The previusly described Barograph was based on the differential MPX5100DP Freescale (Motorola) Semiconductor piezo sensor, which required a substantial degree of preloading. Mercury no longer being available commercially, preloading was done with vaseline oil and the result was a very cumbersome piece of equipment standing in excess of 1.5 m (more than 27") !!
A novel absolute pressure sensor, recently introduced by Infineon and readily available at distributors with a unit price around $ 6.50, has now opened the opportunity of building a DrDAQ + PC based Barograph which can be assembled on less than two inches square on a standard Arduino Shield prototype board.
The KP-235 chip has a dynamic range of 40 to 115 kPa (Kilopascals) or of 400 to 1150 hPa (Hectopascals). The normal range for barometric measurements is approx. 850 to 1120 hPa (650 to 840 mmHg ) so this sensor is ideal for our purpose. The sensor has a guaranteed precision of +/- 12.5 hPa, i.e. approx. +/-1% accuracy over full scale. If the requirement is for relative barometric pressure, then this precision is more than enough and much better than that of presently available devices. If the requirement is for absolute barometric pressure, then a one point or a two point calibration against available (Internet) pressure data will solve the problem. The data sheet for the KP-235 is available here:
The KP-235 is a ratiometric device, i.e. output is affected by supply variations. Therefore a very low power precision, high stability 5 V PSU is required: this will reduce ratiometric error to very low levels. Then, by using a channel of the DrDAQ to monitor the already small supply variations and to generate a suitable correction signal, the ratiometric error can be completely nullified, as will be shown in the course of these presentations.