A low-cost capacitance hygrometer capable of measuring steady-state and some transient changes in the output of medical humidifiers is introduced and evaluated. The method of operation is presented in detail, as is a comparison with the widely used gravimetric standard. The hygrometer produces results within 7% of those given by the gravimetric method when using units of percentage relative humidity (which give the widest discrepancies). This is shown to be an acceptable figure for the proposed application. Problems encountered when converting units of humidity are discussed in detail and methods of conversion explained.