Hi everyone,
Today, I tried to restock my supply of HTU-21D's... but the stock has currently run out in NA. Looking for an alternative, I stumbled across this series of Humidity sensors from Silicon labs. They are the same footprint as the SHT-21 and HTU-21D (DFN-6, i.e. 3x3mm), use the same voltage range (1.9-3.8V), etc. So they are easiest to use with 3.3V systems, like the Due, Pro, or Teensy series.
I haven't looked over the data sheets in detail re: the programming, but many of the offered resolution ranges are similar and the unit allows operation with or without the host CPU being kept blocked. On the Si7013 you can even set the heater intensity (up to 94mA current draw!!!).
FWIW, the si7013 is a strange and interesting creature, i.e. a I2C temp and humidity sensor that additionally features an accessible differential ADC for external NTC temperature measurements, complete with registers for calibration, slope, and offset. Or measure anything else of interest, 0-Vdd for unbuffered and 0.5-Vdd for buffered operation (1.9V < Vdd < 3.8V). Could make a nifty little ADC for things like photoresistors in a external weather sensor system, for example. The ADC offers up to 16 bits of resolution but it's relatively slow (i.e. about 12ms per conversion).
Lest I come off as some sort of marketing drone, I wonder if anyone here has used one of these sensors? They seem like a good substitute for the HTU and SHT series, especially ever since the HTU prices went up 50% at Digikey (from about $8 to now $12+). By comparison, the Si7021 (+/- 2% RH and 0.4*C accuracy) costs about $4 in single quantities and the slightly less accurate si7020 ("only" +/-3%RH) is about 10% less. Both sensors can even be bought with factory-installed PTFE dust shields (add 10% to base price - and look for the -GM1 version instead of -GM model, for example).
The only downside of these chips to hobbyists like us is that a DFN-6 is hard to solder by hand and pretty much requires a reflow solution and a custom-made PCB. Granted, someone like Sparkfun or their affiliates might sell the PCB, but these sensors are definitely not as easy to integrate as the DHT-22, for example. In particular, the Si7013 seems like a bad candidate for hobby use (even if you are experienced with reflow soldering the HTU21D or SHT21 series) because Silicon Labs are cramming 5 pins per side into two sides of a 3mm square package, resulting in 0.5mm OC pin spacing... in a package where you can't observe the pins when the chip is installed.... IMHO, a perfect candidate for blue smoke unless you have a well-calibrated pick and place machine!
Anyhow, I've bought 5 of Si7020's with the PTFE dust cover and I'll report back when I have them re: their compatibility with the Sensiron library. At first glance, it looks like some modifications may be required but setting options, CRC, and so on are similar. Thus, a fork may be possible, something made easier by the fact that the Silicon Labs units can be queried and will reply with the SKU for the model as well as the revision of the firmware on them (!!!).