I don't understand what "The value of the scale is well over" and "The value of the scale does not exceed" mean. This sounds like an issue with the data returned by the scale, which is out of scope for this issue tracker. If you believe the scale is returning the correct data but Chrome is passing it to your application incorrectly please file an issue at crbug.com.

I'm sorry. There's an error because I wrote it using a translator.

Data comes in port.readable.getReader().
But, when the hardware is powered off and turned on, data does not come in port.readable.getReader().

Are you using the RS-232 interface or the USB interface? When the hardware is powered off and powered back on a USB interface will disconnect and you will need to call navigator.serial.requestPort() or navigator.serial.getPorts() to get a new SerialPort object representing the new device connection. The RS-232 interface doesn't have this behavior.

I'm using the RS-232C interface. I'm transferring data from scale to COM 2, and when I connect to COM 2 through navigator.serial.requestPort(), I was able to receive the data.

There may be an error when the scale is powered off or turned back on which would require you to call port.readable.getReader() again. For example, the scale may generate a "break" signal at power-on. Does your code handle errors returned by the reader and request a new reader on the new ReadableStream returned by port.readable in that case?