yeast fermenta tion produces byproducts in small amounts: methanol (causes blindness and d eath), and "fusel oils" such as butanol (which make excellent vehicle fuels by the way - better than ethanol). If I understand it correctly, the folk s who make vodka, etc. know from experience to discard a certain amount of the first distillate liquid, which contain these. And they use batch still s... if you see "triple distilled" then you guessed it, they distilled the liquid three times...

A homemade continuous column would NOT remove the methanol and fusel oils, so be very careful.

How much do you need? Might be more efficient to stock up now before the a pocalypse. (Which could be anything... giant earthquake in Seattle... some geologists think that could happen... Yellowstone blowing up... North Kore a getting lucky...)

ane.asp

Yup, don't you have it when scientific progress advances faster than we can read.

=)

Michael

gree

anol)

trope at around 78 degrees (seems like the exact number depends on which so urce you read 78.2? 78.3? 78.5? I give up)

I'm pretty sure, (though I don't know for sure... like if I did the measurement) that the temperature will change with external pressure, so What's your altitude, and is there a cold front passing through?

I'm thinking of water, where at higher altitudes, you've gotta boil your eggs for more than 6.5 minutes, for that hard white, soft yellow that I like.

George H.

, the better.  If I completely lose control and allow it to get as high as 95 degrees, my column simplifies to a single-stage (allowing way too much water in the distillate) and that completely defeats the purpose of having a column.

g the Mass Transfer (eh, Diffusion Theory) class in college.

my thermometer reads 85 degrees... FULL OPEN!  Whoops, too much cold fee d, now my heating element has stopped boiling and temperature plunges below 78... waiting for the boiler to boil again... drives ya nuts after awhile.