On Sat, 30 Jan 2010 13:15:22 -0000, Gio wrote:

"Gareth" wrote in message
om...

On 30/01/2010 10:57, pete wrote:
Before xmas we bought a new (cheapo) duvet, as it was cheaper
to replace the old one than the cost of getting it cleaned.
Anyway, right out of the bag the new duvet didn't appear as good
as the old one. They were both rated at 13.5 TOF, but the new
one was obviously thinner. It wasn't as warm either (even taking
the colder weather into account) and we needed an extra blanket
on top.
Fast forward to this week and we got another cheapo duvet - this
time from Sainsburys, which again is rated at 13.5 TOG and is much
better then the last one we bought (compares well to the original
one). So purely out of interest, I'd like to test if the TOG rating
of the various duvets measures up - has anyone else tried this?

snip

Any pointers on how to do the sums would be appreciated.

A relative measurement would be quite straight forward, i.e. it will be
easy to compare the two duvets you have bought and see if the
sainsbury's one is better. Doing an accurate measurement will be hard.

A tog is 0.1 m^2K/W. In other words, the thermal resistance in togs is
equal to ten times the temperature difference (in °C) between the two
surfaces of a material, when the flow of heat is equal to one watt per
square metre.

When the temperature under the duvet reaches equilibrium (stops rising)
the heat loss will be equal to the power of the electric blanket, which
I assume you know?. You can easily measure the length and width to work
out the area and you can measure the temperature difference between the
room and under the duvet. What you don't know is how much heat is being
lost through the duvet and how much through the mattress.

You could probably make measurements with one mattress, then the other
and then both and assume the heat loss through the mattress is the same
in all three cases then solve as a set of simultaneous equations to
eliminate the heat loss through the mattress, but I don't suppose you
are really that interested?




This will be quite interesting if the OP posts his findings later because it
is something we have often thought about when buying quilt. As the original
poster says there are differences in heat retention between makes offering
the same TOG rating. Alas we have just dismissed the issue in the past when
by rights I suppose we should have directed it towards Trading Standards ?

Gio


Well, that was an education. Here are the numbers. First for the temperature
between the duvet and the electric blanket, which was under a sheet:

Old duvet, no duvet cover. Reached equilibrium after 90 minutes at 38.5°C
with cover 49.0
New duvet uncovered 51°C
with cover 56.5

measurements taken in roughly the centre of the electric blanket, with
minimal disturbance, sticking my arm in to take the measurements :-)

The outer surface temp of the old duvet was 23.5°C uncovered,
24.5 covered
For the new duvet uncovered 23.5°C
with duvet cover 22.0

measurements taken with one of the £7 eBay non-contact thermometers from
earlier discussions. (BTW, I bought 2, they both read the same values)

So the big thing is delta-T, which comes out at 15, 24.5, 27.5, 34.5
respectively. The room temperature varied between 18 and 19 degrees during
this time. I'm ignoring this as I reckon it's compensated for by the
measurements of the duvet's outside cover measurements.
I'm also assuming that the transmissivity of the white duvet surface and the
darker duvet cover (for the IR thermometer to make accurate measureemnts) is
roughly constant, too.

The electric blanket measured 125 x 116 cm (1.46m²) and I measured it's
power consumption at 61 Watts. So I'm using a figure of 41.8 Watt/m²
Using the formula from Wiki of TOGS = 10 * delta-T / Watts/m² results in
a TOG rating for the old duvet (sold as 13.5TOG) of 3.6 without a cover
and 5.9 when it's inside a duvet cover. The new one measures at 6.6 and
8.3 respectively.

Now although these numbers appear grossly low, I expect that a lot of the
heat from the electric blanket was dissipated in the matress, so I'm willing
to allow an uncertainty of +100% on that basis - which puts the new duvet
in the right sort of value. However the old one is nowhere near it's advertised
value.