So what you what to do is to have a form with 30 machines, and then to enter a meter reading, and keep this value for later use, thus storing multiple value of reading for each machine.
If this is what you want then you would need at least 2 tables.
1st Table
MachineID (Autonumber - indexed)
Machine Number (number or text, whatever you need)
Machine Serial (Text)
2nd Table
MachineID (number)
Meter Reading (assuming number field)
Date (Date/time) - what i was thinking was storing the date/time when the meter reading was entered, this way you are able to do a query to get the last reading entered.
Is there only ever going to be 30 machine, is this static, or is their likely to be an increase/decrease???
I would not put all 30 machines onto a form, cause i think it would look a bit crowded, but that me...
What i would do is to have a drop down box containing all machines, and then the user enters this, and their would be a meter reading text box next to this, and then an update button to write this information to the 2nd table, which auto populates the date and time (i suppose you could also have a user box as well if you wanted)
That is the way i would do it, does that sound about right?
I hope any of this helps.