Model 755A
Instruction Manual
APPENDIX A
VAISALA BAROMETRIC PRESSURE TRANSDUCER
This is applicable if your instrument has a Vaisala Barometric Pressure Transducer installed in it, or, if you are retrofitting a 755A with a Vaisala transducer kit.
The Vaisala PTB100 series Analog Barometer is a N.I.S.T. traceable precision silicon capacitive atmospheric pressure transducer. The sensor delivers an analog voltage output that is inversely proportional to it’s inlet pressure. It has an operational ambient pressure range of 800 to 1060 hPa.
The original 755A instrument was designed for use with a resistance bridge type of pressure transducer requiring an external amplifier with a gain factor of 334. The Vaisala transducer contains an integral gain amplifier that is scaled for a 0 to +5 Vdc output. The 755A “Door Board” can accommodate either transducer type with a single (R97) resistor change.
The 755A “Door Board” pressure transducer interface is configured as a Precision Voltage referenced “Whetstone Bridge” circuit. The Vaisala Interface Circuit board adapts the single ended analog output of the Vaisala sensor into a Precision Voltage referenced, balanced bridge (at 1 atmosphere) input configuration. A “Bridge Balance” pot on the Vaisala Interface Circuit board provides the fine trim “Bridge Balancing” calibration adjustment that is required to set up each individual transducer at a known ambient air pressure. Test points are provided on the circuit board for this adjustment.
An adjustable, precision regulated voltage divider consisting of R1, R2, & VR1 on the Interface Board provide a bridge offset voltage range of + 0.26 to + 0.79 Vdc.
The Vaisala transducer output range is 0 to +5 Vdc.
The Vaisala analog output voltage is fed into a voltage divider formed by R3 and R4 on the Interface Board. This divider reduces the Vaisala 0 to +5 Vdc output to 0 to +0.568 Volts, a reduction factor of 8.8 to 1.
These voltage divided bridge offset and output signals are then applied to the original Whetstone Bridge input circuit at connector J3 on the Door Board.
1.Remove the original transducer, located inside the detector housing (if present).
2.Install the transducer using the mounting bracket provided into the lower right hand corner of the instrument.
3.Install the Interface board using the extended mounting screw and spacer (provided) to the top, left corner of the door board.
4.Connect the
5.Solder the Interface board WHITE wire to door board
6.Replace R97 on the door board with a 2 Meg Ohm, 1/4W, 1% resistor (provided).
Rosemount Analytical Inc. A Division of Emerson Process Management | Appendix A – Vaisala Pressure Transducer |