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'[OT] DS1820 Calibration Variation - How Much?'
1999\01\04@182926 by James Cameron

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Interested in hearing from people who know the DS1820 temperature sensor
quite well.

I've stuck two next to each other and read them using my PIC code.  I
get different results.  How different should these results be?

They say they are factory calibrated.  They claim a few things in the
data sheets that I don't fully understand.  I calculate from observation
that the relative measurement error is about one seventy fifth of a
degree at about 25 degrees centigrade.  I know they make a claim about
the absolute accuracy; and I've seen the pretty graph which shows the
manufactured sensor population culling they might be doing.

--
James Cameron                                      (spam_OUTcameronTakeThisOuTspamstl.dec.com)

OpenVMS, Linux, Firewalls, Software Engineering, CGI, HTTP, X, C, FORTH,
COBOL, BASIC, DCL, csh, bash, ksh, sh, Electronics, Microcontrollers,
Disability Engineering, Netrek, Bicycles, Pedant, Farming, Home Control,
Remote Area Power, Greek Scholar, Tenor Vocalist, Church Sound, Husband.

"Specialisation is for insects." -- Robert Heinlein.

1999\01\05@083513 by James Widman

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James,

>
> Interested in hearing from people who know the DS1820 temperature sensor
> quite well.
>
> I've stuck two next to each other and read them using my PIC code.  I
> get different results.  How different should these results be?

 I've got a handful of these that I tested at both 0.0 & 25 degrees C.
 I found some major differences on the order of 3.6 C. I checked with
 my good old Mercury thermometers and they were all within 0.1 C.
 Now I'm not the greatest at soldering, so it could be argued that I caused
 some damage to them. I found once I applied a "correction" factor to
 the reported temperature they appear to work well. I'm surrounding
 them with a marine epoxy and using as submersible thermometers in an
 aquaculture project.

 I should note that the water baths used for the calibration were well
 mixed and I haphazardly chose the order that the ds1820's reported
 the temperature. So now I have an array of corrections factors that
 match to a particular romid.

>
> They say they are factory calibrated.  They claim a few things in the
> data sheets that I don't fully understand.  I calculate from observation
> that the relative measurement error is about one seventy fifth of a
> degree at about 25 degrees centigrade.  I know they make a claim about
> the absolute accuracy; and I've seen the pretty graph which shows the
> manufactured sensor population culling they might be doing.

Good Luck & Happy New Year,

James Widman
Research Fishery Biologist
National Marine Fisheries Service
.....jwidmanKILLspamspam@spam@mi.nmfs.gov

1999\01\05@113510 by Gerry Cox

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James,

I can't say I know them well but I have played with the Dallas demo kit
which operates  them via a PC parallel port.
I found that the four sensors that  I was evaluating also displayed up to 3
or 4 degrees C difference. Suspecting that the one wire bus wasn't supplying
the 'parasitic' power at a high enough level, I used an external 5V supply
to the VCC pins of the devices. Problem solved - all ok now to within 0.5C.
I didn't trace the real cause but it may be related to my notebook PC which
has a printer port using TTL drivers. These might not have provided the 4.5V
minimum that the DS1820's need.

Gerry Cox
gcoxspamKILLspamdek.com
Weymouth, Dorset U.K.

I haven't go any further with
{Original Message removed}

1999\01\05@224027 by James Cameron

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Gerry Cox wrote:
> These might not have provided the 4.5V minimum that the DS1820's need.

Yes, I've experienced funny readings with unstable power supply.  Also
slowly increasing readings from initial power-up for about 30 seconds.
I suspect the internals of the chip may be warming.

Any others comment?

--
James Cameron                                      (.....cameronKILLspamspam.....stl.dec.com)

OpenVMS, Linux, Firewalls, Software Engineering, CGI, HTTP, X, C, FORTH,
COBOL, BASIC, DCL, csh, bash, ksh, sh, Electronics, Microcontrollers,
Disability Engineering, Netrek, Bicycles, Pedant, Farming, Home Control,
Remote Area Power, Greek Scholar, Tenor Vocalist, Church Sound, Husband.

"Specialisation is for insects." -- Robert Heinlein.

1999\01\06@101047 by jlawson

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We use several DS1820 devices and almost always they require a
correction facter (usually an offset whithin 2 to 3 C).

The biggest problem I have at the moment with these devices is
reliability at negative temperatures. The lowest I have been able to
read a signal from a DS1820 is approx. -13 C, below that and the line
remains high all the time. I am reading out all the registers and the
CRC is always correct if there is a valid temperature reading.

I have played with power, wiring and software to no avail.

Has anybody had good results with DS1820s at cold temperatures?


>   I've got a handful of these that I tested at both 0.0 & 25 degrees C.
>   I found some major differences on the order of 3.6 C. I checked with
>   my good old Mercury thermometers and they were all within 0.1 C.
>   Now I'm not the greatest at soldering, so it could be argued that I caused
>   some damage to them. I found once I applied a "correction" factor to
>   the reported temperature they appear to work well. I'm surrounding
>   them with a marine epoxy and using as submersible thermometers in an
>   aquaculture project.
>

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