Intek RheoVac 940 - Manual (Page 17)

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SECTION 3 — OPERATION
3.1 GENERAL INFORMATION
The RheoVac monitor is compensated and linearized for a wide range of flowing media temperatures,
pressures, and water vapor contents. However, abrupt changes in these parameters can cause the
instrument to temporarily read the flow rate improperly, which could lead to transient spikes in the flow
indication. In particular, if liquid (water) hits the probe tips, there will be high flow indications until all
the water vaporizes. This is a rare occurrence which should not happen if the transducer is properly
installed, according to instructions in SECTION 2 — INSTALLATION.
3.2 SYSTEM START-UP
The RheoVac monitor has been designed for fast warm up following turn-on of power. When power is
first turned on, no differential temperature will exist between the heated and reference probe sensors.
The transmitter is programmed to apply heat to the sensor for a set period of time or until a factory set
value is reached. Then the heater will return to normal. During this initialization period, the display will
have the message '*INTEK, INC.*' on the top line and '*(614)895-0301*', the phone number of Intek,
on the bottom. This message will then change to '*RheoVac*' and the unit's serial number. The bottom
display line will then change to 'initializing'. During this period the status output will be low (alarm
condition) and the analog outputs should be ignored. Further, no data is being transmitted over the serial
ports. The flow value will be monitored internally for stability, which will occur before the display is
set to the normal reading state. After the start-up sequence, program execution is sent to the main
instruction loop. It takes several seconds to execute the initialization sequence.
Upon a "cold" initial start-up of the RheoVac monitor, the transducer may take several minutes to give
accurate air in-leak flow measurements. During this time the probe is transitioning to thermal
equilibrium conditions under vacuum.
3.3 OUTPUT SIGNALS
Standard on all RheoVac instruments are one 2 x 20 alpha numeric LED backlit display, ten 4-20 mA
analog outputs, one 0-15V digital status port, and one serial communication port. Each process variable
is a linear, fully temperature and pressure compensated value on any of these readable outputs. All 4-20
mA output signals are scaled such that 4 mA represents 0% of the rated full scale value and 20 mA
represents 100% of the rated full scale value. The standard full scale values and definitions of all
process variables are listed in Table III.
Note: When the pressure rises above the calibrated range of 10" Hg absolute, all of the flow outputs will
indicate zero. In some instruments the display will read "General Fault Mode 6" at higher pressures,
such as one atmosphere. This is not a problem and the unit will read correctly under vacuum conditions.
Therefore, no flow rates will be reported when the generator is off-line or during hogging until the
pressure falls below 10" Hg absolute. All other variables will continue to be output normally.