I recently received (Sep/09) some
DS1820 temperature sensors from a supplier. In doing test hookups I discovered
that 4 of the 5 sensors were giving erroneous readings.
This is what I saw:
a. Connected to a K145, 3 sensors had constantly changing readings,
fluctuating between roughly -127 and +127
degree C. A 4th sensor
was stuck at a continuous +85 degree
b. Connected to a K190, these same 4 sensors all were stuck at a
continuous +85 degree C.
c. The 5th sensor appeared to be giving okay readings
(when compared to a known good
All sensors were connected either directly to kit terminals, or through short
(6") lengths of cable,
so cable length would not have had an influence.
The 4 "bad" sensors all had
the same numbering on the package, like so:
From this I guessed it might be a bad manufacturing batch. It turns out that's
so, but with a broader scope. See the 1Wire.org
website... under "Reference Materials", have a look at the
"1-Wire Chip Problems" page. Apparently the B7 chip die is known to be
bad. This affects both DS18S20 and DS18B20 sensors.
Sadly, my 5th new sensor also has
the B7 designation, so is suspect, too. My supplier has agreed to provide
replacements, once he's sourced a supply of non-B7 sensors.
For those of you reading this:
forewarned is forearmed.