User Guide - Eurotherm Ltda

nanodac RECORDER/CONTROLLER: USER GUIDE4.16 ZIRCONIA BLOCK OPTIONThis option allows the calculation of Carbon Potential, Dew point or Oxygen concentration. A zirconia (oxygen)probe consists of two platinum electrodes bonded to a pellet or cylinder of zirconia. At elevated temperatures,such a probe develops an emf across it which is proportional to the probe absolute temperatureand to the log of the difference in oxygen partial pressure between its two ends.The temperature of the probe is normally measured using a type K or type R thermocouple. The temperatureeffect on the thermocouple is such, that for successful operation, the probe temperature must be greaterthan 973K (700˚C).4.16.1 DefinitionsTEMPERATURE CONTROLThe sensor input of the temperature loop may come from the zirconia probe but it is common for a separatethermocouple to be used. The controller provides a heating output which may be used to control gas burners.In some applications a cooling output may also be connected to a circulation fan or exhaust damper.CARBON POTENTIAL CONTROLThe zirconia probe generates a millivolt signal based on the ratio of oxygen concentrations on the referenceside of the probe (outside the furnace) to the amount of oxygen in the furnace.The controller uses the temperature and carbon potential signals to calculate the actual percentage of carbonin the furnace. This second loop generally has two outputs. One output is connected to a valve whichcontrols the amount of an enrichment gas supplied to the furnace. The second output controls the level ofdilution air.SOOTING ALARMIn addition to other alarms which may be detected by the controller, the instrument can trigger an alarmwhen the atmospheric conditions are such that carbon will be deposited as soot on all surfaces inside thefurnace. The alarm may be wired to an output (e.g. relay) to initiate an external alarm.CLEAN PROBEAs these sensors are used in furnace environments they require regular cleaning. Cleaning (Burn Off) is performedby forcing compressed air through the probe. Cleaning can be initiated either manually or automaticallyusing a timed period. During cleaning ‘PV Frozen’ is set to ‘Yes’.AUTOMATIC PROBE CLEANINGThe instrument has a probe clean and recovery strategy that can be programmed to occur between batchesor be manually requested. At the start of the cleaning process a ‘snapshot’ of the probe mV is taken, and ashort blast of compressed air is used to remove any soot and other particles that may have accumulated onthe probe. A minimum and maximum cleaning time can be set by the user. If the probe mV has not recoveredto within 5% of the snapshot value within the maximum recovery time set then an alarm is given. Thisindicates that the probe is ageing and replacement or refurbishment is due. During the cleaning and recoverycycle the PV is frozen, thereby ensuring continuous furnace operation. The ‘Pv Frozen’ parameter can beused in an individual strategy, for example to hold the integral action during cleaning.ENDOTHERMIC GAS CORRECTIONA gas analyser may be used to determine the carbon monoxide (CO) concentration of the endothermic gas.If a 4 to 20mA output is available from the analyser, this can be applied to the instrument to adjust the calculated% carbon reading automatically. Alternatively, this value can be entered manually.OXYGEN CONCENTRATIONIn order to measure oxygen concentrations, one end of the probe is inserted into the atmosphere to bemeasured, whilst the other is subjected to a reference atmosphere. For most applications, air provides asuitable reference (reference input = 20.95 for air).Page 150HA030554Issue 7 Nov 12