The issue is with the underlining technology they all use so changing device is unlikely to correct huge errors in readings. There’s plenty of articles about these online but here’s some blarb I just copied and pasted:
All of these devices use light to measure your pulse. They shine a light into the blood vessels in your wrist, and then detect the changes in blood volume that occur each time your heart beats and pushes blood through your body. Sensors on the device detect how much light your blood vessels reflect back: Less reflected light means a higher blood volume.
Experts say devices that use this technology are pretty accurate at monitoring heart rate although they tend to be a little less precise when a user engages in intense activity. Interestingly, some of the makers of fitness trackers do not recommend using their devices if you’re solely interested in monitoring your heart rate during workouts.
Using light to measure a pulse is relatively straightforward when a person is at rest, but becomes challenging when the subject moves around. Ambient light, as well as the movement of the person’s muscles, tendons and capillaries, can all interfere with the measurements.
For this reason, heart rate monitors for exercise have traditionally used chest straps — which measure the heart’s electrical activity — or required users place their fingers over a sensor.
But in recent years, companies have developed algorithms that cancel out a lot of the “noise” generated by people’s movements, and allow sensors to measure heart rate even when a person is on the go.
By algorithms obviously this means the tracker will essentially be estimating your heart rate based how much light it is receiving back and however the algorithm is compensating for your exercise.
Personally, I don’t wear any of this tech. I’ve lost about 3.5 stone in the last couple of years through exercise and maintaining a net of 1500kcal a day. How much I need to eat and still lose weight (fat) was discovered over a period of time through trial and error. I’m now much more in tune with my body on how much food is enough and how much I need to eat after certain exercises.
The rowing machines at the gym (Concept 2 model D’s) do output how many calories you have used during a workout. This is accurate because a calorie is a unit of energy. What people aren’t aware of though is the value on the display is based on a formula which assumes the user weighs 79.5kg. You can’t change this value on the machine but on the Concept 2 website they have a calculator where you can input your workout summary and your actual weight and it will correct the amount of calories burned to your body weight, as if you weigh more than 79.5kg you will be burning more calories than what is displayed (although not by much it has to be said).