Sciencemadness Discussion Board - Inefficient/Bad Hot Plate?

Inefficient/Bad Hot Plate?

zombiedude1 - 24-10-2015 at 15:14

I have a VWR Model 320 Hot Plate w/magnetic stirrer i got from ebay.
(http://www.ebay.com/itm/VWR-SP-35825-Model-320-Phase-1-Hotpl...)

I've been trying to boil 150ml of water on it at full power in a 500ml flat bottom pyrex flask directly on top of it. (even with the stirrer) for almost an hour now, and it won't reach 100C. It stays around 87-90C.

It's older, so it has a mechanical temp. sensor (the connection metal bends under heat to turn off the power, and when it cools it connects again).

So is this hot plate bad?

Sulaiman - 24-10-2015 at 16:03

I have a cheap hotplate with bimetalic power control,
I had to bend part of the switch mechanism to get full power restored, now OK.

Unplug, open casing, short out thermal switch/regulator, power up.
Your hotplate should now be on permanent full power.
If you can now boil water easily then all is well except for the bimetalic switch.
Unplug again, examine switch to see how it works and bend whichever piece is necessary to give control up to full power again.
You may need a couple of attempts, just remember to unplug each time you want to touch the internals.

zombiedude1 - 24-10-2015 at 16:15

I took it apart and unscrewed the tiny allen screw on the main power bar.
Twisted it back to where it now turns on with a very small twist of the dial.
(Previously it was 3 1/2 before it would even turn on)

Put it back together, and it boiled the water within 10-15 minutes. Perfect!
Although, now the dial rests at 3 and is full power at 1; not a big deal.

I just hope I'm getting its full potential now, it is an 865 Watt hot plate!
Thanks for your advice Sulaiman.

alive&kickin - 24-10-2015 at 23:55

zombiedude, I have a vwr375 hotplate stirrer that needed the bimetallic switch replaced. Found out that it's called an infinity switch. Here's the link to the one I used and it works like new. https://www.etundra.com/restaurant-parts/electrical-parts/sw... Hope this helps if you need it.

RareEarth - 25-10-2015 at 19:15

I've noticed my hotplate seems to be quite weak lately. At first I thought it was all in my head.... but this hotplate I have is over 500w and for some reason takes 10+ hours to boil off 250mL of water... mine might have the same problem as yours, even though my plate can reach the full temperature it is designed for.

I may have fried it from spilling stuff on it in the past.

[Edited on 26-10-2015 by RareEarth]

zombiedude1 - 26-10-2015 at 13:35

Quote: Originally posted by RareEarth

I've noticed my hotplate seems to be quite weak lately. At first I thought it was all in my head.... but this hotplate I have is over 500w and for some reason takes 10+ hours to boil off 250mL of water... mine might have the same problem as yours, even though my plate can reach the full temperature it is designed for.

I may have fried it from spilling stuff on it in the past.

[Edited on 26-10-2015 by RareEarth]

Interesting... I'll post an experimental thread to determine efficiency of hot plates where members can participate and compare.

ganger631 - 26-10-2015 at 14:32

Weird. I recently purchased a 500 ml heating mantle off ebay and have been experiencing the same fiasco. My heating mantle cannot even boil water!. However, the ebay seller claims it can reach max of 380c.

zombiedude1 - 26-10-2015 at 16:58

Yeah, it mostly has to do with thermal conductivity.
If your hotplate is able to reach 380C, it should have no issues boiling water, unless there were a conductivity issue.

Is your heating mantle older or newer?
(Temperature controllers vary, as shown with older models having a bi-metallic switch)

Have you tested the surface temperature of the hot plate?

Either way, I'm starting to think these hotplates are highly inefficient after using them, especially for the price you pay! I've been able to heat up to boiling point 10 gallons of an ethanol based mash within 20-30 minutes with no issues using propane, while nearly a 1kw/h hot plate takes approximately 13 minutes to reach the boiling point with 150ml of water?

A 20# propane tank will run 6-8hrs maintaining 12gallons of boiling 12% ethanol/water mix in a large stainless steel keg approximately 5-8x depending on how long/hard I run the turkey burner. At $20 a refill, that's about 50 hours of boiling time for 12 gallons. The cost is an average of .40 cents per hour to maintain a very even and constant boiling temp. with 12 gallons.

At 15 cents per kw/h; and assuming an average for a good hot plate is 800W, and the fact that the hot plate is only really on 1/2 (guess) the time on average after temperature is maintained. So it may cost perhaps 4-8 cents per hour to run. (Also varies greatly on the type of hotplate, electricity rates, etc...)

Considering this, if the factors were linear, it would cost nearly $3/hr to be able to accomplish the same heating power using the efficiency similar to the hot plate. Of course this is impracticable, inaccurate, and not very reasonable.

It would be more practical to heat up larger volumes of flasks in an oil bath heated by a propane burner... if it weren't so dangerous (DONT do this). How about a heating element infused oil bath container w/magnetic stirrer? It seems like these are for sale; but they are pretty expensive. Perhaps I should design my own DIY topic on how to create one. (And make one for myself)

[Edited on 27-10-2015 by zombiedude1]

zombiedude1 - 26-10-2015 at 17:00

Quote: Originally posted by alive&kickin

zombiedude, I have a vwr375 hotplate stirrer that needed the bimetallic switch replaced. Found out that it's called an infinity switch. Here's the link to the one I used and it works like new. https://www.etundra.com/restaurant-parts/electrical-parts/sw... Hope this helps if you need it.

Thanks, but my design is a bit older I think, and may not be easily compatible with that switch. I managed to fix it (I think?), it boils water now. I never got a chance to see if it's using it's full potential due to me not having it when it was new.

[Edited on 27-10-2015 by zombiedude1]

ganger631 - 26-10-2015 at 18:30

Here is the link that i purchased from http://www.ebay.com/itm/500ml-250W-110V-Electric-Temperature...

At this point i dont know what to do. It started smoking when first used probably because of the fiberglass stain and later started to subside. However, now im faced with this problem.

zombiedude1 - 26-10-2015 at 19:51

Perhaps it's a defected unit.
Contact the owner, I've ordered from him before.

Tell him you think it's defected, it won't boil water, and it's smoking.
Ask for him to pay return shipping and to ship you a new one.

It's best to purchase a used unit for around that price. Less problems usually, as the china hotplates/mantles have very bad reliability issues. Either way, no seller likes a negative review on ebay, so tell him what you want and how you want it done.

[Edited on 27-10-2015 by zombiedude1]

RareEarth - 27-10-2015 at 10:28

Quote: Originally posted by zombiedude1

Either way, I'm starting to think these hotplates are highly inefficient after using them, especially for the price you pay! I've been able to heat up to boiling point 10 gallons of an ethanol based mash within 20-30 minutes with no issues using propane, while nearly a 1kw/h hot plate takes approximately 13 minutes to reach the boiling point with 150ml of water?

Propane isn't a good comparison here. Gas based fuels will almost always be able to boil water faster than their electrical counter parts. Not only does it get hoter in general, but the heat/btu output is way higher.

You can get a "small" propane stove for camping that puts out 55000 BTU/Hr.. just that is equivalent to about 15,000 watts or so, roughly.

If you need to boil a large amount of liquid, and you don't have access to industrial power (or stove power, which the traditional kitchen stove uses much higher rated power outlets), then gas is always preferable. The problem herein lies in that many things we boil/reflux when doing chemistry are combustable, meaning using gas can be deadly if you do not take proper safety precautions.

zombiedude1 - 27-10-2015 at 12:07

Quote: Originally posted by RareEarth

Quote: Originally posted by zombiedude1

Exactly.
I don't plan on using gas power to do much of anything chemistry-related due to the use of many combustible chemicals.

I just feel that there are more efficient ways to heat flasks than the old Hotplate + Oil bath methods. A hotplate is good due to being versatile, but most around here are heating up RBF's with hotplates and oil baths, or copper coated bb's, etc... Heating mantles I hear are great, but generally don't provide as good of surface area than oil. (Biggest drawback being they are not as versatile, and are generally more expensive)

Placing a heating element inside the oil bath with a simple controller sounds much cheaper & more efficient than using a hotplate for this purpose. I'm trying to determine whether this is a good route to take, due to my lack of advanced knowledge of science that most here possess.

[Edited on 27-10-2015 by zombiedude1]

JJay - 27-10-2015 at 12:45

Putting a heating element in oil bath with a simple controller is cheap and easy. It's not *that* much more efficient than using a hot plate with an oil bath, but it heats very evenly if the bath is stirred. I got the idea from a practical organic chemistry book that I read a while back... it was an excellent book... I'll try to hunt down the title.

Oh and sophisticated temperature controllers are pretty inexpensive... it would be fairly trivial to set up a fixed temperature oil bath. Pure mineral oil baths can be heated to at least 300 C or so.

[Edited on 27-10-2015 by JJay]

JJay - 28-10-2015 at 08:38

The book was Advanced Practical Organic Chemistry by Leonard et al.

zombiedude1 - 28-10-2015 at 15:16

Great info JJay, thank you.