Model NGA2000 PMD, NDIR

Addendum

748461-B April 2003

VAISALA BAROMETRIC PRESSURE TRANSDUCER

This addendum serves as an amendment to the Model NGA 2000 PMD and NDIR instruction manuals. The following information should be considered part of the manual, if your instrument has a Vaisala Barometric Pressure Transducer installed in it, or if you are retrofitting a PMD or NDIR 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 its inlet pressure. It has an operational ambient pressure range of 800 to 1060 hPa.

The original PMD and NDIR instruments were designed for use with a resistance bridge type of pressure transducer requiring an external gain amplifier with an offset trim The Vaisala transducer contains an integral gain amplifier that is scaled for a 0 to +5 Vdc output. The 661380 “Interface Board” contains the circuitry to convert the Vaisala single ended 0 to +5 V dc output into the referenced bridge 0 to +0.625 V dc output configuration that is required by the Analyzer module’s Computer Analysis Board. The Vaisala transducer kit is backward compatible with the original Rose- mount pressure transducer.

Vaisala Interface Board Circuit Function:

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 con- figuration. 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 indi-

vidual transducer at a known ambient air pressure. Test points are provided on the circuit board for this adjustment.

The Vaisala transducer output range is 0 to +5 Vdc.

The Vaisala analog output voltage is inverted and fed into a balanced summing node that is referenced by a precision regulator. The summing node voltage is 0.000 Vdc at 1013 hPa (1 atmosphere ambient air pressure). The summing node voltage is then buffered, in- verted, and gain reduced by a factor of 8 to produce the Pressure Compensation Output voltage at J12-1 for the Computer Analysis board.

Low noise, Low input current offset Op-Amps, and common substrate resistors are used to reduce gain tracking errors and null thermal coefficient effects.

1-1 INSTALLATION & RETROFIT INSTRUCTIONS

1.Remove the original Rosemount trans- ducer circuit board assembly, located on the motherboard at J12 and J13 (if pre- sent).

2.Install the transducer with the pressure inlet facing the rear of the instrument us- ing the mounting studs provided on left- hand side of the detector housing.

3.Install the Interface board into it’s mating sockets (J12 and J13) on the mother- board.

4.Connect the 4-pin cable (P1) from the in- terface board (noting pin polarity) to the Vaisala Pressure Transducer.

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