Sensebender Micro
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HI, I'm about to experiment with a modded version of the sensebender and I while looking at the schematics of the original sensebender, I see that the crystal does not have any pf capasitors and not connected to GND. Why is that?
All other schematics of atmega or any microprocessor I've seen has dual 22pf caps over the crystal and connected to ground.
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@Magnus-Pernemark
The sensebender was designed to use a 32Khz oscillator for lowpower operation. According to the datasheet (page 33) it is not necessary to have external load capacitors if the crystals datasheet specifies cL below 6pF.
However, we decided early on that an external crystal is not necessary in our application. So that is why it is not mounted, but the pads are still there, in case someone would use it.
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@Magnus-Pernemark
The sensebender was designed to use a 32Khz oscillator for lowpower operation. According to the datasheet (page 33) it is not necessary to have external load capacitors if the crystals datasheet specifies cL below 6pF.
However, we decided early on that an external crystal is not necessary in our application. So that is why it is not mounted, but the pads are still there, in case someone would use it.
@tbowmo Thanks! Yes. I read that it was not needed, but since it said "if you want higher precision" I though that, higher is good, and it would not hurt to place one there. But the crystals I have are 12.5pf so I guess I can't use them, but on the other hand I don't need to buy any other
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Did someone "test" the humidity precision of the Si7021on the sensbender? Of corse the datasheet says max 3% off but how reliable is that?
I have other sensors here and they differ by about 10%. So I did a "Salt-Calibration-Test" and there my two sensbenders or better Si7021 are 6% over the reference value. They report 81% and it should be 75%.
Note: It's my first time that I performed this test and I'm not 100% sure that the (water-salt-ratio) is correct - thats why I'm interested if someone else tested the values?
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@tbowmo Thanks. No worries the electronic won't touch the salt or water. The salt/water is just within the same closed container to bring the enclosed air to exactly 75.3% rel hum (at 25C).
I followed this howto: But there are many other similar ones.
http://www.kingofthehouse.com/hygrometer/ -
@tbowmo Thanks. No worries the electronic won't touch the salt or water. The salt/water is just within the same closed container to bring the enclosed air to exactly 75.3% rel hum (at 25C).
I followed this howto: But there are many other similar ones.
http://www.kingofthehouse.com/hygrometer/@nivoc I setup several sensebenders on battery last summer and noticed they were a few % below two dht22 I had been running for months. I did the saturated salt test on three sensebenders with one dht22 and one mechanical hygrometer and the sensebenders all read 74-75% while the DHT and hygrometer read 78-80%. Not a perfect test and i dont know which devices were correct but one thing I noticed was the sensebenders consistently provided the same result across different devices. Relative humidity is relative.
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@Dwalt
Thanks for the info. I'm currently opening the box every several hours to test the salt soup. My room has something between 50 and 60 rel. (so below 75). And every time I open the box this (see pic). It drops and goes back up to 82 or 83. Than it stabilizes there over the next couple hours. There are two sensbenders in the box - thats what the two line are. And yes they are remarkable equal. But it always goes to 82 - i would hope for 75. But I will repeat this procedure until it starts to stabilizes after every opening a little bit below the value that was there before the opening. Because if that happens i am below the dry point and than I will test in a room with humidity higher than 80 and test if soup can do the same into the other direction and holds it stable for many openings. THAN i know that the salt is performing is job correctly and know that the test is good and the senbender is wrong :-) or hopefully I learn the opposite. -
If the Si7021 is exposed to high humidity for a prolonged period, the RH% measurement will shift upwards, according to the datasheet, page 15, section 4.3
Extract from Datasheet:
4.3. Prolonged Exposure to High Humidity
Prolonged exposure to high humidity will result in a gradual upward drift of the RH reading. The shift in sensor
reading resulting from this drift will generally disappear slowly under normal ambient conditions. The amount of
shift is proportional to the magnitude of relative humidity and the length of exposure. In the case of lengthy
exposure to high humidity, some of the resulting shift may persist indefinitely under typical conditions. It is generally
possible to substantially reverse this affect by baking the device (see Section “4.6. Bake/Hydrate Procedure” ). -
@petoulachi said:
BTW, what is the F() function ? instead of Serial.print("Sensebender Micro FW "); why using Serial.print(F("Sensebender Micro FW ")); ?
It's a way to save some RAM. A simple string constant like "Hello" takes up 6 bytes of RAM and also 6 bytes of Flash (program) memory (6 bytes includes a single "hidden" byte of binary zero as an end-of-text marker). At startup and before your code executes, that bit of program memory is copied to RAM. The F() thing is a "macro" which causes the compiler save only the 6 bytes of Flash (no Ram used). The print function can (through C++ typing) fetch those 6 bytes from Flash at runtime for printing. If you have long or many text constants in your program, this can add up to some useful savings,when you only have 2K of RAM.
See the "F() Macro" on this page: https://www.arduino.cc/en/Reference/PROGMEM
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If the Si7021 is exposed to high humidity for a prolonged period, the RH% measurement will shift upwards, according to the datasheet, page 15, section 4.3
Extract from Datasheet:
4.3. Prolonged Exposure to High Humidity
Prolonged exposure to high humidity will result in a gradual upward drift of the RH reading. The shift in sensor
reading resulting from this drift will generally disappear slowly under normal ambient conditions. The amount of
shift is proportional to the magnitude of relative humidity and the length of exposure. In the case of lengthy
exposure to high humidity, some of the resulting shift may persist indefinitely under typical conditions. It is generally
possible to substantially reverse this affect by baking the device (see Section “4.6. Bake/Hydrate Procedure” ).@tbowmo Good hint. But it is happening to all three sensbeners for at least one month now. (different orders). I now got 4 more so I will test again soon.
Over xmas I tested also with magnesium chloride hexahydrate. magnesium chloride hexahydrate produces exactly 33%rel.
All 3 sensbenders report exactly 39% or 40%. So they are also here 7% off. But almost the same value and very reproducible - so thats great!
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@tbowmo Good hint. But it is happening to all three sensbeners for at least one month now. (different orders). I now got 4 more so I will test again soon.
Over xmas I tested also with magnesium chloride hexahydrate. magnesium chloride hexahydrate produces exactly 33%rel.
All 3 sensbenders report exactly 39% or 40%. So they are also here 7% off. But almost the same value and very reproducible - so thats great!
@tbowmo I'm now almost sure that my test-setup has no faults very reproducable and read many articles - I plan to verify my findings with someone who has a professional calibrated device.
I saw in a museum nearby that they have professional calibrated devices :-)
So I will test against those :-)
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Have you tried to "bake" your sensebenders, in case they have been exposed to high humidity for a longer time?
They are, as far as I know, produced in a part of China, which are known to have very high humidity. (been traveling there a lot some years ago)
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@nivoc
I can confirm your results: I am running two different sensors in the same room next to each other. The sensebender is always above the dht-22. Right now it is
52% to 47.8% rel. humidity, while the temperature is 17.8 to 17.9 (which is nearly equal). -
But I tested tonight with distilled water - so should be exactly 100%. And again (I didn't know that thats possible) I got 104 and 107% so slightly above (still increasing). Thats great b/c that means all values are about 7-8% too high.
At 33% the reading was about 40%
At 75% the reading was about 83%
At 100% the reading was about 106%So reading minus 8% gives a pretty accurate result. In the next few days i will test 3 more sensbender.
Distilled Water (100% expected):
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Read the spec for the humidity sensor used in the sensebender (Si7021 integrated humidity / temperature sensor.)
Precision Relative Humidity Sensor: ± 3% RH (max), 0–80% RH
To my reading this is actually 6% wrong reading to one of the sides - so when you are measuring yours to be 8% off, is caused that you are using the device outside of humidity range 100% and max is 80%Link to spec:
https://www.silabs.com/Support Documents/TechnicalDocs/Si7021-A20.pdfWhat I want you to know, if your device is reading within 6 % of the value - it's still reading correct. and when you compare ex DHT22 that device also have some reading variation error. Do not expect to get 100% perfect value for 2 devices...
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@bjacobse Yes and no :-)
Yes 100% is outside of the spec.
And no. 33% with Magnesium Chloride Hexahydrate is inside the spec and I get 40% instead of 33-34%.
Max off should be 3% above or? Means 37% - but fair enough. Should be ok for household use.
