On Saturday, 6 October 2018 17:41:53 BST Graeme wrote: > I have just installed (separate) temperature and humidity sensors > connected to a Raspberry Pi 2 in my summerhouse. The values from both > sensors are oscillating in parallel: looking at values over an 8 hour > period, max-to-min is typically 14 minutes and min-to-max is typically > 24 minutes. The sensors are being read by a One-wire protocol and are > serially connected by Cat5 cables to a DS2490 USB adaptor. The humidity > sensor is fed by an external 5v source, and the temperature is passive.
If the temperature is actually changing over that cycle, then the humidity will change too, although (all other things being equal) the value for Relative Humidity will go down as the temperature goes up. > The only other electrical device in the summerhouse is a new freezer. A > previously installed temperature sensor (different from the current one) > did not exhibit this behaviour. This seems to implicate the new humidity > sensor, but I can't think how. Can anyone suggest a reason for this > regular oscillation? Hmmm. The corollary of the above is that (all other things being equal) the value for temperature will go down as the Relative Humidity goes up. I can't see how erroneous readings in the RH sensor will affect a separate temperature sensor, unless the one is calculated from the other. (By 'all other things being equal', I mean that the actual water content in the atmosphere stays constant during the period under discussion.) How often are you taking readings? I can't shed any light on the oscillations, but I implemented a three sensor (two DS18B20 probes and the internal CPU sensor) based on the instructions at: http://www.reuk.co.uk/wordpress/raspberry-pi/ds18b20-temperature-sensor-with-raspberry-pi/ We ran this at the WMT for around two weeks, taking readings at 5 minute intervals; I've posted an example of the results at: http://hadrian-way.co.uk/Misc/ There were cyclic variations, but they were over a period of days not minutes and were exactly as you would expect over a given 24 hr period. If you look at the fine grain of the results, there's no way that the variations could be considered to be in step. My software simply wrote the results to a file on the SD Card, so maybe you can see if the variations occur if you simply did this and ignored the USB output. This was a standalone Pi and not connected in any way to the Pi that was installed in the space being monitored. I must say that it seems unlikely that the variations are being introduced in the USB conversion, but it may be that the additional cable lengths and electronics are introducing noise or something. -- Terry Coles -- Next meeting at *new* venue: Bournemouth, Tuesday, 2018-11-06 20:00 Check if you're replying to the list or the author Meetings, mailing list, IRC, ... http://dorset.lug.org.uk/ New thread, don't hijack: mailto:dorset@mailman.lug.org.uk
