On Thursday, 30 November 2017 08:33:15 UTC, Martin Brown wrote:
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.

I'm not 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.

You haven't a clue have you.
You got very confused thinking there is a 1300W setting there isn't



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


where's the 1300W you speak off, closest is at 240V,
the highest voltage here is about 219V.
At the operational voltage your 1300W is less than 1200W
So tell me what voltage you'd need to get this 1300W and what would then be the rated power of the heater at that voltage.





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?

My device has NEVER provided 2kW


Hint: 27 is 66 || 45

Yes I know, I sussed that on day ONE.

and V^2/R IS NOT 1300

So come on you claim to know ohns law how about proving it.
In order to get 1300W disapated in a 45R resistor/element what voltage is required.


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.

Define a decent mains supply .
How long would you expect this 2kW output to be maintained from this decent 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.

Surely you mean halve the voltage by removing either the +ve or -ve cycles.
Are you saying there's a diode in the radiator ?



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!

It is not our job to make the place comfortably warm

Worse
still they are probably overloading the lab ring main in the process.

They would be if we installed all the 20 2kW convection heaters that were brought for us. It was us the technicain that remined them that those heaters have beeen banned so don't order them.



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

We arentl allowed to use fan heaters than were banned even before convection heaters were.


- convection heaters are useless unless you
live on the ceiling where all the warm air accumulates.

How comes those in charge of the buiding services and building managment didn't know this ?


--
Regards,
Martin Brown