Digital tank gauging Systems - the advantage is clear

Reduced cabling and equipment requirements – Instruments are interconnected in a “multi-drop” configuration where the cable goes from point-to-point rather than each instrument being run all the way back to the data acquisition module / central display.

Reduced power requirements – Equipment run in digital mode has a much lower consumption, this enables multiple instruments to be connected via a single power / communications barrier for ATEX compliance instead of each instrument requiring its own barrier.

Equipment can be addressed and calibrated remotely (from any point on the network), no need to be in, or even close to, the tank.

Spares holding reduced as units are interchangeable and configured after installation. Setup and diagnostic functions in the central display means the instruments can be set to work with no special calibration equipment needed.

All parameters stored in non-volatile memory both in the instrument and on the central monitor. This distributed intelligence enhances system reliability since a failure of one unit will affect that unit alone.

Each instrument outputs a comprehensive report of tank status – In engineering units. This can be routed direct to other shipboard systems if required.

Each instrument continually monitors its own health and will provide early warning of problems or of any alarm conditions arising.

The equipment range - defined by experience

PSM has 30 years solid experience of supplying marine tank gauging systems across all classes of Ships including oil /chemical tankers, bulk carriers, cruise ships, yachts, offshore rigs, trawlers, tugs, and workboats.
We are the preferred supplier for a number of the world’s navies, providing equipment fully compliant with the onerous shock, vibration and EMC / RFI standards the military require. The purpose-developed solutions for this most demanding market sector are now available for the wider marine market.
Instrument solutions are based on either hydrostatic or radar technology, each optimised to suit the application. Full details are provided by PSM’s product literature, but to summarise some of the highlights:-

iCT – the intelligent hydrostatic transmitter

Intended for use on service, ballast, freshwater, draught, and smaller cargo tanks, the iCT bridges the gap between conventional analogue transmitters and new generation digital devices. For upgrades or replacements of old instrumentation it can operate as a 0.25% accuracy conventional 4-20mA transmitter but with the added benefits of embedded intelligence for simple user set-up. In dual mode it provides both analogue output and digital communication, and in digital only mode the full benefits of multi-drop connection are realised. The user is free to choose or change the operating mode.

In digital mode the iCT also provides temperature measurement, further reducing the equipment requirement.

Sensor construction is fully submersible and uses a cast corrosion resistant body and ceramic measurement cell to ensure long-term suitability for all in-tank environments. The construction ensures protection against overload and shock pressure conditions.

iCT digital benefits

• Remote access. The full functionality of any transmitter in the network can be accessed from any point.

• Accurate remote re-ranging and/or zero adjustment. There is no need to employ external test equipment and pressure generators. The unit does not even have to be removed from service.

• Accurate temperature measurement. In digital mode, the current process temperature may also be output.

• Monitoring of service conditions and extremes. iCT records the total hours run and the lowest and highest in service pressures and temperatures it has measured to assist in trouble-shooting.

• Tank characterisation. The digital output provides both liquid level and volume data corrected for non-linear shaped storage tanks using the programmable 25-point look-up table. The 4-20mA signal can also be corrected using this look-up table.

• Error reporting. Internal diagnostics continually monitor the health of the iCT. In the event of errors e.g. the 4-20mA output becoming inaccurate, an error flag is set to notify the central monitor. 

• Integral high, low, and deviation alarms with variable setpoint, debounce, and hysteresis

• Rate of change alarms. For the detection of tank leaks.

• Force zero/span. This fixes analogue output to 4 or 20mA within a defined band of these points. The analogue output can also be forced to a programmable fixed value between 3 and 22.5mA for indication of faults.

• Analogue output control. The 4-20mA output can be set to any  value irrespective of the actual input pressure, for proving and calibration of connected display equipment.

In digital mode up to 128 iCT’s can be multi-dropped over a single 2 wire RS 485 network, with a single RFM1 IS+ safety barrier module providing protection for all.

FMCW Radar Transmitters

For use in larger cargo tank the latest generation of FMCW radar transmitter available from PSM is among the most technically advanced on the market. Compared to the earlier generation of 10 GHz units it offers a stable, crystal-controlled high transmit frequency of 26 GHz and a larger bandwidth of 2 GHz providing a Dynamic range more than 100 dB.

The high frequency of 26 GHz provides better concentration and focus of the microwave signals, for improved accuracy, stability, and sensitivity, of the measured value. A 2 GHz bandwidth makes it easier to distinguish valid signals, and measured values are evaluated with greater accuracy. The dynamic range of more than 100 dB is attained using the FMCW principle. This determines the ratio between the strongest valid signals and the weakest possible signals.

Digital signal processing provides very high computing capacity. Complex evaluation algorithms ensure reliable and precise calculation of measured values.

Unique Tank Bottom Following (TBF) mode enables products with low dielectric constants to be measured. For tanks with complex internal structures the Empty Tank Spectrum (ETS) can damp out all unwanted reflections from in tank obstructions.

Remote Function modules

PSM’s range of Remote Function Modules (RFM) provide a simple method of terminating all instrumentation cabling and provide Zener barrier protection to ATEX intrinsically safe standards for both the power and communications circuits in the hazardous area.

The RFM can be specified in a rugged die cast enclosure suitable for mounting on open deck.

RFM’s maximise the multi-drop communications potential of PSM’s instruments providing for a multi-drop point-to-point network. Where the iCT is run in digital mode, its low power requirement means just one RFM1 IS+ barrier will protect an entire ship set of equipment (up to 128 points)  

RFM1

 A termination module providing connection points for the level transmitter, data and power input / output cables, and any local indication.

RFM1 IS & RFM1 IS+  

These two IS versions add a module to the basic RFM1, that regulates the power from ships 12 – 35VDC supply to provide ATEX approved barrier protection for both power and communications cabling into the hazardous area.

These IS modules are functionally identical and are located in the safe area.

The basic or “+” model is selected depending upon the number of  instruments on the data bus and hence the total power requirement.

RFM2 

This adds a processor module and power circuit to the basic RFM1 to provide a local supply voltage for a radar transmitter and convert the radar transmitters output to digital data that is then transmitted over the Modbus network.

Where it is required to monitor conventional analogue signals from other instrumentation in the hazardous area PSM also provide a range of conventional (passive) barriers in the RFM format.