At least one accurate greenhouse thermometer is vital to the successful running of any greenhouse. Undoubtedly the best choice is the maximum and minimum type. This has tiny indicators inside the glass capillary which can be set with a magnet or, in the newest designs, by gravity. There are also dial types. The indicators will show the highest and the lowest temperatures that have occurred during the time of inspection and when they were set. For example, by setting the indicators in the evening you can see in the morning just how low and how high the temperature has been overnight. This is really the only way to properly assess any heating system. It is also invaluable in summer to check whether overheating has occurred during an absence, and the effectiveness of shading.
It is wise to buy a good quality maximum and minimum thermometer. Cheap kinds are often made from metals that corrode in the greenhouse atmosphere, the graduations becoming illegible after a short time.
Other thermometers may be required for propagators or for checking soil or compost temperatures in pots, plunges, and the like. Special soil thermometers are available.
It is hoped that temperatures will eventually be given in the Centigrade scale. This is extremely simple, with freezing point being 0° and the boiling point of water being 100°. In this site both Centigrade and Fahrenheit are given. For measuring air temperature numerous dial-type thermometers have come on to the market. These are usually reasonably accurate. For composts and liquids, glass thermometers with mercury, or spirit coloured red, are the most practical.