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Re: [sensors] Exposing secondary values in the SensorReading

From: Rick Waldron via GitHub <sysbot+gh@w3.org>
Date: Tue, 08 Nov 2016 21:23:34 +0000
To: public-device-apis-log@w3.org
Message-ID: <issue_comment.created-259263942-1478640212-sysbot+gh@w3.org>
> temperature of the pressure sensor in an altimeter

Can you give an example of this? I've implemented drivers for many 
sensor packages that include temperature, pressure and altitude and I 
don't recall encountering anything like "temperature of the pressure 
sensor in an altimeter"—but I obviously haven't worked with _every_ 
sensor package. Generally, an altimeter uses barometric pressure to 
calculate the altitude: pressure at sea level, the pressure as 
measured by the barometer at the place being measured, and the 
temperature at the place being measured. These three values are then 
used to produce the altitude/elevation _from sea level_. To get the 
relative local altitude, the present geographical elevation must be 
known; eg. I'm on the top floor of a 100 story building in Denver, 
Colorado: the relative local is just the 100 stories, and does not the
 geographic elevation. (I'm not really on the top floor of a 100 story
 building in Denver, in fact: no such building exists). 

On the other hand, `bias` is an interesting data point. [In the recent
 discussion about raw magnetometer 
readings](https://github.com/w3c/magnetometer/issues/16), it was 
implied that the individual bias for each axis would be available, 
however that't not true for all magnetometer platform APIs (as later 
stated by @riju) or ICs on the market. For example, the HMC5983 has a 
two configuration bits that allow setting one of two bias modes 
(negative or positive) that are handled by the IC itself (ie. it will 
apply a positive or negative bias current across the axis). If a 
device is made with that chip, or similar, then bias defined as a 
value that is "measured" is not something the sensor is aware of. 

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Received on Tuesday, 8 November 2016 21:23:40 UTC

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