What is the best accurate, reliable and low-power temperature sensor

Hi all,

I have a project to put a temperature sensor inside a refrigerator to send the temperature level each 15 min over the WIFI.
Prototype components
-D1 mini board
-Power source is Battery
-Temperature sensors tested are DHT11, DHT22, DS18B20 and BME280

I am comparing the sensor readings against 2 analog temperature gauges

I have finished everything, but still facing the below issues:
-The temperature readings are not matching the real live temperature.
-The DHT11/22 usually stop sending the readings and then resume sending again by itself.
-The DS180B20 keeping starting from low temperature (~1c) reaching to ~6c in about 50 min, then drop to ~1c and start raising again!

Could it be the board is the source of the issue as it is inside the refrigerator?
I need to use an accurate temperature sensor, any suggestions please?

Thanks

The variations you are seeing are probably real.
Refrigerators are not constant temperature devices. They use on/off control circuitry with temperature hysteresis to avoid 'short cycling' which would otherwise shorten the life of the motor.

If you want to know the temperature of the food then embed the sensor inside the food. You'll probably find that the average is about 4 deg C.

How much money do you have, the simplest and best is in the several thousand dollar range. Simple start be sure all the components are rated to operate at the temperature you are at. You state “The DS180B20 keeping starting from low temperature (~1c) reaching to ~6c in about 50 min, then drop to ~1c and start raising again!” I am assuming when it goes to ~1C it takes maybe 30 minutes or more. This is normal operation for a refrigerator. At this point I believe the DS180B20. If you want to get much closer then 5 degrees you will get into some Complicated control equipment and some changes on the refrigeration side. Battery charge can also be a problem especially at colder temperatures. Why not simply place the probe wire in the refrigerator and the Arduino outside powered from the mains.

GTS_Omar:
The DS180B20 keeping starting from low temperature (~1c) reaching to ~6c in about 50 min, then drop to ~1c and start raising again!

Quite possibly dancing to the tune of the thermostat.

GTS_Omar:
I am comparing the sensor readings against 2 analog temperature gauges

-The temperature readings are not matching the real live temperature.
-The DHT11/22 usually stop sending the readings and then resume sending again by itself.
-The DS180B20 keeping starting from low temperature (~1c) reaching to ~6c in about 50 min, then drop to ~1c and start raising again!

So what were the variences between the DHT11/22, DS180B20 and your '2 analog temperature gauges'

This is like having a few watches and never knowing what time it actually is. My own DHT 22 sensors I have found to be within 0.5 degrees F. To actually check (calibrate) your temp sensor you will need a temperature measuring device more accurate then the DHT 22. You can't rely on any unknown device. Whatever you check your sensor with you want it to be at least 4X to 10X more accurate than your DHT 22. Having a good known accurate standard is the only way you will know what your sensor is really displaying. You also want to test your sensor over the range it will be used in.

Also, forgot to mention, if you have any concerns as to humidity you may want to start looking at saturated salt solutions.

Ron

mikb55:
The variations you are seeing are probably real.
Refrigerators are not constant temperature devices. They use on/off control circuitry with temperature hysteresis to avoid 'short cycling' which would otherwise shorten the life of the motor.

If you want to know the temperature of the food then embed the sensor inside the food. You'll probably find that the average is about 4 deg C.

That is good piece of information.
Thanks

gilshultz:
How much money do you have, the simplest and best is in the several thousand dollar range. Simple start be sure all the components are rated to operate at the temperature you are at. You state "The DS180B20 keeping starting from low temperature (~1c) reaching to ~6c in about 50 min, then drop to ~1c and start raising again!" I am assuming when it goes to ~1C it takes maybe 30 minutes or more. This is normal operation for a refrigerator. At this point I believe the DS180B20. If you want to get much closer then 5 degrees you will get into some Complicated control equipment and some changes on the refrigeration side. Battery charge can also be a problem especially at colder temperatures. Why not simply place the probe wire in the refrigerator and the Arduino outside powered from the mains.

I think the DS180B20 is pretty much good one as it is continuously sending readings to the cloud, unlike the DHT22 which is usually having unstable sending rate.

Thanks

Ron_Blain:
This is like having a few watches and never knowing what time it actually is. My own DHT 22 sensors I have found to be within 0.5 degrees F. To actually check (calibrate) your temp sensor you will need a temperature measuring device more accurate then the DHT 22. You can't rely on any unknown device. Whatever you check your sensor with you want it to be at least 4X to 10X more accurate than your DHT 22. Having a good known accurate standard is the only way you will know what your sensor is really displaying. You also want to test your sensor over the range it will be used in.

Also, forgot to mention, if you have any concerns as to humidity you may want to start looking at saturated salt solutions.

Ron

DHT22 should be able to give readings for temperature between -40 to 85c, but actually, it doesn't give continuous readings, thats why I am using DS180B20 as I think it is more reliable.

I am using my analog gauges as an accurate reference which is usually indicating some differences, which I think it might be related to sensors and its libraries.

Will try to calculate the average readings during one or two hours as I will need to set a threshold which will be the main input to my notification system.

Thanks

GTS_Omar:
DHT22 should be able to give readings for temperature between -40 to 85c, but actually, it doesn't give continuous readings, thats why I am using DS180B20 as I think it is more reliable.

I am using my analog gauges as an accurate reference which is usually indicating some differences, which I think it might be related to sensors and its libraries.

Will try to calculate the average readings during one or two hours as I will need to set a threshold which will be the main input to my notification system.

Thanks

Either and or I guess. Using a DHT 22 set with a 2 second delay in my code it responds to changes quickly and using a standard mercury in glass thermometer for comparison at about 70.0 F I found it to be within a degree. Just a matter of what works best for you. While I have looked at temps inside my own home refrigerator I never tried sending the temps wireless and wonder how well wireless would work in there. :slight_smile: Absolutely use whatever you are more comfortable using. As I recall my fridge really doesn't have any large swings in temp unless the door is left open. Maybe tonight I'll run a temp recording just to see what it does. Haven't done that for awhile.

Ron

The temperature in the refrigerator can wary quite a lot this is the graph from my refrigerator

Ny bitmappsbild.jpg
I'm using a thermistor on a attiny nrf24l01 sensor, and sending it on over wifi with a esp8266/nrf24l01

Ny bitmappsbild.jpg

Just out of curiosity I monitored my refrigerator last night 10:00 PM till 08:00 AM this morning. Not very much to see. The spikes in humidity are interesting against temperature.

Pretty sure the humidity spike was wife getting coffee creamer the riming would be about right. :slight_smile:

Overall not much in temperature swings. This is just a standard 19 cubic food household refrigerator.

Ron