Sciencemadness Discussion Board

question about hot plate stirrer wattage

paxpax - 15-3-2016 at 03:20

i apologize if this question was already answered, i didn't find any info

im looking to buy a hot plate stirrer, but there is something
that i don't understand

what is the difference between hot plate with a higher wattage and one with lower wattage
for example, 2 hot plates stirrer from the same brand that reach the same temperature and with the same heating surface can have different wattage, one at 500w another at 550w

what im supposed to understand ?



[Edited on 15-3-2016 by paxpax]

gdflp - 15-3-2016 at 03:45

A watt is a measure of power, specifically 1 Joule/Second, thus it is energy divided by time. Two hot plates of different power, even if they have the same max temperature, will reach this temperature in different amounts of time. Other factors play into this as well, for example aluminum hot plates will heat up quicker than ceramic ones because aluminum has a lower capacity for thermal energy than ceramic does. In general though, to otherwise equivalent plates will simply have different rates at which the speed up if they use different amounts of power, the higher the power, the faster they reach their max temp.

PHILOU Zrealone - 15-3-2016 at 04:14

There is also maybe a motor under the plate for the magnetic agitator--> extra energy

DJF90 - 15-3-2016 at 04:38

Note that the maximum surface temperature is not the same as the maximum working temperature with a load in place (e.g. heating bath). I've had difficulties reaching 200 *C (sand bath temperature) on a hot plate with a 300 *C max (surface) temp because the power output (wattage) was insufficient.

Maker - 15-3-2016 at 09:41

You would barely notice the difference between a 500 and 550 watt plate.

As has been said, a higher wattage will heat up faster, but it can also heat things to a higher temperature (Even though the surface temperature is the same). If you're working with very large volumes, a higher wattage will be ideal.

Don't base your choice off wattage alone.

paxpax - 15-3-2016 at 14:16

thank you for the responses guys!
Quote: Originally posted by DJF90  
Note that the maximum surface temperature is not the same as the maximum working temperature with a load in place (e.g. heating bath). I've had difficulties reaching 200 *C (sand bath temperature) on a hot plate with a 300 *C max (surface) temp because the power output (wattage) was insufficient.


Please can you explain why you couldn't reach 200°c because of the power output ? if i understood gdflp post correctly it is just a question of time, is there something else i should know ?
Quote: Originally posted by Maker  
You would barely notice the difference between a 500 and 550 watt plate.

As has been said, a higher wattage will heat up faster, but it can also heat things to a higher temperature (Even though the surface temperature is the same). If you're working with very large volumes, a higher wattage will be ideal.

Don't base your choice off wattage alone.


What other factors should i take into consideration beside wattage, the power and speed of the magnetic stirrer, the surface length and type (ceramic...), a LCD with temp and stirrer speed diplay ?



Also i have another question for you guys

what do you think of chinese hot plates stirrer?

Brand names with LCD display a very expensive (200+ $ )

im looking for one that have an LCD with magnetic and temp dislay and Separate Temp and magnetic speed control, with enough surface to heat from 100mL to 1L flat bottom flask
and enough power to heat sand bath, heating block, round bottom flask stand etc... to atleast 200°C




[Edited on 15-3-2016 by paxpax]

Herr Haber - 15-3-2016 at 20:44

IMO the usual price difference for an LCD display would make me go for a normal analog one.
First, because what you'll be measuring is the temperature of the plate: useless. I'm more interested in the temperature Inside the reaction zone or at the top of my distillation apparatus and I have a collection of thermometers for that...

BUT, these high tech stirrers usually come with a port where you can plug in a probe and even sometimes plug them to a computer to gather data or program them (big bucks here!). Then it becomes interesting for obvious reasons.
And yes, some of these probes are Teflon coated :)

I might be tempted by one of these when I have 600 to 1000 Euros to spend. So far, I'd rather make due with what I was given for free by a friend working in a school.

gdflp - 16-3-2016 at 08:13

When you place large items on a hot plate, they will lose heat to their surroundings by both convection and radiation at a certain rate. At equilibrium, this rate is equal to the rate at which energy is being added to the item by the hot plate, which is dependent on the power rating of the hot plate. Greater temperatures will lead to a greater rate of heat loss due to a greater delta T to the surroundings, so the hot plates which are rated at a higher power will reach thermal equilibrium with objects at a higher temperature.

careysub - 16-3-2016 at 15:23

Quote: Originally posted by gdflp  
... Other factors play into this as well, for example aluminum hot plates will heat up quicker than ceramic ones because aluminum has a lower capacity for thermal energy than ceramic does. ...


Aluminum's specific heat (capacity for thermal energy) is in the same range as ceramics. It does however conduct heat much faster.