Pid

Is there an alternate automatic method to precisely fill a tank with water rather than the PID? The process variable is the weight of the tank. (perhaps a timer? How?

I can't imagine why you would want to use PID for this. Simply begin shutting the fill valve when you get to a preset level determined by a comparison statement (< or >). With a reasonably repeatable flow rate you can determine with one or two test the level to stop the valve so that when it closes completely you are at the correct level. If you need really precise control you can use a second cmaller fill valve to "top off" the tank, turning it on as the other one closes.

Self-tuning preact

To build further on Tom's suggestions:

You can make your pre-act self-adjusting. Use a register to store your preact. When the valve is closed, compare the tank level to setpoint. If it's outside of some tolerence level, then adjust your preact by a percentage of the error. You may even want to use two different percentages - a large one if overshooting and a smaller one if undershooting.

With this kind of arangement, if circumstances change gradually over time (in warmer weather, the stuff flows faster), the system will continually compensate for the changes.

Tom's gross/fine valve setup is great for discrete valves. If you have an analog valve, you can treat it like a gross/fine. Open the valve 100%. When at an overly large preact, bring the valve to 1% (or whatever produces a "trickle" flow. When at Preact2, close the valve completely (which should be fast, since it's not open very much).

A third technique I've used with discrete valves is to close them at just before preact, intentionally undershooting, but not by much. Then pulse the valve openclosed, and measure the change. The pulses can be repeated, and perhaps the pulse width changed, to get to your target.

I've seen this method fill a Dixie cup to where the water was OVER the top, yet not spilling a drop. (surface tension)

What Allen is suggesting is sliding mode control

However, I agree with Tom and even a preact seems a little excessive unless the water level increases at different rates. Otherwise one can just shut the valve off a little early to account for delays.

Sometime we should get Tom to explain sliding mode control. Tom brought it up many months back. Allen has made a start. It really is cool stuff and worth knowing. However, there isn't a sliding mode control block in PLCs so no one is ever exposed to it.

Talking about overkill

If you're not happy with PID, sliding mode control or pre-act you could always try fuzzy logic.

Kind regards,

filling a tank

filling a tank with its weight to control is not very precise better is to use a simple toiletfiller. cheaper and faster.
For example to make it more complex use a level only in top of tank and use a valve.
PID is for this not very usable.
Better is on/off or puls control.