For outdoor equipment I was thinking of including a humidity sensor inside the enclosure, as a way to judge whether corrosion might be happening and datestamp and timestamp high exposure levels. However, I noticed some humidity sensors don't even measure high humidity (e.g. the si2071 tops out at 80%RH). Then I found this, which measures high humidity and is only $0.48:
http://www.aliexpress.com/item/HDS10-Dew-point-sensor-Humidity-sensor-Original-authentic/32237816821.html?spm=2114.031010208.3.2.0uCxqF&ws_ab_test=201526_2,201527_2_71_72_73_74_75,201409_4
Is there anything better than that?
Huh? The Si7021 does measure humidity above 80% RH, but with a little higher tolerance. This is pretty much the same with all factory-calibrated digital sensors, too.
The HDS10 is an uncalibrated analog sensor (supposedly manufactured by Aosong, China). Looking at the curve, it does have problems at the top of the range. You can't expect it to be precise above 80-90% either, let alone sampling the sensor properly without a lenghty process of calibration and a high-res ADC to get useful results.
Attached are accuracy figures for both.
A word of wisdom: you get what you pay for.
Ditto what others have said. I would stick with one of the following:
http://www.silabs.com/products/sensors/humidity-sensors/Pages/si7013-20-21.aspx
http://www.ti.com/product/HDC1000/technicaldocuments
Standard I2C interface, already supported by Arduino libraries, breakout boards (bare or populated) widely available.
https://oshpark.com/shared_projects/UazJFmQB
https://oshpark.com/shared_projects/7vhM7yyC
https://learn.adafruit.com/adafruit-hdc1008-temperature-and-humidity-sensor-breakout/overview
http://b.truzzi.me/hdc1000-temperature-and-humidity-sensor-breakout-with-arduino-library/
https://github.com/mlsorensen/SI7021
Hi
I would recommend you to buy a sensor from the company Sensirion. SHT3x or SHTC1. I have quite some experience with them. They are just amazing.
So far I've been happy with the HTU21D as well. The only decent source for them appears to be eBay ($4 on a breakout) unless you want to pay Mouser $11 for the naked chip.
You can, I suppose, also use a si7021 and use its reported temp and RH to compute the dew point.