A Refractometer is a simple and robust way of accurately measuring the salinity within your aquarium and should be used by every serious marine aquarist. Most people when entering the marine aquarium hobby tend to purchase a plastic swing arm hydrometer due to the low price of these items. What many people are not aware or informed of is that this type of product can be notoriously inaccurate and can result in wild swings in salinity which is not normally discovered until there has been a system crash or livestock losses and the water is subsequently tested by a retailer. Bubbles and calcium deposits adhere to the swing arm changing its combined density and producing incorrect readings. The refractometer works on the principle of refraction of light to determine the salinity and is very accurate but also very simple and robust, unlike good quality glass hydrometers which are easily broken. Light enters the instrument at one end and passes through a sample of the aquarium water which causes it to be refracted, (bends the light), depending on the salinity of the water. The refracted light is reflected off a scale inside and can be viewed through an eyepiece on the opposite end of the instrument.