Measuring the TOG value of a duvet

[pete]

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?

From a DIY perspective, I'm planning on measuring the temp. under the duvet, while warming the bed with an electric blanket (and having the room temperature more-or-less constant). The idea is that the greater the thermal insulation, the higher the under-duvet temperature will be. The tricky bit will e converting this temperature to a heat- loss figure and then to a thermal resistance figure. Any pointers on how to do the sums would be appreciated.

[Stuart Noble]

Quite fancy a bit of tog testing myself

[Gareth]

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?

[GB]

There are two ways to do this. One is a steady state approach. Wrap a small (very small) heater in a duvet, wait for the temperature to stabilise, and measure the temperature drop between the inside and the outside. You need to measure the voltage drop and amperage of the heater, as well as the surface area. All rather difficult to do, especially making sure there are no folds in the duvet around the heater, etc. You'll need a fan to keep the room air circulating briskly.

Marginally easier is just to wrap a known weight of warm water in a duvet and plot the temperature drop.

I doubt that you'll get closer than around 50% without cutting the duvet up, which rather defeats the purpose.

[Gio]

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

[Gareth]

Sorry I meant with one mattress, one duvet, then the same mattress and the other duvet, then the same mattress and both duvets.

>

[geoff]

In message , pete writes

See which the cat prefers

or .. get a life

[Owain]

Would the cat prefer the warmer duvet for better self-insulation, or the cooler duvet because it lets out more human heat?

Owain

[geoff]

Have you ever met a cat which prefers "cold" ?

[Fredxx]

A way I'd suggest is similar to Gareth's but I think should be more accurate and should give a direct comparison.

Use an electric blanket on a mattress with the two duvets on top of each other. It might be an idea to use a third thin duvet or blanket on top to trap the top temperature probe. Use 3 temperature probes, one on the electric blanket - 1st duvet interface, second in between the two duvets, and a third on top the top duvet. You'd have to zero, or correct the probes, but the temperature across each duvet should be in proportion to the TOG number. It would be prudent to swap the duvets round and average the results.

Good luck!

[pete]

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.

[Usenet Nutter]

Could it be done with one of those remote thermometers mentioned in here a few days ago. .a Fiver on Ebay ....could you put the duvet(s) in turn on top of the mattress with the electric blanket on and point the thermometer at the duvet(s) and see what the difference in temp was for each one .

[PeterC]

As a bit of an aside, I've a 4.5 TOG that's better than a cheap 9 TOG - the major difference seems to be that the 4 TOG is Hollofill fibre.

I recently bought a 7.5+3.5 TOG combo from Ikea and paid the bit extra for Hollofill. The 7.5 part is lighter than my old 4.5 and just a bit warmer (about 15 years between purchases, to 'newer' material?).

[John]

Do you think the weight and flexibility of the duvet has an effect? If it moulds itself to your sleeping form it may give the impression of being warmer than one which leaves gaps down your sides.

[Owain]

No, but which is warmer - wrapping yourself in a blanket or sitting on top of a radiator? And most cats don't like being *under* duvets.

Owain

[Usenet Nutter]

But that last bit is probably instinct for self preservation so they don't feel trapped and can make a swift getaway .I had a cat that used to burrow under a duvet but only when I was in the bed so presumably it felt safe enough in that situation.

[geoff]

I thought you were putting the duvet on top of an electric blanket

[Dave]

Strange animals are cats. A bit like women ;-)

I've got me coat and I'm off.

Dave

[PeterC]

Yes. I have a top sheet (saves on washing a bit) and have to pull in the bedding (room temperature at ~14C) but did try just the duvet with cover and it didn't have the cold edges.

[Dave]

We used to have three different tog value quilts and when we were heading into winter, would change to a heavier one depending on if Betty swollocks left the bed and lightened them in the same way when the weather warmed up in the spring.

Dave