How would I manually go about getting the next sequence number?
Unfortunately there is no such way, since some databases use identities. Whatever you do, will be database-dependant. I don't understand the part you say "scale provides serial numbers", though. Are you using a table with automatic id or not? And if yes, also didn't understand why you need to take the next value manually? Can't you just create a new record to get the new number and then fill the remaining data?
When I go and get a real weight from the scale the scale has its own internal record of all the weights it has taken and sends it's serial number along with the weight. So for each weighing I record in the my database the weight and serial number I get from the scale along with an identifier for the specific scale that provided the weight. However sometimes users need to record weights manually, for example they might know the tare weight of a truck and just want to type it in without actually weighing it. In this case I have to make up my own serial number which I used a generator for in the past.