OWFS has the ability to do the arcane recalculations for the faulty batches of DS18S20 (after David Lissiuk noted the problem). That's why the die number, trim blanket and all that are included.
Paul On 6/7/06, David Lissiuk <[EMAIL PROTECTED]> wrote: > Vincent & Daryl > > Concerning humidity sensors... > > The HIH Series of sensors are low cost sensors, and perform better than > most similar sensors. However as high humidity ranges are reached > accuracy goes down. After exposure to high humidity they may exhibit > offset problems and need to be re-conditioned (exposure to low humidity > conditions for a period of time) in order to once again respond > properly. > > They do not take well to dew formation on the sensor and ice formation > may cause sensor failure. I've also seen sensors fail due to metal > whiskers and other material failures on the PCB. > > The standard sensor is bulk calibrated, but if you specify the > calibrated (-003 versions as I recall) version you will get a > calibration formula based on that particular sensor to use for getting > the most out of the sensor (I recommend it as it's only a few dollars > more). Most software uses the typical formula which may not be accurate > for your sensor. > > Use of a heating element circuit design to drop the humidity of the air > at the sensor is probably the best solution for outdoor use. This helps > keep the measurement range in optimum sensor measurement range. It can > also help prevent dew or ice formation on the sensing element. Most > 1-wire designs do not include this or other protection elements in the > design, hence the marketing of the sensor for indoor use. (though most > users use it outdoors). > > In addition, the DS2438 is known to be subject to self heating, and > since it's a smt device it measures the temperature off the PCB. In > tests we preformed using our old LP-THS which also had a DS18B20 temp > sensor it could be several degrees hotter depending on PCB temp, though > both were mounted on the same board. (the PCB would heat during the day > and then when cooler air would blow in, it would remain heated. Note > that the DS18B20 works better because it has better airflow mainly... > and its DQ lead is also part of the sensing element as I understand it. > I always try to provide two temp sensors (the DS2438 and a verification > DS18B20 sensor) so the software can read both as needed. > > So its important to know what your working with in any given design. > If theirs enough interest I will do a new design for outdoor use > sometime. > ------ > > Concerning DS18S20 accuracy... > > Consult the data sheet...( 0.5C accuracy from –10°C to +85°C for a > DS18S20) also check if your testing conditions (Same environment, > airflow etc..) If off, then check if your sensor internal calibration > has gotten changed.. there were batches from Dallas that could suffer > from calibration errors. It is possible to re-calibrate a sensor by > using special opcodes. (this is how Dallas does it). Consult > [EMAIL PROTECTED] if you have questions or a problem sensor. > There have been conversations on the Weather list about calibrating > sensors before... > > I typically use the DS18B20 which has better resolution than the > DS18S20. > Again also look at PCB layout issues because it can also effect reported > sensor temp. I've seen sensors placed next to heat generating circuitry > on some boards. I also typically power sensors at 5v regulated for best > performance. > > There is only one brand of DS18S20's. Dallas makes it. The different > variations all use the same chip die internally. DS18B20, DS18S20 etc.. > With the greatest resolution being the DS18B20 sensors. > > Hope this helps somewhat... > Cheers, > David Lissiuk > Sr. Computer Scientist > Springbok Digitronics > > Acting admin for www.1wire.org > > > > > > > _______________________________________________ > Owfs-developers mailing list > [email protected] > https://lists.sourceforge.net/lists/listinfo/owfs-developers > _______________________________________________ Owfs-developers mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/owfs-developers
