Sciencemadness Discussion Board

Suggestions for Melting Point Apparatus

careysub - 5-8-2015 at 17:13

I am looking to put together a melting point apparatus and of course desire high accuracy, but I want to keep the cost down. Lets say I budget $200 for this, max, but would like to spend less if I can. If a component is a versatile multiple-use thing (like a variac) then I might not count it against the cost.

I currently have a Thiele tube, and was considering mounting it in an insulated enclosure with a variac controlled nichrome heating coil, and using at PT-100 probe, and lens assembly for monitoring the sample.

The PT-100 probes are available quite cheap, but I am unsure what would be a good measuring instrument that would have an accurate and convenient read-out.

Other possibilities are to use an aluminum block and nichrome heater with a lens assembly and the PT-100 probe, or to buy a Mel-Temp apparatus on eBay (~$150) and equip with the PT-100 probe.

I build telescopes and so regularly work with aluminum and lens assemblies, so I am familiar with working with those components.

I am also up for an electronic kit assembly if that would bring in a good read-out at a low cost.

Any suggestions are welcome.

AvBaeyer - 5-8-2015 at 18:48

I have a spare Thomas-Hoover apparatus that I will part with. It's really very heavy and so shipping would be quite expensive. However, if you are in or near the SF Bay Area perhaps some hand delivery option could be arranged if you are interested. Send a U2U if you want to discuss further.

AvB

Sulaiman - 5-8-2015 at 21:54

A light/lamp dimmer could replace a variac for power/heating control purposes,
cheaper, lighter, smaller.


If +/- 1C is accurate enough then you could buy something like this
http://www.ebay.co.uk/itm/C100-SSR-Thermostat-Temperature-Co...
with a PT100 probe just as a thermometer.

Wind-your-own nichrome heater is possible but an aluminium clad wirewound resistor is easy and cheap, e.g.
http://uk.farnell.com/arcol/hs50-1k-j/resistor-wirewound-1ko...
easily bolted to the underside of a heating block

shadow - 8-8-2015 at 17:51

There are a couple melting temp devices on ebay right now for $100 and under w/ free shipping. Some are from India, I wouldn't be afraid to deal with them. Another from Colorado, 3 available, might need a thermometer.
I like to use a thermocouple/cheap furnace controller for accurate temp readings. The glass/mercury therms don't make it for me.
Be good.

[Edited on 9-8-2015 by shadow]

[Edited on 9-8-2015 by shadow]

blogfast25 - 8-8-2015 at 18:46

Where is aga when you need him? He put together a working model of such a device. Someone U2U him, immediately...

[Edited on 9-8-2015 by blogfast25]

careysub - 8-8-2015 at 21:39

I ordered an Omega Engineering process controller on eBay for $60 (list price $600) which is supposed to be in good condition. Haven't seen it yet. With a PT100 probe it should be accurate to, and read to, 0.1 C.

My plan is to wrap the heating arm of the Thiele tube in 35 gauge nichrome wire cemented with sodium silicate/vermiculite dust cement, wrapped with fiberglass tape, and heat it with a variac (needed for mantle heating anyway).

I am going to attach the tube to a piece of mineral board used for kilns, and put it in a mineral wool insulated container, with a mounted magnifying lens.

Anyone care to hazard a guess as to what would be a suitable heater wattage range? Thiele tubes aren't very big. I am thinking of wrapping with 100 ohms of resistance, which with a variac dialed from 10 V to 120 V would go from 1 watt to 144 watts. I am thinking 50 watts (70 V) as a likely max.

Is mineral oil good to 300 C for melting point determination? The tube would never be kept at a high temperature for long (since you have to cool it to take more MP determinations, if any).

HeYBrO - 9-8-2015 at 03:12

@ blogfast: here is the thread i think you're referring to.

aga - 9-8-2015 at 10:12

wow. that was me.

Seems like decades ago.

Edit:

Given the low wattage requirement, you'll be better off and a LOT safer using 12v soldering iron elements.

Silicone heat compound is specifically designed to transfer heat between uneven surfaces, so use some of that as well.

The PID thing can easily be done on an ardunio.
https://www.youtube.com/watch?v=i2nxqLs9wBg

[Edited on 9-8-2015 by aga]

AvBaeyer - 10-8-2015 at 18:31

Careysub,

Mineral oil is only good to about 150C where it starts to smoke pretty badly. It also rapidly darkens with consistent high temperature use. There are some cooking oils that have smokepoints around 250C (500F). You can find lists of these oils on the internet. As I recall, highly refined olive oil (so-called extra-light) is one of them. Grapeseed oil also comes to mind. The best oil is high temp silicone oil which is somewhat hard to find and fairly expensive. I recall it is good to >350C or maybe higher. The good thing is you do not need too much. Perhaps someone here has a lead to a source for small amounts.

AvB

careysub - 11-8-2015 at 07:34

Quote: Originally posted by AvBaeyer  
Careysub,

Mineral oil is only good to about 150C where it starts to smoke pretty badly. It also rapidly darkens with consistent high temperature use. There are some cooking oils that have smokepoints around 250C (500F). You can find lists of these oils on the internet. As I recall, highly refined olive oil (so-called extra-light) is one of them. Grapeseed oil also comes to mind. The best oil is high temp silicone oil which is somewhat hard to find and fairly expensive. I recall it is good to >350C or maybe higher. The good thing is you do not need too much. Perhaps someone here has a lead to a source for small amounts.

AvB


I looks like avocado oil (refined) can reach 270 C. A significant number of organic compounds have high MPs than this (anthraquinone is 286 C).

On eBay I notice lots of listing for two types of silicone oils - shock absorber oil, and polydimethylsiloxane (PDMS, dimethicone) intended for cosmetics.

Both of these products have some significant viscosity: the lightest shock absorber oil is 10 WT (10 centistokes at 70 C, maybe 20 cSt at 40 C), I see PDMS as light as 350 centistokes (room temp) which is similar to castor oil in thickness, about half that of glycerine. Of course I would expect this to be lower at high temperature, but lots of chemicals have MP around 40 C.This is a problem in a system that is stirred by convection only. Avocado oil would have no viscosity problem.

I don't see any data on the temperature stability of the shock oil (but suspect it is pretty good); PDMS is highly temperature stable, over 400 C for sure.

Neither of the silicone oils are terrifically expensive given the small amount required.

From this I am looking a 10 wt silicone shock oil, which has pretty low viscosity and I will bet a higher stability temperature than 270 C.

Praxichys - 11-8-2015 at 07:42

DOT 5 brake fluid does the job in many of my oil baths.

careysub - 11-8-2015 at 07:50

Quote: Originally posted by Praxichys  
DOT 5 brake fluid does the job in many of my oil baths.


I see that DOT 5 has a minimum BP of 260 C, of course a given product could have a much higher one in reality. How high have you pushed it?

AvBaeyer - 11-8-2015 at 19:18

Here is an article on bath fluids that you may find generally useful.

AvB


Attachment: Choosing Bath Fluids.pdf (108kB)
This file has been downloaded 607 times

TGT - 17-8-2015 at 02:22

High temperature silicon oil can be purchased at Sears in the treadmill department. It is used to lubricate the rubber walker as it circulates along the frame and pulley wheels. It is quite expensive, but this is the only place I was able to find it for my plasticization purposes.

TGT

careysub - 17-8-2015 at 10:21

I have been doing additional research:

https://books.google.com/books?id=QbPxCAAAQBAJ&pg=PA111&...

and

http://pubs.acs.org/doi/abs/10.1021/ac60110a058

I have decided to abandon the Thiele tube (too messy) and instead use a 2"x2"x3" aluminum block based loosely on the ACS scheme, above.

Current plan:
1. drill a 1/4" hole all the way through the center long axis of the block
2. drill a 1/4" hole in the center of the side of the block to intersect the first hole (but no farther)
3. Insert 1.5" glass rod in long axis hole to form the melting stage.
4. Attach two 50 W aluminum chassis mount power resistors to either side of the block.
5. Place a bright white LED under the glass rod.
6. Insert the RTD sensor probe (which are 1/8" wide) in the top hole, along with the melting point capillary (drilling two holes side-by-side is impossible without real machining equipment).

Some experimentation will be required to get the viewing details worked out I expect. I think a 1/4" hole 1" long will provide adequate access with a viewing lens. I am thinking of attaching a mica sheet to viewing side with screws.

aga - 17-8-2015 at 11:10

Sounds familiar ...

If you can find a big Al bolt, then the machining is a lot easier.

Steel takes a bit of practice.

Al is much better at thermal conductivity as well.

[Edited on 17-8-2015 by aga]

careysub - 17-8-2015 at 11:51

Quote: Originally posted by aga  
Sounds familiar ...

If you can find a big Al bolt, then the machining is a lot easier.

Steel takes a bit of practice.

Al is much better at thermal conductivity as well.

[Edited on 17-8-2015 by aga]


I have some practice with making stuff from aluminum (telescope parts), doing even a 3" hole is easy, you just take your time, backing out frequently, with a drill press with a sufficiently long drill bit.

Working aluminum has more in common with working wood than it does steel (which I avoid, except for drilling sheets).

Also aluminum is inexpensive. A 3" length of 2" aluminum 6061-T6511 square bar is $8.46 from OnlineMetals.

[Edited on 17-8-2015 by careysub]

aga - 17-8-2015 at 12:15

Bolts tend to be a lot cheaper than chunks or bars.

JJay - 15-5-2016 at 13:27

I guess I could make a capillary tube with a blowtorch and a piece of broken glass tube, attach it to a thermometer with a rubber band, and then heat it in a beaker of oil?

I'm a bit concerned that the substance might dissolve in the oil... are there any convenient sources for closed-end capillary tubes?

hyfalcon - 15-5-2016 at 16:37

Heat seal them yourself. Doesn't take a great deal of heat no bigger then a capillary tube is.

JJay - 15-5-2016 at 17:36

Quote: Originally posted by hyfalcon  
Heat seal them yourself. Doesn't take a great deal of heat no bigger then a capillary tube is.


I will try that, but eliminating all taper from the tube could be tricky, so perhaps I should seal it at the bigger end....

Dr.Bob - 16-5-2016 at 06:14

I have a few tubes of MP tubes left. They are pretty cheap, and I have some with both ends open, but also some with one closed end (the normal kind). Not sure what the open ended ones are for. If someone wants the open ends ones, I can make a good deal (mostly, you pay the postage).