Micrologix 1400 Programming Question

Introduction: I'm tasked with created a rslogix program to communicate with a SCADA based PH monitoring and equalization project for our wastewater.

The contractor is doing the SCADA programming.
In the chemical pump dosing routine they're asking the following:
For the dosing pump repeat cycle timer the operator shall be able to enter a dosing time (TON preset variable) for open and one for closed. In the reaction repeat timer ON time is active, the dosing time shall be initiated and the air supply open and remain open until dosing time expires at which time the valve shall close. The closed time shall be based upon a "Lookup Table' such that when the difference between measured PH and PH set point is large, the valve closed time shall be shorter; when the diff between the measured PH and PH setpoint is small, the Valve closed time longer.

I'm confused with some of their thought process, especially with the Lookup table.

Thoughts?

TIA

What they are trying to describe is called a "split range time proportional" (SRTP) control, but the description is a little weird.

The usual way to do SRTP is to consider it a percentage between 0 and 100.

Then you choose a time period. Let's just use 1 minute.

A 10% command turns the pump on for 6 seconds, and off for 52 seconds. A 50% command turns the pump on for 30 seconds and off for 30 seconds.

It doesn't seem sensible to instead make an operator enter independent "on" and "off" times. Then they're the ones who have to calculate the percentage of dosing.

Their description also sounds contradictory to me: they both say that the "OFF" time shall be entered by the operator, and also that it shall be calculated from a lookup table. Which one do they want ?

A "lookup table" is one way to do proportional control. Do they want the operator to also fill out table entries that determine the proportion of "off time" to the delta in pH ?

I think you need to take a step back and have them determine what is being controlled by the SCADA system, and what is being controlled by the MicroLogix.

If they just want proportional control of a dosing pump, they should choose the percentage of ON-time and the time interval.

Contradiction is exactly what I was thinking.
I have dozen of pages of technical notes by the engineers who will be doing the programming for the SCADA. Mostly the SCADA is for interface and data logging.

This is till in it's infant stages but I'm just getting a head start before they throw it officially on me and say have it doe Friday.
I guess when the time comes I'll have to sit down and have the re-iterate what they are wanting.

Thanks!