A free program for Android, by Health Fitness AI Lab.

Heart Rate Monitor – The App For Your Fitness is a professional tool that can be used to measure, track, and record your heart rate to get an accurate reading. This app is designed to make measuring your heart rate a simple and easy task. All you need to do is select a measurement, press the start button, and the measurement will begin.

Once the measurement is done, you can view the results in a chart, and you can also export the results to a CSV file. This app has a built-in heart rate zone function.