--- title: "On-Site Tests & Parameters" description: "Both Fernox on-site tests check inhibitor levels and produce an instant report through the Fernox App. The difference is scope: the Express Inhibitor Test checks inhibitor only, while the Instant System Test adds turbidity, corrosion and pH for a full picture of system water." url: https://help.fernox.app/en/articles/129-on-site-tests-and-parameters collection: "On-Site Water Tests" updated: 2026-11-05 language: en alternates: [es, de, nl, fr-be, nl-be, fr, sv, tr, sk, ro, pl, cs] --- # On-Site Tests & Parameters Both Fernox on-site tests check inhibitor levels and produce an instant report through the Fernox App. The difference is scope: the Express Inhibitor Test checks inhibitor only, while the Instant System Test adds turbidity, corrosion and pH for a full picture of system water. ## The Express Inhibitor Test The [Express Inhibitor Test](https://fernox.com/residential-solutions/express-inhibitor-test/) answers one question: is there enough inhibitor in the system? Dip a strip in system water, compare its colour with the chart on the pot, and read the concentration. A reading of 100 ppm or above is a pass. The strips detect molybdate, so they work with all Fernox Protectors and any other molybdate-based inhibitor. One strip tests one system. ## The Instant System Test The [Instant System Test](https://fernox.com/product/instant-system-test/) runs the same inhibitor test plus three more: - **Turbidity:** a tube shows how much solid material is suspended in the water, graded A, B or C. High levels point to ongoing corrosion. - **Corrosion:** a strip detects dissolved metals, a sign of active corrosion inside the system. - **pH:** the same strip shows whether the water is too acidic or alkaline, both of which speed up corrosion. Together, these show not just whether the system is protected now, but whether damage is already happening.

On-site Chemical Fields	Instant System Test	Express Inhibitor Test
Turbidity	✓	✕
Corrosion	✓	✕
pH	✓	✕
Molybdate (mg/L)	✓	✓

## The reports compared Both tests produce a lab-style PDF report the moment you submit in the Fernox App. Each report is emailed to you, can go to your customer and is stored in your account. Both record the same job details: submission date and kit reference, customer and project references, pass or fail status, reason for sampling, address, and property details such as appliance, system type and inhibitor used. ## Which test to use when **Choose the Express Inhibitor Test when:** - You're doing a routine service on a system with no known problems. - You've just dosed or topped up inhibitor and want to confirm the level. - You need quick proof of inhibitor level for a warranty or Benchmark record. **Choose the Instant System Test when:** - You're commissioning a new system or taking on one you haven't seen before. - The system shows signs of trouble, such as cold spots, noise, sludge or discoloured water. - You want a fuller BS 7593:2019 check covering water quality, not just inhibitor. - The system is older or has mixed metals, where pH and corrosion matter most. If either test fails on inhibitor, you can dose the system, retest and issue a passing report on the same visit. If results point to a deeper problem, Fernox's postal laboratory tests, such as the Water Quality Test or System Health Check, give a full diagnosis. ## The bottom line The Express Inhibitor Test is the quick check for confirming protection. The Instant System Test is the full on-site health check. Many installers carry both: the express test for routine visits and the full test for new, older or problem systems. ### Read more [ IST Contents & Preparation The Instant System Test is a convenient, easy-to-use method of testing the system water quality at the job site. After testing a PDF certificate will be generated automatically and sent to customers and chosen contacts. ](https://help.fernox.app/en/articles/127-ist-contents-and-preparation.md)[ EIT Contents & Preparation What is included in the Fernox Express Inhibitor Test and how to prepare a central heating system water sample. ](https://help.fernox.app/en/articles/128-eit-contents-and-preparation.md)