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02-16-2017 02:19 PM
I wanted to know who much time the heater on the Si7021 temperature and humidity sensor needs to be turned on in order to remove the 'memory' effect or in order to remove the condensation when used at dew point. Also, how often does this have to be done. For the sake of calculations, let's assume the HTRE bit is set in the user register and the value in the heater control register is set to '0000' meaning drawing 3.09 mA.
The reason for these questions is that I want to calculate how much current the sensor will draw from a small button cell and for how long will the battery last.
02-16-2017 02:32 PM
The heater is not compatible with coin cell operation.
In a situation where power is not limited it is useful if the heater is run continuously in high humidity environments.
Simply pulsing the heater on and off does not really help and for coin cell operation and long term exposure to high humidity conditions (which will cause the memory effect) has to be avoided by the user.
02-16-2017 02:39 PM
Thank you for the reply.
What kind of battery is recommended for the heater operation. What should be the minimum rating of the battery? Assuming we are using 3V Lithium Cell (non-rechargeable and readily available from consumer retail walk-in stores), what will be the on timings for the heater and the current drawn?
02-16-2017 03:14 PM
I do not think heater operation is practical for any non-rechargeable battery. The heater would have to be run continuously in high humidity environments to be effective. So worst case would be the environment is always high humidity and the heater would have to run all the time. 3mA draw would imply an impractical battery size unless it is rechargeable.
The only practical solution I know of is for the user to avoid high humidity conditions. Brief exposure (<1 day) is generally OK.
02-16-2017 04:19 PM
Thank you for your reply.
If the humidity is not always high (say it peaks twice every day), how long should the heater be felt on? How do you determine when to start and when to stop the heater?
02-16-2017 04:26 PM
For heater use we recommend turning it on when the humidity is above 80%. The heat will reduce the humidity as described in AN607. The heater would normally be turned off when the humidity after correcting for heating drops back under 80%.
Again, most people would not use the heater in a battery powered application. It can take a long time (minutes) for the device temperature to settle between heater on and heater off and the power is quite high.
The heater is more often used where AC power is available and it is generally just turned on and left on.
02-17-2017 08:04 AM
For heater use we recommend turning it on when the humidity is above 80%. .... The heater would normally be turned off when the humidity after correcting for heating drops back under 80%.
just to save someone not thinking some frustration: do not use the same humidity as off and on DO have some hysteresis
02-17-2017 08:43 AM
Thank you for the comment. Our application note AN607 has the correction formulas so that humidity can be corrected for heating. I think it is a good idea to add a little hysteresis, but the large hysteresis that would be required if the correction is not done can be avoided if the correction is done.
Some customers to elect just to put in a large hysteresis and not bother with the correction. 1C of heating produces about a 5% humidity change so they hysteresis should be around 20% for 4C of heating if it is done this way.