Scope
Where this method applies
Use for irrigation water, prepared nutrient solution, or other compatible liquid samples. Use a method appropriate to the substrate when evaluating media or root-zone pH.
Root-zone measurement · THC SOP
pH Meter Calibration & Measurement
Produce traceable pH readings with a clean, calibrated meter and enough context to interpret the number correctly.
Frequency
When to run it
Calibrate at the interval specified by the meter manufacturer and whenever verification checks, storage history, or measurement behavior suggest drift.
Preparation
Tools and preconditions
Required tools
- pH meter with appropriate probe
- fresh calibration buffers appropriate to the meter
- clean rinse water
- clean sample cup
- record sheet or digital log
Before starting
- Confirm the probe has been stored and maintained according to the meter instructions.
- Use uncontaminated calibration solutions and clean containers.
- Allow sample and buffers to reach a stable working temperature when temperature differences could affect the reading.
Procedure
Run the method in order
Step 1
Inspect and rinse
Inspect the probe for visible residue or damage, then rinse it without wiping or abrading the sensing surface.
Record: Record abnormal probe condition or maintenance performed.
Step 2
Calibrate
Run the meter's calibration routine with the required fresh buffer points, following the manufacturer's sequence and acceptance prompts.
Record: Record calibration date, buffer values, and whether calibration passed.
Step 3
Prepare the sample
Mix or collect a representative sample in a clean vessel without cross-contaminating it with buffer solution.
Record: Record sample identity, source, and relevant treatment or batch.
Step 4
Measure
Immerse the probe to the proper depth, provide gentle movement if the meter instructions call for it, and wait for a stable reading.
Record: Record pH, sample temperature when available, and time.
Step 5
Rinse and store
Rinse the probe after measurement and return it to the storage condition specified by the manufacturer.
Record: Record any cleaning or storage exception.
Quality control
Verification checks
Calibration routine passes without an error or out-of-range warning.
A check buffer or known reference is within the meter's stated tolerance when verification is required.
Repeated readings of the same well-mixed sample are reasonably stable.
Evidence record
Fields to preserve
- meter identification
- calibration date/time
- buffer values
- sample identity
- pH
- sample temperature if available
- operator or observer
- notes/exceptions
Interpretation boundary
Limitations
pH does not directly report nutrient concentration.
Substrate pH depends on the sampling/extraction method; do not compare unlike methods as though they are identical.
Connected learning