Breaking News
Comments
Newest First | Oldest First | Threaded View
David Ashton
User Rank
Blogger
Converting temperature readings
David Ashton   12/20/2013 5:17:38 AM
NO RATINGS
Just a note - in page 1 just above the picture, I said

"I got the FFFF part to display 0.X degrees by dividing it by 25"

I should have added that I had first multiplied it by 16 which gave me a value between 0 and 240 decimal, in steps of 16.  Dividing this by 25 (and ignoring any remainders) gives a value between 0 and 9 which can be used for the digit to the right of the decimal point.   Some values will be one decimal point out compared to the values if you were to round off properly, but with a sensor accuracy of only 0.5 degrees this is not a big deal.

David Ashton
User Rank
Blogger
Re: Measuring temperature
David Ashton   12/19/2013 10:30:48 PM
NO RATINGS
Thanks, Aubrey.  Good points about accuracy and calibration.  I would imagine the DS18B20 is similar to the older DS1820 and certainly ANY heated tracks which can conduct heat to any of the leads will affect the reading somehow.   In my case, the device has an accuracy of +/- 0.5 degree C, which for any purpose I want to use them for is adequate.  This also makes the resolution of 0.0625 degree a bit of overkill in some ways.   That's why I didn't worry too much about rounding errors in my conversion of the fractional part of the temperature to a 0.1 degree resolution for my display.

Compared to Max's thermostat errors (which were in the region of 5 degrees (though that's Fahrenheit, it's about 3 degrees C) the DS18B20 is a very accurate device.  I know what I would use if I were to design an Airconditioning thermostat!

I don't possess any super-accurate temperature calibration devices but obviously when you are using these it's important to have good thermal coupling between the calibration sensor and the DUT.  And you could not do things like put a DS18B20 into an ice bath without insulating it properly, which would probably add some thermal insulation as well.   Fortunately as I said, the basic 0.5 degree accuracy of the DS18B20 is fine for me!

 

 

antedeluvian
User Rank
Blogger
Measuring temperature
antedeluvian   12/19/2013 8:40:11 PM
NO RATINGS
Hi David

Good article, as usual. Something to consider when using this (or any device) is what is actually being measured. Apparently the DS1820 (an obsolete version) measured the temprature of the GND pin- that was how the heat was conducted to the temprature sensor. A TI FAE recently said that this was quite common for solid state temperature sensors.

This came as a rude shock to one of our engineers. He had used the TO92 package up in the breeze to measure the temperature of air floating by. However there were some resistors heating the PCB nearby and it contributed to a measurment error.

Of course that brings us to an issue when it comes to calibrating a temperature sensor- how do you know what is the right temperature, and what exactly is your sensor measuring. Max once did a blog when he faced this problem with his home thermostat: I'm becoming hot and bothered



Flash Poll
Top Comments of the Week
Like Us on Facebook
EE Times on Twitter
EE Times Twitter Feed

Datasheets.com Parts Search

185 million searchable parts
(please enter a part number or hit search to begin)
EE Life
Frankenstein's Fix, Teardowns, Sideshows, Design Contests, Reader Content & More
Max Maxfield

Energizing the Young Engineers of Tomorrow
Max Maxfield
15 comments
It doesn't seem all that long ago when I was a bright-eyed, bushy-tailed young engineer. Now I feel like an old fool, but where are we going to find one at this time of the day (LOL)?

Jolt Judges and Andrew Binstock

Jolt Awards: The Best Books
Jolt Judges and Andrew Binstock
1 Comment
As we do every year, Dr. Dobb's recognizes the best books of the last 12 months via the Jolt Awards -- our cycle of product awards given out every two months in each of six categories. No ...

Engineering Investigations

Air Conditioner Falls From Window, Still Works
Engineering Investigations
2 comments
It's autumn in New England. The leaves are turning to red, orange, and gold, my roses are in their second bloom, and it's time to remove the air conditioner from the window. On September ...

David Blaza

The Other Tesla
David Blaza
5 comments
I find myself going to Kickstarter and Indiegogo on a regular basis these days because they have become real innovation marketplaces. As far as I'm concerned, this is where a lot of cool ...