Multimat’s (www.multimat.uk.com) self-inflating and foam mats feature among the vital pieces of kit taken by many expeditions to the harshest environments on Earth. From the Caudwell Xtreme Everest Expedition in 2007 to the Kaspersky Commonwealth Antarctic Expedition earlier this year, Multimat’s reputation has led international teams heading for the Earth’s frozen extremities to come knocking at their door before embarking on their frozen adventures.
However, there’s one recurring issue that confuses even some of the most seasoned explorers and that is the different ways of expressing a product’s thermal insulation used around the world.
On the face of it there are two different measurements - the Tog and the R-value or “Thermal Resistance value”. But what is often overlooked is that the R-value in the USA is significantly different from the R-value used by the rest of the world.
The R-value is usually expressed around the world in the SI Units – that’s 'metric' to most people. It is the amount of heat that passes through one square metre of a material each second when the temperature on one side is one degree higher than on the other. Its units are 'square-metre kelvins per watt' or m²·K/W – but not in the USA. America still uses US Customary units, an imperial system similar to the British Imperial system, based on old fashioned square feet, Fahrenheit and British Thermal Units. So in America, R-values are expressed as ft²·°F·h/Btu. The result is two completely different figures.
It is particularly easy to confuse SI R-values and American R-values, because they are often cited without their units, e.g. R-2.8. Adrian Stammers, managing director of Multimat, said, “It’s the poor consumer who suffers from this misleading situation – and not just ordinary campers but serious expeditions too. We make bespoke mats to order for some of the best known polar explorers and mountaineers in the business – some of them household names - but we still have to explain it to them. It also undermines Multimat and other UK-made products as many American mats have an R-value nearly six times what it would be if they were measured using the SI system that we and the rest of the world use."
There are conversions that can be done to enable campers to compare one product with another, though you will need a calculator. A US R-value is 5.7 times the UK R-value, so divide the US R-value by 5.7 and you get the equivalent UK R-value.
Adrian added, “At Multimat we usually quote tog values to avoid misleading comparisons. Tog (from the Latin word toga) was developed for the British textile industry in the 1960s as an easier system to use for textiles’ thermal insulation qualities. 1 tog is 0.1 RSI or 10x the R-value when expressed in SI units. As a rule of thumb, our Trekker 75 self-inflating mat in our All Seasons range has a tog value of 10.0. That is equivalent to an R-value of 1.0 using UK (SI) R-values and is pretty much as warm a mat as you can get. If a camper is quoted a figure significantly higher than that for a mat they should check with the manufacturer.”