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androidbluetooth-lowenergybluetooth-gattheartrate

BLE Heart Rate Senser Value Interpretation

I have an Android App where I get Heart Rate Measurements from a Polar H10 Device. I'm totally lost on how to interpret the heart rate. Various links to the bluetooth.com site are resulting in 404 errors unfortunately.

The characteristics value is i.e. [16, 59, 83, 4]

From what I understood the second byte (59) is the heart rate in BPM. But this does not seem to be decimal as the value goes up to 127 and then goes on -127, -126, -125, ... It is not hex either.

I tried (in kotlin)

characteristic.value[1].toUInt() 
characteristic.value[1].toInt()
characteristic.value[1].toShort()
characteristic.value[1].toULong()
characteristic.value[1].toDouble()

All values freak out as soon as the -127 appears.

Do I have to convert the 59 to binary (59=111011) and see it in there? Please give me some insight.

### Edit (12th April 2021) ###

What I do to get those values is a BluetoothDevice.connectGatt(). Then hold the GATT. In order to get heart rate values I look for

  • Service 0x180d and its
  • characteristic 0x2a37 and its only
  • descriptor 0x2902.

Then I enable notifications by setting 0x01 on the descriptor. I then get ongoing events in the GattClientCallback.onCharacteristicChanged() callback. I will add a screenshot below with all data.

From what I understood the response should be 6 bytes long instead of 4, right? What am I doing wrong?

On the picture you see the characteristic on the very top. It is linked to the service 180d and the characteristic holds the value with 4 bytes on the bottom.

The characteristic 0x2a37

Solution

  • It seems I found a way by retrieving the value as follows

    val hearRateDecimal = characteristic.getIntValue(BluetoothGattCharacteristic.FORMAT_UINT8, 1)

    2 things are important first - the format of UINT8 (although I don't know when to use UINT8 and when UINT16. Actually I thought I need to use UINT16 as the first byte is actually 16 (see the question above) second - the offset parameter 1

    What I now get is an Integer even beyond 127 -> 127, 128, 129, 130, ...