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Manual: 152-082010 301-305-306_303-307 Series Page 24 of 35
3.8. Electronic Circuitry
The 300 Series flow meters employs a thermal transfer principle (capillary tube described in section 3.2)
to measure the flow through the sensor which is proportional to the total flow through the instrument.
The sensor develops a differential voltage output signal proportional to flow, which is amplified to
produce 5 VDC at full scale flow. The amplified output can be measured on pins 6 and 1 of the external
“D” connector. If a Hastings power supply is employed, the 5 Volt output is also sent to the terminals on
the back and to the decoding circuitry in the display, which converts it to a numeric output. The
optional 4 - 20 mA analog output is available in lieu of an output voltage. The addition of a 4 - 20 mA
current loop transmitter on a secondary PC board (mounted parallel to the main pc board) is required to
provide this current loop. A jumper change is made on the secondary PC board to establish the selected
output mode.
3.9. Instrument Performance
The combination of these principles embodied in the 300 Series give inherently linear and highly
accurate output and the typical response time from zero to full flow is also quite good. The meter
settling time of a 1000 slm full scale flow meter is typically under 0.5 seconds. Flow controller response
is less than 2 seconds.