Why is my temp off?

Ok ... New to all this electronic stuff. Bought Uno couple years ago ... collected dust while I looked at it ... Now got a sensor kit and trying out stuff.

Sitting here in my living room .. Wall thermometer reads 69*F and this thermometer reads 59.74... I'm within 8ft and 2 ft lower than the one on the wall.

Can anybody shed information on how to:

Procedures they do to ensure that readings are accurate.
Procedures to calculate an offset?

Just information in general.

Sensor I'm using is a DS18B20 from this kit: Click Here!

I've found the same thing with sensors.
Have you checked to see if it can be calibrated?

Well the kit was pretty cheap and finding information about the actual discrete parts used is not something I've got great skill @.

Was kind of wondering if I was being an idiot or if I'm just another chump in the crowd lol. Being new to something makes you question your intelligence alot.

first off what is the wall thermometer an old bulb type (red Liquid) or the furnace thermostat. You are 8 feet from it you are measuring air temp while it is measuring wall temperature. To get a totally accurate reading you need to physically attach the two together i.e. tape them together or something. To get a 99% accuracy you would need a laboratory grade thermometer. What is normally done is to put both sensors in a glass of water either with ice, room temp or boiling water then compare them. And different thermistors read different values at same temperature. Your temp sensors needs to be calibrated to read right. If this topic interests or concerns you study Arduino thermistor and various libraries and the different formulas to read them

rogertee:
first off what is the wall thermometer an old bulb type (red Liquid) or the furnace thermostat. You are 8 feet from it you are measuring air temp while it is measuring wall temperature. To get a totally accurate reading you need to physically attach the two together i.e. tape them together or something. To get a 99% accuracy you would need a laboratory grade thermometer. What is normally done is to put both sensors in a glass of water either with ice, room temp or boiling water then compare them. And different thermistors read different values at same temperature. Your temp sensors needs to be calibrated to read right. If this topic interests or concerns you study Arduino thermistor and various libraries and the different formulas to read them

Not sure but I think throwing this circuit into a glass of water would be detrimental to my uno. I thought of that as well ( putting it near the wall thermometer btw it's a digital one ) just to remove that variable.

Thanks for responding.

I would be more inclined to trust the DS18B20 than the thermometer on the wall, and no, they don't need to be calibrated. They are not thermistors.

iwrk4dedpres:
Well the kit was pretty cheap and finding information about the actual discrete parts used is not something I've got great skill @.

Was kind of wondering if I was being an idiot or if I'm just another chump in the crowd lol. Being new to something makes you question your intelligence alot.

Don't worry...everyone starts somewhere. I'm only slightly ahead of you. Still a vast amount to learn.
But I have found that some of the Chinese sensors can vary by so much that they're almost useless.

Some work pretty good...some are garbage. I usually order 3 or 4 or any sensor I get and see which ones I'll keep and which ones are paper weights.

Nick_Pyner:
I would be more inclined to trust the DS18B20 than the thermometer on the wall, and no, they don't need to be calibrated. They are not thermistors.

Here's a thread on calibrating the sensor
http://www.avrfreaks.net/forum/calibrating-ds18b20-temperature-sensors-better-accuracy

Well I tend to think that if 2 things read the same thing then they should read the same value.

I get that they won't be the same .... but in the same GENERAL environment readings of 68F and 59F seem to be wildly out of place ( i'm in my living room ).

Trying to read the datasheet for the D18B ic now.

quamikazee:
Here's a thread on calibrating the sensor
http://www.avrfreaks.net/forum/calibrating-ds18b20-temperature-sensors-better-accuracy

Well holy bat crap batman .... that's way more information than this general layperson has the ability to understand. LOL.

But I appreciate the link. BTW .... I had to break out the crayola's with my UNO. KISS applies to me at least the "stupid" part

my quick $.02 - the quality of the sensor matters - in terms of library support and usability. I spenmatht way to many hour cursing temp sensors, and getting BS readings, until I switched the sensor type I used.

I'm now using an AM2302. cost a few bucks more, but worth it in that it works, and gets working quickly without a ton of funny calculations.

FullOfBadIdeas:
my quick $.02 - the quality of the sensor matters - in terms of library support and usability. I spenmatht way to many hour cursing temp sensors, and getting BS readings, until I switched the sensor type I used.

I'm now using an AM2302. cost a few bucks more, but worth it in that it works, and gets working quickly without a ton of funny calculations.

yeah I'm sure after sometime, if I keep this up, I'll come to the same conclusions. For now just trying gain understanding to what I'm doing. Will look up your part reference.

Far as the library support ... yeah I keep looking at the libraries that are used like the dallasxxxxx something or other and wish I understood what was in it. But that will take time as well. Sticking in searial prints where I'm trying to understand something.

iwrk4dedpres:
Well I tend to think that if 2 things read the same thing then they should read the same value.

This is true, but not enough of a reason to doubt the DS18B20. You make no comment about rigorous comparison, and are just talking generalities. The wall thermometer may be reading a convection stream rather than the typical room air. One thing you can be sure of is that any perceived NEED to calibrate DS18B20 is pure horseshit. These things are one of the most popular sensors on this forum, and for good reason. If any calibration was needed, everybody would have known about it for years - and would be using something else. Indeed about the worst thing that can happen with the DS18B20 is that people have complained that they are having a hard time with what they think is a DS18B20, and it turns out to be something else. In that event, no amount of calibration will fix the problem.

I have a couple of dozen DS18B20s and I have to admit that I do test every one of them in water against a lab-grade 0-50 mercury thermometer. I also have to admit that I don't actually have a good reason for doing this, as I have never found one that doesn't do what it is supposed to do.

The DS18B20 sensor does not require any 2-point calibration like LM35 to find gain and offset. The DS18B20's reading is directly mapped to normal decimal value (natural binary). The Following Table has been extracted from the data sheets of the sensor.

Hi,
You do have the 4K7 resistor fitted?

Can you post your code please?

Tom... :slight_smile:

TomGeorge:
Can you post your code please?

+1

Also it's bad physics to assume that different parts of a room are at the same temperature, as stated above.

Last couple of months. I've played with a few sensors. BME280, BM280 and MCP9808 from Adafruit. Then purchased a BMP280 from China. Even the China version worked great!

Yes, there may be a 10-degree difference from the wall to where ever. The wall can radiate heat/cold by that much!

iwrk4dedpres:
Well I tend to think that if 2 things read the same thing then they should read the same value.

you are correct in one of your assumptions, incorrect in the other.
two things that read the SAME thing should read the same.
however, a sensor on a surface and a sensor some distance away are not reading the same thing.
a thermostat is mounted on a wall and the wall has mass that heats and cools. the thermostat, in the trade, is called a wall temperature sensor. but also a space temperature sensor. as soon as any difference is noted, the WALL part becomes important.
for kicks and giggles, get 5 of those DS18x20 sensors.
put them all in a box, do not wire them for parasitic power.
see how close they are.
then put one on the floor, one on the wall, one on a table top, one hanging about hip height, one hanging about head height and one on the ceiling.
what you are learning is that your old assumptions are to be blown away once you start measuring things.
some fun with physics.
in the winter, tape a string across a doorway of a large room , about a foot above the floor. not too close to the front door. Best if you have a room on the other side of the house, but on the same floor at the front door.
hang some toilet paper to it so the TP does not touch the floor but is close.
do the same at the top of the door, about a foot of TP
open the front door for 30 seconds.
watch the paper
when you fill a glass of water, the denser material will displace the less dense. With a glass of water, you see the liquid, not the air. but the air moves all the same.
hang some TP over your TV set and over the chair in which a person sits.
it will be in constant motion from the convection currents.

Nick_Pyner:
This is true, but not enough of a reason to doubt the DS18B20. You make no comment about rigorous comparison, and are just talking generalities. The wall thermometer may be reading a convection stream rather than the typical room air. One thing you can be sure of is that any perceived NEED to calibrate DS18B20 is pure horseshit. These things are one of the most popular sensors on this forum, and for good reason. If any calibration was needed, everybody would have known about it for years - and would be using something else. Indeed about the worst thing that can happen with the DS18B20 is that people have complained that they are having a hard time with what they think is a DS18B20, and it turns out to be something else. In that event, no amount of calibration will fix the problem.

I have a couple of dozen DS18B20s and I have to admit that I do test every one of them in water against a lab-grade 0-50 mercury thermometer. I also have to admit that I don't actually have a good reason for doing this, as I have never found one that doesn't do what it is supposed to do.

To be honest just posted this question ( did look for other temp threads ) to get information. I'm just beginning to play with UNO and I'm not hip to all the ins and outs of this stuff. Just wanted to see if I was missing something.

TomGeorge:
Hi,
You do have the 4K7 resistor fitted?

Can you post your code please?

Tom... :slight_smile:

Just looked for the sketch I was using and I didn't save it yesterday. However, component I'm using is a small board that has the resistor and the component on a small board with 3 headers coming off it. Its schematic states it does have the resistor.

iwrk4dedpres:
Just wanted to see if I was missing something.

You're probably not. Just go ahead and use it without being distracted by all this calibration stuff. The board you are using sounds a reasonable and convenient way to go for measuring air temperature. Be aware that the DS18B20 comes in different forms