1535 Relative Humidity Transmitter - Novatech Controls

!2.24 DUST IN THE DRIERThe preferred method of mounting an oxygen probe for dust laden applications is facing vertically downwards withthe filter removed. With no filter some errors in gas calibration can occur when the gas sample has an oxygen contentsignificantly different from that of the flue gas. Allow for this when setting auto-calibration tolerances in the set-upmode. An occasional automatic back purge is helpful with probes mounted at 30° downwards with or without a filter.Normally heated probes are supplied with filters for dusty applications. The probe response time should be testedwhen the probe is first installed and then regularly until it remains constant for a significant period. A partiallyblocked filter will slow the response of the probe. Many probes in the field operate continuously in this way formany years with an occasional filter purge. To test the probe response time, use a stop watch to obtain the time for aprobe to achieve a 63% change from one reading to another.If a probe filter blocks completely in a short period of time, then there is no option but to use the probe without thefilter. Trial probes with filters are available to test whether filter blockage is likely to occur.2.25 STRATIFICATIONIf the transmitter and probe have been fully tested and the oxygen readings in the drier are incorrect, gas stratificationmay be occurring.The phenomenon cannot be anticipated for any particular installation. The problem is normally solved by moving theprobe to a new location.NOVATECH HUMIDITY TRANSMITTER Page 31