I re-designed it with a Silicon-Labs Hygrometer chip which has a better hygroscopic filter than the Honeywell part used before. The specs of the two parts are similar with the new part having +-3% error between 0% to 80%RH, whereas the Honeywell HIH6131 had a 5% error over 10-90%RH . In the end I think that the numbers are pretty much the same since they are given over different ranges and temperatures. You can read all about the Si7021 here in it’s Datasheet.
While the previous sensors performed well, the filter was just a glued on a micro mesh screen. It had the tendency to fall off and then the sensing element was exposed to dust which causes wrong readings and potentially failure of the device. This filter is different and it is printed on over the entire top surface of the chip. I think this one will not come loose. It’s still a good idea to protect the entire module from dust and dirt though. I have made some little micro mesh bags that one can use to keep the module clean, even in a dusty environment.
For the barometer the Bosh BMP085 part remains in place. It has worked well and I have not had a single one go bad so far. When checking the modules against the local airports instruments I see the most 0.03″ Hg error on these barometers, while most are within 0.02″ Hg. That’s pretty dang good !
One nice feature of the re-deign is that I was able to switch to a different connector. The module now has a 6 pole modular connector and can use a standard FAX cable (straight, not crossed) as it’s connection to the base PCB. This makes it a lot easier to change the length of the connection and the placement of the module. The cable can be up to 20 feet, but needs to be kept away from heavy noise sources and power lines.
The new module is plug-and-play compatible with the station firmware. For main boards that don’t yet have the 6 pole modular connector I can provide a modified cable.