Set-point (SP) for the tank level node red

[Colin]

Post the flow containing just the pid node and the surrounding logic so I can see what you have done.
To post a flow here, select the nodes then Ctrl-E (or menu Export), copy to clipboard, and paste it here. In order to make code readable and usable it is necessary to surround your code with three backticks (also known as a left quote or backquote ```)

``` 
   code goes here 
```

See this post for more details - How to share code or flow json