盐度计的工作原理 The basic principle behind the operation of this very commonly used instrument is the fact that pure water has a very high electrical resistance and therefore a very low conductivity, to the extent that absolutely pure water can be considered as non-conductive. If sea water is added to pure water, the resistance to the passage of an electric current fails, i.e. the conductivity increases. This instrument measures the conductivity of water such as in boiler feed system by passing a sample through a small cell containing two platinum electrodes and measuring the current flow that occurs due to the conductance effect of any salts present. Readings are based on the assumption that the conductivity is due to sodium chloride (Nacl)or CI as the case may be, but other salts such as magnesium chloride and calcium chloride will almost certainly be present. It is assumed for convenience that all conductivity is due to sodium chloride, and although this is not strictly accurate, it is sufficent so for practical purposes.