Integrating dosing and measurement technology (pH/redox) cleanly
An automatic pH and redox control system is only as reliable as the way it is integrated into the water circuit. Most faults do not originate at the measuring instrument, but in the pipework connecting the measuring cell and the injection points. This article shows you how to integrate the sample water, probes and dosing correctly into the pool hydraulics.
Where measuring and dosing belong in the circuit
Measurement and dosing technology is installed on the pressure side downstream of the filter. Only here is the water clear enough that the probes are not falsified by suspended solids. The order of the components is decisive: first the sample water is drawn off, then the heater follows, and only at the very end are the dosing agents introduced. This way, undiluted chemicals reach neither the heat exchanger nor the probes.
| Position in the pressure line | Component | Reason |
|---|---|---|
| Downstream of the filter | Sample-water take-off (bypass) | Clear water, stable measurement |
| After that | Heat exchanger / heat pump | Heating before dosing |
| After the heater | pH and disinfection injection points | Protects the exchanger; chemicals dilute on the way to the pool |
| Before the return | Salt chlorinator cell (if fitted) | Disinfectant produced goes straight into the pool |
The pipe dimensions typically used in the plant room are OD d50 (DN40) or OD d63 (DN50). The actual sample-water side stream is considerably smaller and branches off from this main line.
Measuring cell in a bypass: creating a pressure differential
The measuring cell is not installed directly in the main line, but in a side stream (bypass) running parallel to it. The advantage: a steady, small flow of sample water that you can shut off and service independently of pool operation, without stopping the circulation.
For any water to flow through the bypass at all, you need a pressure differential between the two branch points. To achieve this, set two tapping points with a tee into the OD d50 (DN40) pressure pipe and install a defined resistance between them. The following have proven effective:
- a slightly throttled ball valve in the main line between the branches,
- a fixed orifice plate, or
- a spring-loaded cone check valve whose opening pressure creates the required differential.
The resulting pressure difference pushes the sample water from the front branch through the cell and back into the rear branch. Important for the pipework detail: the measuring cell is mounted vertically and the flow passes through it from bottom to top. Only in this way can air bubbles escape and the probe tips stay permanently wetted. Shut-off valves before and after the cell, together with unions, make later probe replacement possible without having to open any solvent-cemented joints.
Caring for and calibrating the probes
pH and redox probes are wearing parts with a limited service life. The pH probe works with a glass membrane, the redox probe with a platinum tip – neither must ever run dry. Always store removed probes wet in their storage solution (potassium chloride), never in distilled water or dry.
The pH probe is calibrated with buffer solutions pH 7 and pH 4, while the redox probe is checked against a redox buffer solution. Carry out the calibration at the start of the season, after every probe change and at regular intervals thereafter. Deposits and coatings make the readings sluggish or inaccurate – careful cleaning according to the manufacturer’s instructions helps here. A typical pH setpoint is in the range of about 7.0 to 7.4; set the appropriate redox setpoint on a system-specific basis so that sufficient disinfection performance is achieved.
Placing injection points correctly – after the heater
The injection points for pH correction and disinfectant are located as the last components in the pressure line, downstream of the heater and the sample-water take-off. There are two reasons for this: the heat exchanger is not attacked by aggressive, undiluted chemicals, and the added chemicals do not reach the probes in pure form, but only in diluted form via the pool.
When placing the injection points, observe the following:
- Separate injection points with spacing: never introduce pH-minus and chlorine product at the same location. If the two meet undiluted, dangerous chlorine gas can form.
- Injection valves with a non-return function: these prevent pool water from pushing back into the dosing line.
- Install as high up on the pipe as possible: this prevents chemicals from collecting in the line during standstill.
- Serviceability: a union at the injection point makes later cleaning of the injection valve easier.
Dry-run protection and dosing enable
The most important safety mechanism is coupling the dosing to the circulation. If the filter pump is not running, no dosing pump may deliver – otherwise highly concentrated agents collect in the stationary line and, at the next start-up, are pushed into the pool as a surge or attack the material.
Proven solutions for the enable signal are:
- a flow switch or flow monitor in the sample-water line that only enables dosing when flow is confirmed,
- a volt-free contact from the filter pump or the controller that switches the dosing along with it,
- combined level and flow monitoring.
In addition, empty canisters should be detected via a suction lance with an empty signal so that the dosing pump does not run dry. The electrical integration of enable contacts and pumps belongs in the hands of a specialist company and must comply with the relevant electrical regulations.
Avoiding false alarms and control fluctuations
Erratic readings and recurring alarms almost always have hydraulic or maintenance-related causes. Check in the following order:
- Air at the probe: vent the cell and ensure flow from the bottom.
- Sample-water flow too low: check the bypass pressure differential and the throttling.
- Probe condition: remove coating, check calibration, replace an aged probe.
- Dosing too aggressive: regulate more gently via proportional and pause times so that the value does not overshoot and undershoot.
In addition, keep the probe and signal cables away from power cables and ensure a clean equipotential bonding of the system – this too is a task for the qualified electrical contractor. A plausibly set run-on and delay time prevents short, normal fluctuations from being reported immediately as a fault.
Integrating the measuring cell in a bypass and the injection points correctly
- Define the take-off point Plan the sample-water take-off on the pressure side downstream of the filter, where the water is clear. The main line is usually OD d50 (DN40) or OD d63 (DN50).
- Set the branches with a throttle point Fit two tees into the pressure pipe and install a defined resistance between them (throttled ball valve, orifice plate or spring-loaded check valve) so that a pressure differential develops.
- Install shut-off valves and unions Fit ball valves and unions before and after the measuring cell so that the cell can be shut off and disconnected for probe replacement.
- Mount the measuring cell vertically Install the cell upright with flow from bottom to top so that air escapes and the probe tips stay permanently wetted.
- Insert the probes and vent the cell Insert the pH and redox probes, vent the cell and set the sample-water flow to a steady, small value via the throttle.
- Set the injection points after the heater Arrange separate injection points for pH correction and disinfection with spacing downstream of the heater, each with an injection valve with a non-return function, as high up on the pipe as possible.
- Wire the dry-run protection Couple the dosing enable to the circulation via a flow switch or a volt-free pump contact. Leave the electrical integration to a specialist company.
- Calibrate and test run Calibrate the probes with buffer solutions, set the setpoints as well as the dosing and pause times, and check the system in a test run for steady, plausible readings.
Bill of materials
Example – please verify with a specialist. Parts and quantities depend on the installation; sizes shown are examples.
| Component | Size | Art. No. | KLN No. | Qty |
|---|---|---|---|---|
| 90° Tee, solvent socket, equal (PVC-U) | d50 | 70022 | 51512912 | 2 |
| 2-way ball valve M1, solvent socket (PVC-U) | d50 | 128814A | 51511559 | 2 |
| Cone check valve S4, solvent socket (PVC-U) | d50 | 121041 | 51510312 | 1 |
| Union Ocean, solvent socket / solvent socket | d50 | 170397 | 51512461 | 2 |
| Griffon cleaner / degreaser | – | 1 | ||
| Griffon UNI-100 PVC-U adhesive | – | 20023 | 51512566 | 1 |
| pH/Redox measuring cell (flow-through fitting with probe ports) | – | – | – | Install vertically, flow from below, with air bleed |
| pH and Redox probe | – | – | – | Wearing parts; store moist and calibrate regularly |
| Measuring and control unit with dosing pumps | – | – | – | With an enable input for dry-running protection |
| Injection valves with non-return function | – | – | – | Separate valves for pH correction and disinfection, spaced apart |
FAQ
Why is the measuring cell installed in a bypass and not directly in the main line?
The bypass delivers a small, steady flow of sample water and can be shut off and serviced without stopping the circulation. In addition, the probes remain protected from pressure surges and air.
How often do I have to calibrate the pH and redox probes?
At the start of the season, after every probe change and at regular intervals thereafter. If the readings are sluggish or implausible, clean and recalibrate immediately.
Can pH-minus and chlorine go into the same injection point?
No. Both agents must be introduced at separate injection points with sufficient spacing, otherwise dangerous chlorine gas can form.
How do I create the pressure differential needed for the sample-water flow?
Through a defined resistance in the main line between the two branches, for example a slightly throttled ball valve, an orifice plate or a spring-loaded check valve.
Is dry-run protection absolutely necessary?
Yes. The dosing enable must be coupled to the running circulation, otherwise concentrated chemicals collect in the stationary line and can damage the material and the pool.
This guide provides general technical orientation and does not replace professional planning. Safety-, electrical- and permit-relevant work must be carried out by qualified specialists.
