Troubleshooting Guides · Free Tool

pH Sensor Fault Finder

Choose the pH measurement problem and get likely causes in the electrode, reference, cable or process, with checks and fixes.

Low slopeOffset errorSlow responseDrift

What do you see?

SymptomCalibration fails: low slope

Most likely causeAgeing or dehydrated glass membrane

First checkSlope below about 90 %

Causes in order of likelihood

Cause	How to check	Fix
Ageing or dehydrated glass membrane	Slope below about 90 %	Rehydrate in buffer or storage solution; replace if no recovery
Coated membrane	Visible coating; slow response	Clean with a suitable cleaning solution
Old or contaminated buffers	Try fresh buffers	Replace buffers

Before You Start

Work under a permit, inform the control room and put the loop in manual or bypass the trip before touching a live instrument.
Check buffer solutions are fresh and within expiry.
Read the slope (%) and offset (mV) from the last calibration.
Check the temperature compensation reading is correct.

pH Sensor Fault Finder: All Symptoms

Calibration fails: low slope

Likely cause	How to check	Fix
Ageing or dehydrated glass membrane	Slope below about 90 %	Rehydrate in buffer or storage solution; replace if no recovery
Coated membrane	Visible coating; slow response	Clean with a suitable cleaning solution
Old or contaminated buffers	Try fresh buffers	Replace buffers

Large offset (zero point error)

Likely cause	How to check	Fix
Poisoned or clogged reference junction	Offset beyond about ± 30 mV	Clean the junction; replace the sensor if needed
Reference electrolyte depleted	Check electrolyte level (refillable types)	Refill or replace
Ground loop through the process	Reading changes when equipment starts	Use solution ground or an isolated transmitter

Slow response

Likely cause	How to check	Fix
Coated or ageing membrane	Compare response time in buffers	Clean or replace
Low conductivity water	Check process conductivity	Use an electrode designed for pure water
Low temperature	Check process temperature	Allow longer stabilisation

Reading drifts

Likely cause	How to check	Fix
Reference junction clogging	Drift continues after calibration	Clean or replace the sensor
Temperature compensation fault	Compare temperature reading with a reference	Repair or replace the temperature sensor
Cable moisture or damage (high impedance signal)	Inspect connectors and cable	Dry and reseal connectors; replace cable

Noisy or jumping reading

Likely cause	How to check	Fix
Damaged or wet sensor cable	Move the cable and watch the reading	Replace the cable; keep connectors dry
Electrical noise from nearby equipment	Check routing and grounding	Route away from power; use a solution ground
Air bubbles on the glass	Check installation angle	Mount at least 15° from horizontal, avoid air pockets

Key insight: slope tells you about the glass, offset tells you about the reference: low slope means clean or replace the glass, large offset means look at the reference junction.