The Garmin 235 differs from the previous model 230 by having an added optical sensor. This increases the reliability of results of wrist based heart rate monitoring due to the difference in how the data is retrieved from the body.
Rather than send electrical impulses through the body, the optical sensor measures blood flow rates. This technology is best used when doing activities under 160 beats per minute (BPM), so keep that in mind when selecting a monitor based on the intensity of your activity level. It is very difficult to get as accurate readings as wearing a cumbersome chest strap, but you pay a price for comfort, convenience, and style.
This new optical sensor was developed by the Garmin tech team in-house which is another example of how the manufacturer is increasingly becoming self-reliant in creating proprietary wearable technology.
The issues that some users experienced with the Garmin Vivosmart HR have been resolved in the Garmin 235, so this updated technology is certainly worth noting.