Sciencemadness Discussion Board - Hot plate temperatures

Hot plate temperatures

sternman318 - 16-6-2011 at 15:53

How hot do normal hot-plates get on their highest settings? I have a hot plate graciously given to me by my biology teacher that, using an IR thermometer, gets to atleast 400 degrees farenheit on its highest setting. Yet, does not boil solutions in a standard 500 mL beaker, even when using very small amounts. The solutions get to about 180 degrees. Could using it outside, where there is much free flow of air, cool the solutions so much?

Neil - 16-6-2011 at 16:19

Yes. When I use a hotplate outside I have to use a aluminum foil skirt to contain the heat around the pot.

hkparker - 16-6-2011 at 16:31

Quote: Originally posted by Neil

Yes. When I use a hotplate outside I have to use a aluminum foil skirt to contain the heat around the pot.

Same, makes a big difference

sternman318 - 16-6-2011 at 16:36

Thanks, I will give that a try next time!

redox - 16-6-2011 at 16:53

Do you own a propane torch?

If you need to reach temperatures higher than your limited hotplate can handle, use it.

Look at hkparker's videos for the technique.

My hotplate is quite nice. I picked it up cheap on ebay, and it reaches 500 Celsius.

The WiZard is In - 16-6-2011 at 16:55

Quote: Originally posted by sternman318

How hot do normal hot-plates get on their highest settings? I have a hot plate graciously given to me by my biology teacher that, using an IR thermometer, gets to atleast 400 degrees farenheit on its highest setting. Yet, does not boil solutions in a standard 500 mL beaker, even when using very small amounts. The solutions get to about 180 degrees. Could using it outside, where there is much free flow of air, cool the solutions so much?

If you cannot boil water with a hot plate at 400o F. Four reasons
come to mind.

a- your in a deep sea compression chamber
b- your in a cave 27 856 feet below sea level
c- your on another planet
d- now you know why he gave it away for free

given the contact area with a 500ml beaker I suffer
from grave doubts that a wind you could stand up in
would cause problems.

I have put - checking the temp on my hotplate on my
thing to do on the morrow list. I don't think my IR thermometer
goes that high, however, I have a Fluke.

Neil - 16-6-2011 at 16:56

It's quantity not quality

I have one that gets to 500~F but outside with no skirt it can't get a water bath hot enough to boil methanol.

The WiZard is In - 16-6-2011 at 18:01

Quote: Originally posted by The WiZard is In

I have put - checking the temp on my hotplate on my
thing to do on the morrow list. I don't think my IR thermometer
goes that high, however, I have a Fluke.

Ok at no extra charge I have moved up this on my schedule.
I have just check out my hot plate, an old as the hills
Corning PC 35.

Naturally the battery in my Fluke Model 87 VOM is dead so
I cannot use my Fluke 80T-IR non-contact temperature
attachment. I know - it's lame, however, it was gift from
back-when, when - IR thermometers were very expensive.

With my Fluke 51 II digital thermometer I get to 822o F.

So .. my advice is Patience grasshopper.

sternman318 - 16-6-2011 at 18:24

Quote: Originally posted by The WiZard is In

Haha,, i've cracked up reading this a few times now. I might have to go with D though. Unless New Jersey is considered extraterrestrial

UnintentionalChaos - 16-6-2011 at 19:10

Quote: Originally posted by sternman318

Quote: Originally posted by The WiZard is In

Haha,, i've cracked up reading this a few times now. I might have to go with D though. Unless New Jersey is considered extraterrestrial

Greetings also from New Jersey. Hotplates vary wildly in how much power they can deliver, and max surface temp is not usually a good indicator of this, IIRC. Increasing the surface area in contact with the vessel, using a sand or oil bath, tenting with foil, or wrapping in insulation (I use woven fiberglass cloth doubled over a few times) can help retain heat and improve performance.

My Corning PC-351 for example has no problem boiling lots of solutions but it usually can't do it when I try to get things to boil much above 200C. Using a fractionating column or other distilling implements also increases the needed power.

peach - 17-6-2011 at 00:38

Look on the back of the plate for a data label.

The temperature alone is like stating a voltage alone. E.g. "This air ioniser will get to 2,000V!"

I can put my finger on such an ioniser and all that will happen is that it'll sting a lot, because there's next to no current flowing at that voltage.

Conversely, "This microwave power supply runs at 2,000V!". If I put my finger in that, there'd be a good chance I'd have a heart attack, because the supply outputs amps at that voltage (thousands of times more than the ioniser).

Another example, the inside of fusion reactors reaches hundreds of millions of degrees Kelvin. The lining is made from metal. That temperature will boil every known substance on contact. Yet the plasma doesn't all pour out and melt the universe because there's only a small amount of energy present at that temperature.

The plate may reach the required temperature when it's sat in still air with nothing on it, but it takes power to sustain the temperature. A typical hotplate for boiling hundreds of mls to a litre or two will be rated at 200 - 400w.

If the wattage is somewhere around there, there's a possibility the plate is damaged in some way. If it's a none digital plate, repairing such damage would be a lot easier than it would be with the high tech ones (the digital IKA's turn up quite frequently on eBay, damaged, and buying new PCB's for them is expensive! I seem to remember there are two in an IKA, and they're both over £100 each).

Let us know the wattage first however, and if possible, take a photo with a rule or pen on it so we can see it. If the wattage isn't written on it, look for any kind of model numbers and check what the fuse rating is in it.

[Edited on 17-6-2011 by peach]

Neil - 17-6-2011 at 05:52

Ohms law anyone?

Also make sure you are using the thickest shortest extension cord you need to get power from point A to point B.

peach - 17-6-2011 at 07:04

The mains leads shouldn't make any difference for a standard hotplate, unless you're wiring it up with extensions for christmas lights.

watson.fawkes - 17-6-2011 at 10:15

Quote: Originally posted by sternman318

Could using it outside, where there is much free flow of air, cool the solutions so much?

Yes, in the general direction you're headed. The advice to use Al foil is correct, and there are other related things you might do.

The input power (energy per unit time) is only one aspect of your problem. The output power, which in this case is heat lost through radiation and convection, is the other. Heat losses increase with increasing temperature, no matter what the mechanism. This is why you're seeing a temperature maximum. You have a constant power input and a power output that increases with temperature. At some point the two of these match and you get a steady-state temperature. To get a higher ultimate temperature, you have to decrease heat losses.

At some point, radiation loss always dominates because it goes up by the fourth power (the Stefan-Boltzmann Law). The Al foil technique is a radiant energy insulator; it works by reflecting at high efficiency. You can make something more reusable up out of Al sheet and pop rivets; it works on the same principle.

Simple conduction is linear with temperature, although that's not much at play here.

Convection is rather more complicated, because it involves the geometry of gas flow and surrounding temperatures. The basic rule, though, for preventing convection is to prevent air flow, which means restricting flow either at the top or bottom of each vertical channel.

Where NOT to put aluminium foil

peach - 17-6-2011 at 10:44

Who's seen Flight of the Navigator?

The foil tee-pee method works brilliantly.

However, what you see above is the result of me putting a piece of foil over the surface of the plate to give a clean background to take a photo against.

I wasn't using the plate at the time to heat anything, but must have switched it on without removing the foil.

The result is quite clearly not a positive one.

This plate didn't have any form of feedback or thermal cut out. Even though it's normal maximum temperature was around 300-400C, it has managed to get all the way up to 660C and has melted the top, for the sake of a piece of tinfoil and some trapped air being in the way of convection.

I haven't actually dissected this plate yet. I know the stir function still works and I'm pretty sure the element neon was still on as it went into melt down. So that may also work.

Either way, it served me well and was starting to get a bit touchy anyway, I wasn't all that upset to see it self-destruct.

But I thought I'd post these photos because I know quite a few people want to keep their plate box fresh and using foil on the surface is one way to keep splatters off. Which may end with it looking like this.

[Edited on 17-6-2011 by peach]

The WiZard is In - 17-6-2011 at 12:50

Quote: Originally posted by sternman318

... standard 500 ml beaker... The only 500 ml beakers I
am here of here in the US of A are Tall form. A
standard 600 ml beaker would have more bottom surface
contact with la hot plate.

sternman318 - 17-6-2011 at 14:17

Wiz- I aplogize, I meant a 400 mL beaker.

As for the plate, here is some photographs:

So, as for 200-400 watts... how does 865 sound?

Guts

Control side view ( heat control in on right, stir bar control is on left)

A close-up of the heat control at lowest temperature

A close-up of the heat control at the highest temperature

I think the head controlling element is the issue. It seems very primative, but as you turn 'up' the temperature, it closes the gap on the metallic piece behind it ( just look at the last two pictures). Does it work merely on the basis of presure between the contacts? I think I need to adjust the calibration ( by adjusting the metal collar with the grub screw protuding from it), as contact only begins at about 4 on the dial( it goes to 10). Should I worry about potentially frying the hot plate by adjusting the calibration of the dial?

And after Peach's post, I am a little scared to use a foil skirt- what is the proper 'procedure'?

[Edited on 17-6-2011 by sternman318]

watson.fawkes - 17-6-2011 at 14:26

Quote: Originally posted by sternman318

It seems very primative, but as you turn 'up' the temperature, it closes the gap on the metallic piece behind it ( just look at the last two pictures). Does it work merely on the basis of presure between the contacts?

It's a bimetallic strip. It changes shape based on temperature, so it's the sensor in your thermostat. In the cold state, it should turn on early on the dial.

sternman318 - 17-6-2011 at 14:33

That's what I thought! But it didnt look all curly like in thermostats, so I discounted that thought. So I should go ahead an calibrate it then i guess! Simple, but a really neat idea now that I think about it!

[Edited on 17-6-2011 by sternman318]

peach - 18-6-2011 at 05:16

Holy moly, 875 watts is enough to cook your dinner on! You've got no shortage of power there. And that's a nice present to get.

Before adjusting it, have a quick look at the contacts on the end of the strip.

If they're badly oxidised, that may be why you're not getting enough current through to the element.

Grab a nail file and use the fine side to give them a very gentle, short wipe. Or use some high grit sand paper.

Sealed relays use blobs of mercury to help them create a good contact when they flip. But exposed mains rated contacts that are routinely flipping are prone to oxidation issues.

Now we have mondo solid state relays reasonably cheap, the mercury's on it's way out.

If the contact only closes at 4, it does need tightening up a bit. You want it so it's closes just as you raise the dial from 0.

These are another one to watch out for in terms of oxidation and faults - mains contactors. If they still snap when the close voltage is applied, they're still working and just need a clean.

Solid states have no moving or exposed parts, so the mechanical issues are gone. These are about £10.

Couple the above to one of these (a £20 PID controller that usually includes a thermocouple) and you've got yourself a digital PID hotplate.

[Edited on 18-6-2011 by peach]

m1tanker78 - 18-6-2011 at 10:50

The contacts look fairly worn. Peach put forth a good method to clean the contacts. I would add a couple of things.

1. If you clean the contacts with an abrasive, do so after you make any adjustments to the spacing by carefully inserting the abrasive sheet in between then manually closing the points. It should only need a couple of 'swipes' on each contact (if single sided sheet is used).

2. Most times, these mechanical contacts are magnetic. This is done to prevent excessive arcing (retard contact fouling) and to reduce switching bounce. As the points heat up, they lose some of their magnetism and open the circuit. Degradation of magnetic contacts will result in shorter ON times. If you aren't careful with the adjustment, the contacts may stick during operation and cause a meltdown.

3. I noticed that the base of the hot plate seems to be missing the feet. If fresh air can't enter the device from the bottom, the heat inside the device won't be able to dissipate as quickly which will result in shorter ON times. This will also shorten the lifespan of the internal components (few as they may be).

4. Like the label says, unplug the unit before servicing.

Good luck,

Tank

peach - 19-6-2011 at 01:08

Well spotted on the feet.

There are threaded holes in the corners, see if you can grab some from eBay RSS or Farnell.

If the screw on ones are too pricey, get some generic self adhesive ones from a pound shop.

[Edited on 19-6-2011 by peach]

sternman318 - 19-6-2011 at 16:34

So I did the adjustment, and everything seems to be working well. A light goes on when it is heating, and I watched it go on/off/on ect when it reached temperature. And guess what... it boiled

Thank you for the suggestions Peach, but I will stick with the original hardware until it becomes necessary to replace it.

And I will work on getting those feet, thank for spotting it!

And as for the magnetism, I didn't quite see them doing a little magnetic-like pull towards each other, but Ill keep an eye out.

But one question still remains, how do I apply the aluminum foil? Do wrap it around the glass, loose or tight ( immature people giggle), ect. I don't want to make a little saucer like Peach did hehe.

peach - 20-6-2011 at 06:25

Good to hear it's going again! I'm sure it will serve you well with all those watts.

Suspend a flask over the hotplate, leaving a small gap under it so it's not resting on the plate. If you're using a clamp stand and a big flask, add bricks to the base of the stand to make sure it's stable.

Get yourself some budget line, ultra cheap foil from the supermarket. It's usually best to wrap with two separate pieces, one from each side because getting one long piece round in one go can be tricky.

Squeeze or scrunch the long edge around the openings at the top and begin draping it around the flask. Use your fingers as poking sticks and hone up your dress making skills by straightening the foil up, such that there is a big air pocket around the flask. It doesn't have to be perfect, some bits are inevitably going to end up resting on the glass, but you want a decent sized void.

Run the skirt down to the surface of the plate, but don't go down any further. Leaving some gaps here is a good thing as it allows convection to move air through the tent. Wrapping it all really tightly will encourage the melt down effect and slow down the heating of the flask; as stagnant pockets of air will overheat and not move past the flask to warm it up.

Leave some openings around the necks to allow the hot air to convect through.

When the girl in the kinky 60's style Red Indian outfit appears, the skirt is complete.

If a reaction starts going awol, you can easily and quickly cool it again be gently removing the foil (which won't be all that hot, or stay hot for more than a few seconds).

The foil is not being used as a conductor, it's being use to contain and reflect the heat. Foil is used simply because it can get hot, it's cheap and easy to shape into the tent.

As my other photos demonstrate, get hot it can.

The foil TP has never given me any problems in terms of plate doing what it did in the other photos.

Please keep in mind when looking at my plate, that is years worth of daily, harsh abuse followed by a complete meltdown. The TP alone shouldn't do that. But laying the foil flat on the plate will when the element is up high. I have a suspicion yours would also be susceptible to a melt down, as the bimetallic is not on the plate surface it's self.

It is far, far safer, cleaner and easier than using oil baths, and can reach the same temperatures. With big flasks, it may take a while to warm up, but so do cold oil baths and it will get there if the skirt is loose and well wrapped.

Hot air guns are handy and cheap things to have. Those can reach about 400C. They're good for developing stains and heating test tubes.

There are also a few interesting threads related to warming flasks on here.

This is a great thread by smuv, in which he takes apart one of his mitten style heating mantles to fix it. This shows you precisely how they are made. Again, getting in touch with your girly side is required, as it involves fabric, cutting and stitching.

In this one, cyrus suggests a thing to go on the top of a plate made out of steel pipe with some mesh over it for the flask to rest on.

This is a similar idea to the heating inserts available for mantles, which have been introduced to remove the oil baths from labs. The product is a block of aluminium that sits on top of the plate. They come with different bowl shaped inserts that drop in to change the flask sizes. It is a really nice idea (I was thinking of making some of these myself), but they are kind of expensive (a few hundred dollars).

[Edited on 20-6-2011 by peach]

m1tanker78 - 20-6-2011 at 10:02

Sternman: Glad you got it going. If you can't get hold of some feet for it, you can temporarily prop it up with 4 pieces of tile at each corner. Make sure the unit doesn't become unstable if you do.

Peach: Nice writeup! BTW, there are other ways to melt aluminum for casting, ya know?!? That mamasita in the photo is hot enough to melt tungsten.

Tank

Neil - 20-6-2011 at 10:53

I love these peachy little informational interludes, good job peach.

sternman318 - 21-6-2011 at 15:08

Thanks Peach!!! I also enjoy your posts haha