Accuracy of the measurement system shall be ± 1% of the measured value. This may be achieved by the use of additional sensor pairs in the transducer unit giving multiple measurement paths. 4.4.4 Thermal Flow Meter Thermal flow meters shall be used where there is a requirement for continuous mass flow measurement. As such, these meters are particularly suited to gas measurements where density may vary significantly due to other factors. Flow is measured by monitoring the cooling effect of the fluid as it passes over a heated transducer. As the fluid passes through the instrument, it passes over two temperature sensors. One temperature sensor is used conventionally, while the other is used as a heater. The temperature transducer monitors the fluid temperature, while the self heated transducer is maintained at a constant differential temperature above the measured fluid temperature by varying the current passing through it. Higher flow will increase the cooling effect and hence higher current will be required to maintain the temperature differential. The thermal flow meter shall consist of a transducer and transducer may be either a flanged pipe section or a boss Insertion types may have an integrated transmitter unit or interconnecting cabling. Flanged pipe section transducers shall with interconnecting cabling. a transmitter unit. The mounted insertion type. remote transmitter with use a remote transmitter Flanged pipe section transducers shall be used for installations in small diameter pipe work (up to 150 mm diameter). The transducer shall be an inline type with two PT 100 resistance temperature detector sensors fitted. Insertion transducers shall be suitable for mounting in larger diameter pipe work (greater than 150 mm diameter) and shall be of suitable length for the size of pipe they are to be inserted into. The transducer shall be fitted with two PT 100 resistance temperature detector sensors. The transmitter shall be a microprocessor based unit, with an integral operator interface, and shall be housed within a corrosion resistant enclosure. The operator interface shall have an alphanumeric display, bar graph and simple to use push button interface. Error codes, variables, configuration data or other internal values shall be accessible and capable of being displayed on the integral operator interface. It shall be possible to limit access to configuration data parameters by means of a configuration password. The transmitter shall be powered from a 110 V AC or 18 V to 30 V DC supply of sufficient current rating to power the heater. Output shall be by 4 mA to 20 mA current loop which shall be proportional to the measured flow. A digital output shall also be provided, configured to generate an alarm in the event of an instrument fault. The instrument shall give measurements at an accuracy of ± 5% of the measured value. 4.4.5 Orifice Plate Flow Meter Orifice plate flow meters shall be used for continuous volumetric flow measurement of liquids with low solids content in closed pipes where high head losses can be tolerated. In simple terms, an orifice plate is installed within the pipe work causing a constriction, resulting in a pressure differential across the orifice plate. The pressure drop will be proportional to the square of the flow rate. The flow rate is obtained by measuring the pressure differential and extracting the square root. The flow meter shall comprise a transducer, a separate transmitter unit and an interconnecting cable. The transducer shall take the form of a section of pipe with the orifice plate installed within the section. Pressure sensors shall be preinstalled upstream and downstream of Page 119