The VISA Read is returning the correct number of bytes that are in the serial buffer. Why the number of bytes would vary is possibly due to a slow response from the instrument. If the instrument does not return a termination character, then you probably want to use a different VI. What I often do is place the VISA Bytes at Serial Port in a while loop. When the number of bytes is greater than 0, exit that loop and go to another while loop. In that loop, another VISA Bytes at Serial Port is used with a VISA Read and that loop will exit when the byte count is equal to 0. This way, you don't have to use a fixed delay and when the last loop finishes, you know you have completely flushed the buffer of all characters. It's also a good idea to provide some sort of timeout in the first loop so that if no response comes, the loop will eventually finish.
