On 29/11/2017 13:59, whisky-dave wrote:
On Wednesday, 29 November 2017 12:28:44 UTC, Martin Brown wrote:
On 29/11/2017 10:48, whisky-dave wrote:

Anyone with any insight would have known that 202/240 should give you the fraction that you use to multiply by so :-

No it doesn't.

Yes it does.

No wonder UK engineering is in such a mess if you are typical of the
people who are supposed to be teaching it.

A heater of 2KW at 240V will consume 2000W
A heater of 2KW at 202V will consume 1683W

So if it's consuming 700W already we have 1683-700= 983W NOT 1300W that you expected.

He is more nearly right that you are.

No he is NOT because those are the real world results I get.
This is ONE of the reasons why we run both theory and practicals.

WOW! I knew engineers could be pretty dumb but I didn't think there
would be anyone claiming to be an engineer who couldn't grasp Ohm's law.

Anyone that understands the basics
of Ohms law will know that the power dissipation in a resistor R is I*V

= V^2/R

OK here's the resitances at cold of the relivant parts.
at 240V at 230V (the speicifed operational volatge)
I 66.4R 867W 797W
II 44.9R 1283W 1178W
III 27.1R 2125W 1952W

Don't you think that my estimate of 29R when hot is a rather good
approximation to your case III when the device is providing 2kW?

Hint: 27 is 66 || 45

R won't quite be constant but it won't vary by all that much either.
R might be a percent or so lower at 1.4kW since the element is cooler.

which element ? element I or II

The combination III where the device is supposed to output 2kW on a
decent mains supply.

Cheap and nasty resistive electric heaters are usually configured
series, single and parallel to present loads of V^2/2R, V^2/R and 2V^2/R
to the mains being low, medium and high heat settings respectively.

Yep and this is what I first suspected of this ~£25 heater.

It is the standard way of doing it. Electric blankets have their lowest
setting with a grubby diode in series to make it slightly less deadly in
the event of insulation being compromised and halves the power.

You perhaps ought to worry if your lab mains voltage is as low as you
say it is something somewhere must be dropping nearly 30V.

I think it's the heaters as the lab was NEVER designed to be heated by heaters connected to the labs mains supply.

It really, is that simple.

So we have an engineering lab staffed by people who are too dumb to
figure out for themselves how to make the place comfortably warm! Worse
still they are probably overloading the lab ring main in the process.

No wonder that UK industrial productivity is so pathetic.

If you want to warm the place up quickly you heat the *AIR* in the room
with a powerful fan heater - convection heaters are useless unless you
live on the ceiling where all the warm air accumulates.

--
Regards,
Martin Brown