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ADT7410 Temperature Inaccuracies
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Re: ADT7410 Temperature Inaccuracies

by lunch_box on Sat Sep 14, 2019 5:58 pm

I've been letting the sensors run for a few weeks now. During most of that time, I was away from home and the AC was off almost the entire time. I was seeing smooth, beautiful temperature swings throughout the day. But as soon as I got back home a few days ago and started running the AC again, the temperature spikes returned.
Honestly, I'm at a complete loss. I think this rules out my self-heating theory. There's something environmental going on, but the ADTs are responding much more drastically than the DHT22 or the BMP388.
Despite the legend on the graph, all sensors are in the same location.
Screen Shot 2019-09-14 at 5.45.56 PM.png
Screen Shot 2019-09-14 at 5.45.56 PM.png (263.28 KiB) Viewed 170 times

lunch_box
 
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Re: ADT7410 Temperature Inaccuracies

by millercommamatt on Mon Sep 16, 2019 12:43 pm

Download your raw data and make sure you're not seeing an artifact of the Adafruit IO plotting.

I'll make up a plot of BMP388 data from my office.

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Re: ADT7410 Temperature Inaccuracies

by lunch_box on Thu Sep 19, 2019 9:45 am

millercommamatt wrote:Download your raw data and make sure you're not seeing an artifact of the Adafruit IO plotting.

I'm certain that's not the case. The period of the temp spikes in on the order of 10 or more minutes. On shorter timescales, they look more like gradual rises. Many of the spikes consist of dozens of steadily increasing measurements over the course of 10-20 minutes.

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Re: ADT7410 Temperature Inaccuracies

by lunch_box on Mon Sep 23, 2019 5:09 pm

Not sure I'm going to get to the bottom of this any time soon. I bought a bevy of new sensors to evaluate, including a K-type thermocouple that I can calibrate against a water bath reference, then use as a temperature standard for all the other sensors.

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Re: ADT7410 Temperature Inaccuracies

by lunch_box on Sun Sep 29, 2019 4:06 pm

I ran a new test with a new sensor, an MCP9808. Initially, I ran the sensor on a breadboard, and it behaved as expected. However, I then soldered the board in place and it began behaving exactly as the ADT7410s have been behaving. I also wired an ADT7410 on a breadboard, and it behaved normally. So it doesn't seem to be environmental after all. I'm starting to think that heat is radiating from the voltage regulator on the ESP8266. My IR thermometer seems to show this as the warmest part of the board. The temperature spikes may correspond to periods of increased WiFi power output- and therefore increased regulator load- perhaps when RF interference or other environmental factors strain the 2.4GHz link quality. Unfortunately, that's going to be tough to isolate. I'm thinking about rigging up a shunt resistor to measure current draw and see if that corresponds with the spikes in temperature.
I also took a look at the power supplies I'm using. In the case of the breadboard, I'd been using an FTDI USB-serial dongle to power it all, but using a 5v wall wart to power the other boards. The wall wart showed what appears to be a 600Hz AC power harmonic at roughly 20mV p-p. Not anything I'd consider problematic. The FTDI cable showed some noise at roughly 11.1KHz. I'm guessing that's just bleedover from a clock or oscillator inside the controller. Again, nothing that I'd expect to cause any problems.
At this point, I see two paths forward: 1) redesign the board to increase the separation between the ESP and the sensor (or extend it out on a wire), or 2) rewrite the firmware to close the WiFi connection and put the ESP to sleep between measurements.
I think I'll explore option 2), because I like having the board nice and compact. I don't really want loose probes floating around my house. I'm not sure how practical is to open and close a WiFi link every 30 seconds though. If anyone has any insight about that, I'd love to hear it.

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Please be positive and constructive with your questions and comments.