Info from Polar on the altimeter in the s72x watch

[Dr_Crash]

I have been asking Polar why their user manual includes a disclaimer about climbing with their watch:

 

I am interesting in getting a s720i or s725 watch. The manual that you have online says:

 

"It also measures altitude and temperature in cycling. No other use intended or implied. The altitude and temperature functions are not designed to be exclusive measuring equipment in aviation, climbing sports, water sports or their equivalent."

 

I would like to get more information about that. The main appeal of the altimeter for me is to use it for climbing (not as a single instrument, of course, but in conjunction with a map and compass) and skiing. I wonder why your manual has the above disclaimer. Is the altimeter not precise enough for you to recommend for such uses? Do you have its specifications (operating temperature, range of elevetion, and precision)?

 

Thanks,

Dr. Crash

 

Here is Polar's answer:

 

Dear Dr. Crash,

 

Thank you for your email.

 

The S720i/S725 are designed so that the unit is mounted on a handlebar.

Both can record your altitude and temperature in running and cycling modes.

However while running or hiking the monitor is worn on the wrist and your body heat will affect the temperature readings.

 

The altitude readings are based on the measurement of atmospheric pressure, essentially a barometer. Since air pressure changes constantly due to various weather conditions, the calculated altitude of a certain spot may differ with the weather.

 

Variation of high- and low- pressure can cause a change of up to 80 mbars which means approximately 700m/2000ft change in the calculated altitude.

For example in sea level it is normal to have change of 900-1500 mbars, which can cause +/- 100 meter variation in the measured altitude, which is normal for altitude readings based is based on air pressure.

 

The altitude is measured from air pressure with the pressure sensor.

Therefore, when measuring the altitude, the weather conditions will cause some error in the measurement. For example, when the starting point altitude is set in the S710/S720/S725 correctly and if one is on that same level several hours, the value may drift several meters depending of weather change. Therefore, the altitude measurement made by the pressure sensor cannot calculate absolute altitude value, only the altitude difference during exercise.

 

There are also internal errors which affect the altitude measurement:

 

The pressure sensor has some normal nonlinearities, which cause errors in pressure measurement and therefore also to the altitude.

 

The pressure sensor which is used in the S710/S720/S725 (and also in our competitors devices) is sensitive to temperature changes. Therefore, the pressure sensor is temperature compensated at the pressure sensor factory, but some temperature-based error does remain. Therefore if the unit is worn on the wrist the altitude readings may be affected by your body heat.

 

The value that is measured with the pressure sensor is converted to digital value, i.e. digitized. The altitude is calculated from this value using the

ISO2533 standard. The calculation contains some inaccuracies due to the digitization, resulting in calculation error.

 

Therefore, there is four main error sources; 1. nonlinearity of the pressure sensor 2. temperature based error on pressure sensor 3. altitude calculation based error.

4. weather conditions change based error

 

The sum of errors 1 and 3 during measurement can be maximum +/- 2% .

However, the typical error is inside of +/- 1%. The temperature is presumed constant +25 degrees.

 

Error 2 is calculated in the temperature range of -10 to +60 degrees. The temperature based error depends on the altitude. On the sea level maximum temperature change based error is about +/- 8m, at 1000m level it is +/- 9m, 3000m it is +/- 12m, 5000m level it is +/- 14m, and at 7000m it is about +/- 17m. However, at high altitudes the temperature changes are normally much smaller (for example +25 --> +15), and therefore also the temperature based error is also smaller.

 

Error 4 depends on the weather, i.e. how fast weather is changing from the low pressure (rainy) to the high pressure (sunny) or vice versa.

 

For example, if one is riding from the starting point at 300m to the end point at 800m, and the starting value is set at 300m, the maximum error of the altitude value can be calculated as follows:

 

Errors 1 and 3:

The altitude difference is 500m (300m --> 800m), therefore the maximum error is +/- 10m (+/-2% of 500m).

 

Error 2:

The maximum temperature change based error, in temperature range -10 - +60 Celsius degrees, is about +/- 9m at 500m level

 

The total maximum error is sum of these two errors, if we assume that the weather conditions have been stable. The maximum error is +/- 19m

 

If the weather conditions have not changed, the measured value at the end point (800m) can be between 781m - 819m. However, if the weather conditions (error 4) have changed, the altitude error can be greater.

 

If you need further assistance please feel free to contact Customer Service at 800-227-1314 or 516-364-0400.

 

Thank you for choosing Polar.

 

drC