--- title: Proof Test Intervals (PTI) as Proxy for Reliability description: Proof test intervals as a proxy for design and diagnostic-coverage image: https://blog.tri-sen.com/hubfs/red%20alligator%20with%20text.png --- [![26TRIS065-Tri-Sen 50 years logo-R2](https://blog.tri-sen.com/hs-fs/hubfs/Assets/26TRIS065-Tri-Sen%2050%20years%20logo-R2.png?width=1476&height=573&name=26TRIS065-Tri-Sen%2050%20years%20logo-R2.png "26TRIS065-Tri-Sen 50 years logo-R2")](https://tri-sen.com/) # Tri-Sen Blog # Proof Test Intervals (PTI) as Proxy for Reliability By [Harry L. Cheddie, P.Eng.,CRE, CQE, CFSE](https://blog.tri-sen.com/author/harry-cheddie) on Nov 4, 2025 2:56:55 PM ![Harry L. Cheddie, P.Eng.,CRE, CQE, CFSE](https://blog.tri-sen.com/hubfs/Harry%20Cheddie.png) Share this post - [Tweet](https://twitter.com/share) It's been a minute, so to get back in the swing of things, we've asked  guest blogger, [Mr. Harry Cheddie, ](https://blog.tri-sen.com/author/harry-cheddie)P.Eng. CRE, CQE, CFSE; an industry-recognized expert in industrial safety, to comment on the relationship between reliability and proof test intervals. ## Why Longer Proof Test Intervals Signal Greater Reliability: In functional safety, the proof test interval (PTI) is derived to minimize the probability of a dangerous undetected fault interfering with the operation of a safety system, in order to meet a defined Safety Integrity Level (SIL) target. So another way of looking at the PTI is as a proxy for the system’s inherent reliability — a reflection of how confidently we expect it to remain free of dangerous undetected faults over time. > **For a given SIL, a longer PTI implies a more reliable system.** If a system can maintain SIL performance while undergoing proof testing less frequently, it is inherently more reliable. Let’s look at why this is the case and what it means for design and maintenance. ### Understanding the Relationship Between PTI and Reliability:** ![beer without background with labels v1.1](https://blog.tri-sen.com/hs-fs/hubfs/beer%20without%20background%20with%20labels%20v1.1.png?width=510&height=243&name=beer%20without%20background%20with%20labels%20v1.1.png) ** A system’s Probability of Failure on Demand (PFDavg) is a key measure of reliability. It depends on both the dangerous undetected failure rate (λDU) and the proof test interval (T). ![pfd_equation](https://blog.tri-sen.com/hs-fs/hubfs/Assets/pfd_equation.png?width=150&height=44&name=pfd_equation.png) > *Note: in this post, “reliability” is used in the probabilistic safety sense — as expressed by PFDavg — which is closely related to availability but not identical.* In this relationship, smaller is better. Increasing "T" generally increases PFDavg — unless the system’s λDU is low enough to compensate. That’s the key insight: > #### If a system can maintain its SIL at a longer PTI, it must have a lower dangerous undetected failure rate, making it inherently more reliable. ## Why a Longer PTI Implies a More Reliable System: ### 1. Lower Failure Rate Is Required Maintaining SIL at a longer PTI means the system’s inherent failure rate is lower or it's fault detection is better, reflecting stronger design, better components, or more effective diagnostics. ### 2. Less Frequent Testing Signals Higher Confidence Reliable systems can go longer between proof tests without exceeding PFDavg limits. If a system requires more frequent checking, everything else being equal,  that’s a sign of higher risk. ### 3. Design Strength Over Test Dependence Frequent proof tests can mask weak reliability. Robust design — through higher-quality materials, redundancy, or advanced diagnostics — enables longer PTIs without compromising safety. > *Note: proof test interval is only meaningful in light of test coverage — if the test doesn’t detect a significant share of dangerous failures, then PTI by itself is an incomplete indicator regardless of frequency . > > * ## PTI in System Comparisons Two systems with the same purpose may both meet SIL-2, for example, but if one requires proof testing every year and another every twenty, the latter is carrying more of the reliability burden in its design. Differences in PTI often reflect engineering choices such as: - Selection of components with proven reliability and clear suitability to purpose. - Improving diagnostic coverage to detect dangerous faults without additional layers of testing. In that sense, PTI provides a way to compare systems that meet the same SIL target by different means — distinguishing those that are compliant from those that are more inherently reliable. ## Conclusion A system that maintains SIL compliance with a longer PTI is statistically more robust, probably better designed, and safer over time. That said, there are many considerations in selecting a safety system. PTI doesn’t tell the whole story — but it remains an important and practical specification to consider when comparing similarly purposed systems. **Further Reading** - [Harry's books on Amazon](https://www.amazon.com/s?k=harry+cheddie&i=stripbooks&crid=2N0DVZB4BWLA4&sprefix=harry+cheddie%2Cstripbooks%2C93&ref=nb_sb_noss) As always, your likes, comments, and questions are appreciated. Thanks for reading — see you next time!   Topics [ turbine safety ](https://blog.tri-sen.com/topic/turbine-safety) , [ safety systems ](https://blog.tri-sen.com/topic/safety-systems) , [ PFD average ](https://blog.tri-sen.com/topic/pfd-average) , [ proof test interval ](https://blog.tri-sen.com/topic/proof-test-interval) , [ PTI ](https://blog.tri-sen.com/topic/pti) , [ TSODS ](https://blog.tri-sen.com/topic/tsods) , [ overspeed detection ](https://blog.tri-sen.com/topic/overspeed-detection) ## The Tri-Sen Blog... About once a month, we'll discuss a turbomachinery controls topic that we were either asked about recently, or a topic that we think you might be interested in learning more about.  Topics that are included or that will be: - Compressor Controls - Turbine Controls - Turbine Safety - Relevant API directives - General Industry Happenings ### Subscribe Here! ### Recent Posts ### Posts by Tag - [compressor control (10)](https://blog.tri-sen.com/tag/compressor-control) - [surge (9)](https://blog.tri-sen.com/tag/surge) - [recycle (4)](https://blog.tri-sen.com/tag/recycle) - [process protection (3)](https://blog.tri-sen.com/tag/process-protection) - [quench (3)](https://blog.tri-sen.com/tag/quench) - [refrigeration (3)](https://blog.tri-sen.com/tag/refrigeration) - ["Jim Jacoby (2)](https://blog.tri-sen.com/tag/jim-jacoby) - [PFD average (2)](https://blog.tri-sen.com/tag/pfd-average) - [PID (2)](https://blog.tri-sen.com/tag/pid) - [PID control (2)](https://blog.tri-sen.com/tag/pid-control) - [TMC (2)](https://blog.tri-sen.com/tag/tmc) - [process control (2)](https://blog.tri-sen.com/tag/process-control) - [safety systems (2)](https://blog.tri-sen.com/tag/safety-systems) - [turbine safety (2)](https://blog.tri-sen.com/tag/turbine-safety) - [turbomachinery controls (2)](https://blog.tri-sen.com/tag/turbomachinery-controls) - [API612 (1)](https://blog.tri-sen.com/tag/api612) - [API670 (1)](https://blog.tri-sen.com/tag/api670) - [MTTF (1)](https://blog.tri-sen.com/tag/mttf) - [PTI (1)](https://blog.tri-sen.com/tag/pti) - [TSODS (1)](https://blog.tri-sen.com/tag/tsods) - [axial compressor (1)](https://blog.tri-sen.com/tag/axial-compressor) - [compressor efficiency (1)](https://blog.tri-sen.com/tag/compressor-efficiency) - [hydraulic trip (1)](https://blog.tri-sen.com/tag/hydraulic-trip) - [incipient surge (1)](https://blog.tri-sen.com/tag/incipient-surge) - [overspeed detection (1)](https://blog.tri-sen.com/tag/overspeed-detection) - [polytropic (1)](https://blog.tri-sen.com/tag/polytropic) - [proof test interval (1)](https://blog.tri-sen.com/tag/proof-test-interval) - [stonewall (1)](https://blog.tri-sen.com/tag/stonewall) - [surge curve (1)](https://blog.tri-sen.com/tag/surge-curve) - [surge detection (1)](https://blog.tri-sen.com/tag/surge-detection) - [surge margin (1)](https://blog.tri-sen.com/tag/surge-margin) - [temperature control (1)](https://blog.tri-sen.com/tag/temperature-control) - [trip block (1)](https://blog.tri-sen.com/tag/trip-block) [See all](https://blog.tri-sen.com/proof_test_intervals#) ** HELP ###### HeadQuarters - North America  Engineering 109 Magellan Circle Webster, TX 77598 Tel:  +1-832-632-1211 Fax: +1 281-724-1226 ###### Netherlands Verlengde Zuiderloswal 4 Hilversum, NH 1216 BX Tel: [+31 88 011 9292](tel://+31%2088%20011%209292) ###### Singapore 56 Kallang Pudding Road #06-10 HH@Kallang  Singapore 349328 Tel: +65 6381 9188 Fax: +65 6381 9199 ### Search ** ### Stay in Touch - [** ** ](https://www.linkedin.com/company/tri-sen-systems-corporation/) [Privacy Policy](https://tri-sen.com/privacy) | [Terms and Conditions](https://tri-sen.com/terms) Copyright © 2019. Tri-Sen All rights reserved. Contact Us