Digital RF speed sensor / TTL Pickoff

for the sustainable upgrade of existing turbines

In some cases, existing turbine flow meters that are still running well are taken out of service because the associated instrumentation system does not perform with reliable flow readings any more. These faults, for example indicate readings at now flow and the other way round. More serious are flow rate readings with increased uncertainty. Malfunctions are more likely caused by the electronic part of the measurement system rather than by the turbine flow meters itself.

RF (radio frequency) pickoffs are very common on the market to detect the turbine rotor speed. They engage, in comparison to magnetic pickoffs, no drag to the turbine rotor. However to operate an RF pickoff, a specific driver circuit is required. To keep the turbine flow meter small and to protect the driver circuit electronics from harsh environment the electronics are fitted remote very often. Due to the resulting cable length which may easily be up to 10 meters the oscillating driver circuit is exposed to interference which could affect the performance and reliability of the low pulse output signal.

The new DDM Smart Pickoff ideally overcome this potential failure source. The RF (radio frequency) circuit is highly miniaturized integrated in the DDM Smart Pickoff. The low signal pulse is amplified to an high level TTL digital output. The digital open-collector TTL pulse output can be transmitted safely to DDM flow computers (VCA & VCA-T) for linearization and viscosity correction.

Turbine flow meters in general are viscosity-dependent. To compensate for this, a type-T thermocouple is built into the DDM Smart-Pickoff also. The thermocouple is used to determine the media temperature and transmit digital readings to flow computers connected. Up to 10 media characteristics (viscosity by temperature) can be stored in any DDM flow computer (VCA & VCA-T). User may easily select the appropriate media tool free by means of an RFID tag. Together with the actual media temperature the flow computer calculates the fluid viscosity and corrects the flow rate output signal in real time.

An ”old” turbine flow measurement system susceptible to interference effects is transformed into a very robust, reliable, linearized and viscosity-corrected measurement system by using DDM smart pickoffs and flow computers (VCA or VCA-T). The turbine retains its value and is upgraded to the current state of the art.

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