Design Information

It should be remembered that the timescales for the hardware and software design and construction have have been very tight. Moreover, there have been some difficulties in agreeing measured parameters, the specifications for the sensors, and the software operation (keyboard or barcode data entry).

Sensors

The system has been designed using existing industrial sensors and instruments, chosen because of their superior performance, construction quality and ruggedness. In particular, thermohygrometer, anemometer and CO2 gas sensor are highly sophisticated instruments capable of the required accuracy in measuring these difficult parameters. The choice of these instruments has resulted in a system which is somewhat bulkier and heavier than at first envisaged but the superior accuracy and reliability should compensate for this.

Data Acquisition Hardware

A National Instruments AI-16XE-50 DAQ card was chosen partly because of its high resolution and accuracy but also because its design permits operation with different channel gains for each sensor.This simplifies and streamlines the software design. The Channel Wizard makes the specification of input channel sensitivities very flexible and straightforward.

In view of the very short development timescale and to give the necessary flexibility it was decided to interface the DAQ card, which has a subminiature 68 way SCSI connector, to the group of sensors using a National Instruments Breakout Box. This robust unit converts the mechanically vulnerable connections of the DAQ card to sensible screw terminals to which the sensor cables may be attached. The breakout box could be dispensed with some additional hardware development but this was not possible within the existing timescale.

Data Acquisition Software

LabView was chosen as the software development environment because it is ideally suited for this type of application and allows rapid and flexible development. The use of LabView keeps many important design options open right through the project life cycle.

Standalone executables can be created that will run on machines not equipped with the LabView development software.