Pressure switches are not measuring or control devices; accordingly, they are not set to a scale but with the aid of a pressure gauge. Any scales which are provided are for the purpose of coarse setting or to enable a setting to be found again easily after the switching point has been changed. At the time of first commissioning, after lengthy pauses in operation or in cases where pressure fluctuations are present which go far beyond the switching points, it must be expected that unavoidable switching-point variations will be encountered to a greater or lesser degree, depending on the type of pressure sensor involved. In cases where particularly high requirements are placed on switching accuracy, test-running of a sample unit under actual operating conditions is recommended. There are three types of pressure switches.