Overview
Users sometimes observe that their HRV (Heart Rate Variability) data plots seem to lag behind their actual breathing—for example, the graph may trend downward while they are already inhaling. This experience is most common when using Bluetooth Low Energy (BTLE) sensors.
Why Does a Delay Occur?
The amount of “latency” (delay) you experience depends largely on the type of sensor connection you are using:
-
USB Sensors: These provide the fastest data transfer and are considered close to real-time.
-
Wired Lightning Sensors: These introduce a very slight delay due to the processing required by the mobile device connection.
-
Bluetooth (BTLE) Sensors: These have the most noticeable delay (typically 2 to 4 seconds). This is because the standard Bluetooth protocol sends data in intervals to conserve the sensor’s battery life and reduce processing overhead. This is a characteristic of almost all heart rate sensors using BTLE technology.
Data Processing vs. Real-Time Plotting
It is important to distinguish between the HRV graph and Coherence calculations:
-
HRV Plotting: The line graph you see on the screen reflects the raw pulse data being received. With Bluetooth, this data arrives in “packets,” causing the visual delay.
-
Coherence Scores: Both the emWave and Inner Balance applications calculate your coherence levels every 5 seconds. This calculation is based on an accumulation of data from the previous 60 seconds, ensuring that the score reflects your overall physiological state rather than a single heart beat.
Can the Delay Be Fixed?
Currently, the delay is a technical limitation of the Bluetooth Low Energy protocol used across the industry. While the HeartMath Engineering Team is constantly looking for ways to optimize data flow, the 2–4 second lag is necessary for the sensor to maintain its small size and long battery life.
Summary for Users
While the visual lag can be slightly distracting, it does not impact the accuracy of your session data or your final coherence scores. The system successfully captures every heartbeat; it simply takes a moment longer for those beats to travel from the sensor to your screen wirelessly.