Under normal and recommended operating conditions, the monitor will display negative measurements when no water is present. The reason for this is that the air present in the flow cell without water has less background fluorescence than water. This is a normal situation; however, the analog output, which will be 4-20 mA, will be limited to 4 mA or 0 ppm, if so programmed, even when no water is present. Thus, the remote indication will be 0 ppm in the event of no water flow.
The monitor will also display negative numbers if the measurement is less than the value of the blank. Although negative concentrations are only theoretical, a display of a negative concentration is very important diagnostic information for our monitor since fluorescence is never measured from an absolute zero, but in reference to a defined zero. Thus, a negative measurement in the presence of water indicates that the defined zero, which the customer selects, is not appropriate. If the display was truncated to zero, this situation, which will occur in water obtained from natural sources, will never be detected. It could also cause false measurements from the monitor if not corrected.