The Series 1800 Monitors alarm circuit is unique. If the instrument logic detects sample concentration is increasing at such a rate that alarm 2 value will be reached within an hour, a warning signal is activated. This signal is a one second alarm every four seconds which increases in frequency as a concentration approaches alarm level. Also, if absolute sample concentration is 90% of alarm 2 value, intermittent warning signal is activated to alert operator that high concentration limit is being approached. If this feature is not desired, the alarm 1 should be used to set external alarm.