Temperature indicator strips or, second best because they are not so easy to fit, metal melting tabs. The UK happens to have the world’s leading manufacturer of temperature strips: Thermographic Measurements Ltd. of Connah’s Quay, North Wales, which makes products which are distributed to the remanufacturing world by Dave Hadley Engineering Ltd. The company has been manufacturing temperature sensitive labels for more than 25 years and claims to have 90% or more of the UK market.

Thermographic’s "Thermax" strips are used very widely in the automotive world, and similar devices can also be found checking temperatures at which chocolate is distributed and sold, ensuring that electronic components do not overheat, and showing the temperature distribution on the surface of gas turbines. Formula 1 racing is also a major user.

Russell Booth, General Manager of Thermographic says the company has supplied temperature strips to all the F1 teams at one time or another to check when recommended operating conditions for engines, engine management units, radiators and a host of other parts are being exceeded. Though perhaps the most interesting uses of strips will never come to light, as engine and vehicle manufacturers use them to help iron-out design faults on new products.

But how do they work? The self-adhesive strips each contain several small white paper squares, each impregnated with a thin layer of an organic, microcrystalline wax-based chemical which turns black at its set temperature. The strips most commonly used in automotive engine applications contain squares which turn at 88, 99, 110 and 121ºC. In an engine, the most commonly asked question is this: has this engine ever dried-out or not? A strip firmly stuck onto the core plug or elsewhere will provide the answer - if the 110ºC square is black then the engine has overheated at some time since the strip was fitted.

These strips are irreversible, that is any black areas stay black, preserving the evidence of overheating until someone chooses to look. They are also impervious to oil and water.

Dave Hadley says: "With temperature strips, I’m basically selling insurance. A strip can mean the difference between a successful insurance claim and one that is turned down. And if a strip can save you the cost of one replacement engine a year, it has been well worth it."

And how well do they perform? Thermographic say their strips are accurate to within plus or minus 1ºC below 100ºC and to within plus or minus 1% above that. To prove it, Russell Booth showed ER&R how they are tested. Strips are stuck on to a silver block which is heated electrically. The tester notes the temperature at which each part of the strip begins to and completes turning black - this is usually between 0.5 and 1ºC - and the mean of these two readings should match the strip ratings.

Temperature is measured using an embedded thermocouple to accurately record the temperature of the plate. The whole process is carried out in an ISO 9000 environment on testing equipment which is accredited and recalibrated every six months by independent certification authority NAMAS. The tests we witnessed were spot-on.

Thermographic manufactures an enormous number - 32 million a year - of temperature labels and a huge range of specially-designed products covering temperatures from -30 to +1270ºC. Products include labels for which the color change is reversible and irreversible, devices based on thermochromic liquid crystals and a series of paints, inks and crayons which change color according to temperature.

Oddly, there simply isn’t a chemical ready to turn black at every point on the thermometer, although Thermographic is forever developing and testing new ones, and thus the use of squares around 10ºC apart on the automotive strips.