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[on:]

Most sensors have some kind of respnse time.  Temperature/PH/Humidity
I'm currently looking at the SHT75 - Digital Humidity Sensor http://www.sensirion.com/en/01_humidity_sensors/06_humidity_sensor_sht75.htm
it says it has a response time of 8 seconds.
What does that actually mean?
I think it means from the time it "turns on" it takes 8 seconds to get to an accurate reading (or any reading).

Since arduino just quick grabs the output it doesnt check the value for 8 seconds before showing on the serial interface. 

So can I assume I am getting accurate readings as long as power is always supplied to the sensor even though arduino is just checking the value once every 30 minutes?

And to follow on that.  This will be a battery operated sensor.  So I want it to sleep.  At which point the sht75 will(should) be off.
I'm assuming I can just turn on the sht75 - sleep 8 seconds - then poll the data?

[Reply #1 on:]

Humidity, unless buckets of water are involved, tends to be slow to change. Therefore, the sensor doesn't have to be extremely quick to pick up a difference in humidity.

So, those 8 seconds relate to the example given, but what it really means is when you physically change the humidity, it will take the sensor 8 seconds to put out the actual value read. Understood?

A simple example is, imagine that the humidity sensor is in fact a light sensor. So if you turn a light switch, it will take the sensor 8 seconds to tell you that the light has been turned on.

[Reply #2 on:]

"The device automatically returns to Sleep Mode after measurement
and communication are completed."

"recommended supply voltage is 3.3V."

"3.1 Start up Sensor
As a first step the sensor is powered up to chosen supply
voltage VDD. The slew rate during power up shall not fall
below 1V/ms. After power-up the sensor needs 11ms to
get to Sleep State. No commands must be sent before
that time."
"3.3 Measurement of RH and T
After issuing a measurement command (‘00000101’ for
relative humidity, ‘00000011’ for temperature) the
controller has to wait for the measurement to complete.
This takes a maximum of 20/80/320 ms for a 8/12/14bit
measurement. The time varies with the speed of the
internal oscillator and can be lower by up to 30%. To
signal the completion of a measurement, the SHT7x pulls
data line low and enters Idle Mode. The controller must
wait for this Data Ready signal before restarting SCK to
readout the data."

So you power up, have to wait until you can send a command, send the command and then poll a line to see when its done.
8 seconds typical "Time for reaching 63% of a step function, valid at 25°C and 1 m/s airflow."
If you're not big jumps in humidity would expect less - but you need to poll the line anyway to see when its done.