Am 03.10.2013 22:33, schrieb Håkan Elmqvist:
> I have a remote DS1820 that consistently reads exactly 85 degrees C,
> both in cached and uncached modes,
>
Possible misunderstanding: there is no cached or uncached "mode". OWFS
always caches sampled values. The difference between the two directories
is, when you access the normal "cached" directory, you may get the value
from cache instead from the sensor, depending how the caching timeout is
set and at which time you ask for it. When you access the "uncached"
directory, you always trigger a new sampling for the particular value
(which then is copied into the cache and returned.)
-> When testing the hardware, always use "uncached".
> when the real reading should be
> around 3 degrees.
> Has anyone seen this kind of failure before, is there anything I can do
> about it remotely or do I have to go there and put in a new sensor?
>
This is a FAQ. 85°C is the power-on value the sensor reports before the
first measuring is completed. As OWFS always triggers the measure,
waits, then reports the value when you sample the node directly, the
sensor must have accidentially gone through a power-cycle during the
measuring.
As it worked before, I would look for loose cables. In addition,
measuring low temperatures with parasite power is bad, as it heats the
sensor and thus, gives you slightly wrong temperature values. (How much
wrong? Noone can tell, depends on the bus load!)
Kind regards
Jan
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