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Technologies

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Intrinsically Safe Meters



There are two methods for the safe installation of flowmeters in hazardous areas. Either the electronics must be housed in heavy flame-proof enclosure with expensive armoured cable connections (Eexd/Flameproof) or the electronics can be intrinsically safe (IS). The preferable approach is that the flowmeter is intrinsically safe and is not capable of igniting volatile gases. A normal low-cost, non-armoured cable is used to connect the flow-meter to the safe environment where isolator barrier devices are installed. These devices limit the energy passing to the IS transmitter electronics.
The Litre Meter flowmeter transmitter electronics is available in an intrinsically safe form. Originally, approval was granted by INIEX in 1978. The present design conforms to ATEX standards. Litre Meter operates an approved quality control system for the licensed manufacture of intrinsically safe equipment.

The flowmeter can be used for extremely volatile gases (where temperature T5 class applies) in a Zone 0 hazardous area. Approval is to EEx ia IIc T5 to EN 50 020 and to EN 50 014.
The transmitter electronics is mounted in a robust, IP65 or IP68-rated, metal enclosure. On some units a 'quick-release' feature allows the entire sensing system to be removed from the flowmeter while it remains under service pressure. The highest pressure rating types have a meter cap with an integral sensor and removable pick-up cartridge.

Isolator Devices

The function of the isolator devices is to keep the electrical parameters of the circuit in the hazardous area within specified, safe limits. These are 28V, 100mA or 1 Watt.

For zener barrier installations two DC voltages are applied. The power supply to the flowmeter is at 15 volts and for the output signal line, 9 volts is applied. The output is a negative going voltage pulse. The zero volts of the system is earthed at the zener busbar. Zener barrier installations may be 4-wire which includes a self-test function facility.

For galvanically isolated installations an MTL device is used (either 4032 or 5032) The device provides a totally isolated positive going voltage pulse output varying from 0 to 20Vdc (approx) appearing on terminal (+) on drawing below with reference to terminal (-). Alternatively a negative-going pulse from 15Vdc quiescent to 0 VDC is obtained at terminal marked (-) when a power supply of 15VDC (maximum) is applied to terminal marked (+). Using a 3-wire system based on the DC power supply to the device a problem may arise concerning the trigger level which denotes a low on the output.

An IS earth is not required which can be a major installation advantage to the user.

The output pulses via the barriers/isolator may be used directly by data acquisition systems or via Litre Meter.

Hazardous Area Instrumentation

Various system configurations allow use of IS counters, batchers and rate indicators in the hazardous area with optional pulse and flowrate output. Alternatively, 'Stand-alone Ex d' enclosed systems powered by 24V DC with internal galvanic isolator devices offer local indication functions and outputs in the hazardous area.

Outputs





PLEASE NOTE: We have now uprated all of the Litre Meter sensing electronics. They now function at voltages up to 27V dc (rather than 17V dc). If higher voltages than 12V are applied then the value of the 4K7 resistor may need altering to produce a voltage swing down to +2V

Output Signals

The standard output signal from the flowmeter is a strong, relatively noise immune, 3mA pulse that may be run over long distances. As shown in the diagram above it can be converted to a voltage swing. The amplitude of the pulse is from the quiescent 'high' at the DC supply voltage to a minimum 'low' of 2 volts.

Signal Processor Unit FPod

The FPod interface range provides industry standard outputs. The units are field mountable in IP68 tubular enclosures. Alternatively they can be installed adjacent to a computer.
Various outputs are available:
1. Analogue outputs: Conventional 4-20 mA analogue output calibrated proportional to flowrate. Accuracy better than 0.1% of full scale; zero offset <10 microA; 16 bit resolution 0.32 microA



2. Scaled pulse outputs: while computers are undoubtedly fast, valuable machine time can be used in monitoring fairly high frequency turbine-type flowmeter output signals. Litre Meter have answered this problem by offering scaled pulse outputs working in lower frequency engineering units. These have 50/50 'on' time. The pulses can be NPN Open Collector (28V max, 150mA max)[Standard, factory set], TTL (0 to +5V) or PNP. Impedance >2.2 kOhms; Accuracy < 0.1% of reading, resolution 0.018Hz/step; linearisation latency 2.5 to 3.5 msec plus period of input; 18 bit. Full scale frequency range 50 to 4800 Hz.



3. Pulse outputs: As scales pulse outputs but at raw meter frequency.



4. Data communication MODBUS is a standard option on the FPod. The Eexd version has HART, Profibus and Fieldbus options as well. [RS232: Baud rate: 19200. Win 95 and above, Windows NT and above compatible. Data input: manually / electronically Data output: Full data file stored in board and on disk. RS485 monitor link optional]