Overview
Culture's pH control system uses a PID controller to correct pH by sparging CO2 (to lower pH) or dosing base (to raise pH, when enabled). To prevent acid and base from competing with each other and to avoid overly frequent corrections, control actions are gated by deadbands - defined thresholds above and below the pH setpoint. The controller only activates when pH drifts outside those thresholds.
A consequence of this design is that once pH crosses the deadband boundary, the PID controller's error term is calculated from the original setpoint - not from the deadband edge. This means the controller "sees" a larger error than the actual drift beyond the boundary, and can respond more aggressively than intended. This is especially noticeable when deadbands are wide or PID gain terms are high.
The Deadband Setpoint Offset parameters give you a way to soften this initial response for CO2 and base independently.
How Deadbands and Error Calculation Work
When pH rises above the positive deadband threshold, CO2 sparging is enabled. At that moment, the PID controller calculates how much CO2 to apply based on the error - the distance between the current pH and the setpoint.
For example, with a setpoint of 7.0 pH and a positive deadband of 0.2 pH:
The controller activates when pH reaches 7.2
The error is calculated as
7.2 − 7.0 = 0.2 pH— a relatively large valueThis produces a proportionally large initial CO2 response, which can cause pH to overshoot downward before stabilizing
The diagram below illustrates this behavior. On the left side (Positive Deadband Setpoint Offset = 0), the full 0.2 pH distance from setpoint is used as the error, resulting in a large CO2 burst and a sharp pH drop. On the right side (offset = 0.15), the virtual setpoint is shifted up to 7.15, so the controller sees only a 0.05 pH error at activation and responds much more gently.
Diagram: pH control with deadband and Positive Deadband Setpoint Offset
The same logic applies in the downward direction: when pH drops below the negative deadband and base dosing activates, the error is measured from setpoint rather than from the deadband boundary, which can similarly produce a larger-than-expected initial base dose.
The Deadband Setpoint Offset Parameters
These parameters add an offset to the actual pH setpoint to create a virtual setpoint used only for PID error calculation when CO2 or base is active. They do not change the deadband boundaries or the setpoint itself.
By shifting the virtual setpoint toward the deadband edge, you reduce the error the controller sees at the moment a correction begins — resulting in a gentler initial response.
CO2 (positive direction): The Positive Deadband Setpoint Offset shifts the virtual setpoint upward, reducing the error seen when CO2 sparging first activates.
Base (negative direction): The Negative Deadband Setpoint Offset shifts the virtual setpoint downward, reducing the error seen when base dosing first activates.
How to interpret the CO2 parameter (example):
Positive Deadband Setpoint Offset | Virtual Setpoint | Error at activation | CO2 Response |
0 (default) | 7.0 pH | 0.20 pH | Large |
0.15 | 7.15 pH | 0.05 pH | Small |
0.20 (= deadband) | 7.20 pH | ~0.00 pH | Minimal |
Assumes setpoint = 7.0, positive deadband = 0.2
Setting the offset equal to your deadband means the virtual setpoint sits right at the deadband boundary. The controller starts with near-zero error and ramps up its response only as pH continues to drift further. This is the most conservative setting and is useful when your process is sensitive to overshoots.
When to Adjust These Settings
Consider adjusting the Deadband Setpoint Offset if you observe:
Sharp pH drops shortly after CO2 sparging activates, or sharp rises after base dosing
Oscillating pH that overshoots past setpoint after a correction
Small drifts beyond the deadband triggering a disproportionately large response
A good starting point is to set the offset to match your deadband value, then reduce it gradually if the controller feels too slow to respond.
Where to Find These Settings
These parameters can be set in two places:
Script Visual Editor (SVE)
Open the pH control function block and expand Advanced Settings. You will find:
Positive Deadband Setpoint Offset — controls the CO2 correction path
Negative Deadband Setpoint Offset — controls the base correction path
Values set here apply for the duration of the run as scripted.
Live Controls
From an active run, open Live Controls and expand the Advanced pH Control section. You will find:
pH Control CO2 Positive Deadband Setpoint (delta pH) — equivalent to the Positive Deadband Setpoint Offset
pH Control Base Negative Deadband Setpoint (delta pH) — equivalent to the Negative Deadband Setpoint Offset
Live Controls allows you to adjust these values mid-run without modifying the Script.
Note: The parameter names differ slightly between SVE and Live Controls but control the same underlying behavior. Consistency between the two will be improved in a future release.
Summary
pH control is gated by deadbands to prevent overcorrection and acid/base competition
When pH exceeds a deadband, the PID error is measured from the original setpoint, which can cause a large initial CO2 or base response
The Deadband Setpoint Offset parameters shift the virtual setpoint used in error calculation, softening the initial correction
Setting the offset equal to your deadband produces the gentlest possible start; reducing it from there increases responsiveness
These settings are available in the Advanced Settings of the pH control block in SVE, or under Advanced pH Control in Live Controls
For questions or help tuning your pH control parameters, contact our support team at support@culturebiosciences.com or reach out via the messenger in Console.