"Martin Brown" wrote in message
...

[snip]

A quick and dirty rule of thumb is that the cost to
maintain the temperature an extra 5C above ambient roughly doubles.

That does not make sense. What would it, according to that rule, cost
to keep the temperature just 5 deg C above ambient? Nothing?
In fact, the cost does not vary exponentially, but is is roughly
proportional to the desired temperature differential.

Franz

Franz Heymann wrote:
"Martin Brown" wrote in message
...

A quick and dirty rule of thumb is that the cost to
maintain the temperature an extra 5C above ambient roughly doubles.

That does not make sense. What would it, according to that rule, cost
to keep the temperature just 5 deg C above ambient? Nothing?

It is an engineering rule of thumb, Franz, not a law of physics!

If it costs X to maintain 5C above ambient it costs 2X for 10C and 4X
for 15C. It climbs quite a bit faster than proportional to the
temperature difference because of convection kicking in.

In fact, the cost does not vary exponentially, but is is roughly
proportional to the desired temperature differential.

Only in the draft proof ideal world of the physics lab.

Regards,
Martin Brown

In article ,
Martin Brown writes:
| Franz Heymann wrote:
|
| In fact, the cost does not vary exponentially, but is is roughly
| proportional to the desired temperature differential.
|
| Only in the draft proof ideal world of the physics lab.

Hmm. Until you get up to the point where radiation losses start
to be dominant, it is a pretty good approximation. All of the
usual causes of loss (including forced and unforced convection,
conduction, air and other material exchange etc.) are very close
to linear in the differential.

However, the cost CAN increase 'exponentially' when the desired
range is below the maximum external temperature, and there is no
cooling. This isn't because the losses aren't linear, but is an
artifact of the way that the cost is asymmetric.


Regards,
Nick Maclaren.

"Janet Baraclough" wrote in message
...
The message
from "Jodie" contains these words:

Ah, most helpful

Ignore the demented troll, Jodie.

Janet
--------------
Trying...I'm trying :-)
Thanks BTW
Jodie

"Martin Sykes" wrote in message
...
"Alan Holmes" wrote in message
...

"Jodie" wrote in message
...
Have a nice new electrical greenhouse heater. Its set to automatically
start and stop with temperature.
So as to calculate power use and hence cost of greenhouse heating anyone
know of a good inexpensive system of recording the duration the heater
was actually on?

Connect a cheap electric clock in paralell with the heater, then all
you have to do is read the time!


But the clock won't be switched on and off by the thermostat if it's built
in to the heater.

It's not impossible to connect to the thermostat.

--
alan

reply to alan(dot)holmes27(at)virgin(dot)net

This Watt Meter should provide the complete answer to the original question:

http://www.machinemart.co.uk/search.asp?q=watt+meter

Fred

Appolgies if this post appears twice - my news server is playing up.

This Watt Meter should provide the complete answer to the original question:

http://www.machinemart.co.uk/search.asp?q=watt+meter

Fred

"Martin Brown" wrote in message
...
Franz Heymann wrote:
"Martin Brown" wrote in message
...

A quick and dirty rule of thumb is that the cost to
maintain the temperature an extra 5C above ambient roughly
doubles.

That does not make sense. What would it, according to that rule,
cost
to keep the temperature just 5 deg C above ambient? Nothing?

It is an engineering rule of thumb, Franz, not a law of physics!

My first degree was an engineering degree. We learnt all about
Newtons law of cooling in the first year. I did a laboratory
experiment to verify it. It actually is an empirically verified law.
The exponential law which you quote is one of the many myths which
need to be exposed and expunged.

If it costs X to maintain 5C above ambient it costs 2X for 10C and
4X
for 15C. It climbs quite a bit faster than proportional to the
temperature difference because of convection kicking in.

Nope. The power required to heat a greenhouse does most certainly not
rise exponentially with the temperature differential.
If you looked up Newton's law of cooling, you would find that for
small differences, the power required is proportional to the
temperature difference between the object and the ambient temperature.
Newton's law of cooling has been tested quite extensively for
centuries.
Actually, if the tempersature difference becomes larger, the power
begins to rise roughly as the 4/3 power of the difference. It never,
never becomes an exponential function.
And internal convection does not have a threshold.
--
Franz
One Galileo in 2000 years is enough. Pope Pius XII

That's exactly what I want...thanks Fred ...now, lets see if I can pick up a
second hand job.
------------

"Fred" wrote in message
...
Appolgies if this post appears twice - my news server is playing up.

This Watt Meter should provide the complete answer to the original
question:

http://www.machinemart.co.uk/search.asp?q=watt+meter

Fred

The considerable fluctuation of normal usage would make such a
comparison rather meaningless in the context of what Jodie is trying to
achieve.

Regards,
Emrys Davies.



"Richard Brooks" wrote in message
...
Jodie wrote:
Have a nice new electrical greenhouse heater. Its set to
automatically start and stop with temperature.
So as to calculate power use and hence cost of greenhouse heating
anyone know of a good inexpensive system of recording the duration
the heater was actually on?
Thanks

Could you not use a cheap paraffin heater for a few days (to keep the
cacti
happy in the meantime), measure each days power usage on your meter
for
normal usage, then plug your new electric heater in, monitor the power
each
day for the same period as without electric heating then take two
averages,
without electric heatng and with electric heating ?


Richard.