TCO & Maintenance
The calibration interval nobody budgets for
The purchase order covers the instrument. It does not cover the annual verification, the electrode, the couplant, the diaphragm lifted out of a wet well, or the hour the plant spends with a reading nobody trusts.
Three lines that decide the real cost
Consumables on a schedule. A pH electrode replaced every 15 months, an ion membrane every two quarters, a rotor bearing every three years: these are predictable, and predictable is cheap. The alternative is reacting to a failed sensor at the worst moment.
Access. An instrument mounted where a technician needs a scaffold is not a cheap instrument. A 300 mm extension spool that puts the sensor at hand height costs less than one lift.
Verification method. A meter that can be checked against a portable clamp-on, without draining or cutting, will be checked. One that has to be removed and sent away will not be, and after two years of no checks the number on the screen is a guess.
Add those three to the instrument price before comparing two offers. The difference is usually larger than the price gap, and it lands on the maintenance budget in year two rather than on the purchase order in year one.