Re: Dreaded 85 degrees C read from DS18B20 located outside
Eloy Paris <peloy <at> chapus.net>
2011-03-02 21:07:20 GMT
On 03/02/2011 04:41 AM, Mick Sulley wrote:
> Hi Eloy,
> I am using a mix of DS18B20 and DS18S20, my experience has been that
> they will work reliably when powered and also in parasitic mode if you
> link Gnd and Vdd, but if you don't they are a bit hit and miss on a
> sensor by sensor basis, one may work fine but another reads 85. Also
> when floating they don't seem to be sure if they are powered or not,
> some say they are and others say they are not.
> My advice therefore is to link Vdd to Gnd.
I think this is sound advice even if things have worked well for me with
12 sensors that all have Vdd floating.
> Why it should be affected by
> humidity or thunder storms I don't know, but given that they seem to be
> a bit 'on the edge' with Vdd floating I guess it may change the
> resistance or capacitance slightly and push it over the edge. The
> capacitance effect will also be influenced by objects and materials in
> close proximity.
Thanks for the insight! Good stuff in this thread; I'm glad I asked even
if it was a bit off-topic for owfs
> Also does anyone know if the 85 reading is irrespective of units? My
> setup is in degrees C, but if it was in F would the error still be 85 or
> would it be 185F? If it stays at 85 irrespective it is a way to
> differentiate an error from a genuine 85 reading.
The 1820 only reports in degrees Celsius and it is owfs the one
providing degrees F capabilities, right? If that is the case then I
would expect 85 C to be reported by owfs in the equivalent degrees F.
So yes, that is a good question -- how does one tell the difference
between true 85 C and error 85 C? (not that I am expecting to read a
true 85 C, although I suspect things get pretty hot in an attic in the
> Best of luck
> On Tue, 2011-03-01 at 12:54 -0500, Eloy Paris wrote:
>> Hi list,
>> Apologies for the slightly off-topic message but I figured someone here
>> may have some suggestions for me on this non-owfs issue I am having...
>> I have a DS18B20 sensor located outside. It is in a location that is
>> safe from water (unless it rains horizontally). My bus master reaches
>> this sensor through a cat5 bus that has about 10 more sensors. The
>> sensor, its pins, and the small PCB the sensor is soldered to, are all
>> exposed (to wind and humidity, but not to direct water). Parasitic
>> powered, and Vcc pin is floating (not connected to ground). This setup
>> has been pretty stable (no 1-wire errors or bad temperature readouts in
>> 6 months of operation).
>> Last night there was heavy rain and thunderstorms in the area and for a
>> period of about two hours I was getting the dreaded 85 degrees C from
>> this sensor. All other sensors were fine during this time and did not
>> report 85 degrees. The problem fixed itself a bit after rain and
>> thunderstorms passed.
>> I think it is humidity what causes the 85 degrees C readout since we've
>> had a few warm and humid days/nights recently (but no rain) and I've
>> gotten a few 85 degrees C readouts here and there from the same sensor.
>> But nothing like last night when I consistently got 85 degrees C
>> readouts for about two hours.
>> Next time this happens I plan on grounding Vcc to see if that helps, but
>> I also wanted to ask the list for theories on what could be causing
>> these 85 degrees C readouts when it is humid (or when there are
>> thunderstorms in the area).
>> If grounding Vcc does not help I think my next step should be to protect
>> from the elements the sensor, its pins, and the small PCB. Any ideas on
>> how to accomplish this; is there some coating I can spray on the
>> sensor's pins and on the PCB that will protect them from humidity?
>> Thanks in advance for any suggestions.
>> Eloy Paris.-
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