At Mid-West Instrument, we manufacture differential pressure gauges with tight internal tolerances to ensure precision performance and long-term reliability. A key component of our design is the piston assembly, which must allow for accurate movement while controlling media bypass. One of the critical performance characteristics we test and certify is the allowable leakage rate. Because of the precision sizing of our piston and body bore, we guarantee that leakage across the piston will not exceed 32 SCFH (Standard Cubic Feet per Hour) of air at 100 PSID (Pounds per Square Inch Differential) under ambient conditions. This specification ensures dependable operation across a wide range of industrial applications. Below is a detailed breakdown of what this value means, how we measure it, and why it matters for performance, safety, and reliability.

What’s the Purpose of Allowable Leakage Rate?

The allowable leakage rate refers to the maximum amount of gas that can bypass the internal piston in a differential pressure gauge during normal operation without impacting function or accuracy. In our gauges, we specify that no more than 32 SCFH of air can leak across the piston at a pressure differential of 100 PSID, measured under ambient temperature and pressure.

SCFH, or Standard Cubic Feet per Hour, is a volumetric flow measurement corrected to standard conditions, typically 14.7 psi and 70°F. This standardized unit allows for consistent comparison across various products, media types, and testing environments. By using SCFH as our baseline, we ensure that customers have a clear understanding of how our instruments perform under defined and repeatable conditions.

The 100 PSID testing point is chosen to represent a meaningful differential pressure where leakage would most likely show measurable impact. Operating at this pressure level during testing confirms the integrity of the internal piston seal under real-world load conditions. By limiting leakage to 32 SCFH at this point, we help ensure that gauge readings remain accurate and responsive to true process conditions.

Ambient conditions mean that the testing is done at normal room temperature and atmospheric pressure. These variables are controlled so that the leakage rate measurement reflects performance without influence from external factors like extreme temperatures or fluctuating barometric pressure.

Engineering Design Behind Low Leakage Performance

Our ability to maintain a low allowable leakage rate starts with the way we engineer our piston and bore assembly. Each component is manufactured to tight dimensional tolerances, with precision grinding, honing, and inspection processes applied during production. The piston and bore are specifically sized so that the clearance between them minimizes internal leakage without creating excessive friction.

We use specific material combinations to enhance sealing characteristics and wear resistance. Many of our gauges use hardened stainless steel or specialized polymers to ensure dimensional stability and low friction operation over time. We avoid over-reliance on elastomeric seals. Instead, we allow the metal-to-metal or polymer-to-metal fit to provide a long-term, mechanically controlled seal. This approach helps reduce drift, minimize hysteresis, and extend service intervals.

In addition to material selection and machining accuracy, our assembly process ensures that each piston aligns perfectly within its bore. This prevents binding and allows smooth, repeatable movement in response to pressure changes. Because internal leakage can impact response time and pointer accuracy, we carefully tune this interface to stay within the 32 SCFH limit while maintaining responsiveness.

Each of our gauges is tested before shipment to ensure that leakage across the piston remains within allowable limits. We use calibrated air flow test setups with digital mass flow sensors to verify compliance at the specified pressure differential. If any unit exceeds the leakage limit, it is rejected or reworked before leaving our facility.

Impact on Accuracy, Reliability, and Application Suitability

Controlling leakage across the piston is essential to maintaining the accuracy and long-term reliability of our differential pressure gauges. If leakage exceeds acceptable levels, the pressure differential across the piston would equalize faster than expected, leading to inaccurate pointer readings. This would compromise the user’s ability to monitor process changes or identify system performance problems.

By limiting piston leakage to 32 SCFH, we ensure that the measured differential pressure reflects the true difference between the high-pressure side and the low-pressure side. The gauge provides stable, consistent readings without drifting, sticking, or reacting slowly to pressure changes. This is especially critical in applications where the gauge is used for monitoring filters, flow, or level, all of which rely on real-time, precise differential measurements.

Leakage also affects the gauge’s ability to maintain its calibration over time. Excessive leakage can cause wear on internal components, lead to temperature-related expansion issues, or cause pressure drops in low-volume systems. By holding leakage within a tightly defined range, we extend service life, reduce recalibration frequency, and ensure safe operation in critical environments.

Our customers use these gauges in demanding industries such as oil and gas, water treatment, HVAC, aerospace, power generation, and chemical processing. In each case, the low leakage performance of our instruments ensures compliance with quality management systems and safety protocols. Operators can trust that our gauges provide accurate data to support efficient system control, process safety, and regulatory compliance.

Compliance, Testing Protocols, and Industry Standards

We define our allowable leakage rate according to both internal performance goals and customer-driven industry standards. The 32 SCFH at 100 PSID benchmark aligns with common expectations for high-performance mechanical differential pressure gauges in critical applications.

We test each gauge under controlled conditions using air as the test media. Our test setups simulate a pressure differential of 100 PSID and measure flow using calibrated equipment. The environment is kept at ambient temperature to remove temperature variables from the results.

Although 32 SCFH may seem like a small value, over time even minimal leakage can accumulate, especially in systems where the process media is expensive, hazardous, or reactive. By testing and certifying our instruments to remain below this limit, we protect system integrity and ensure long-term measurement consistency.

Many of our customers also operate under third-party standards such as ASME, API, or ISO. Our leakage rate specification helps customers meet these requirements when pressure instrumentation is part of the overall compliance scope. We can provide documentation, material traceability, and test results on request to support customer audits and validation programs.

Mid-West Instrument

Mid-West Instrument designs and manufactures our differential pressure gauges with precision-engineered piston and bore assemblies that tightly control internal leakage. Due to our exacting production standards and material selection, we ensure that leakage across the piston does not exceed 32 SCFH of air at 100 PSID under ambient conditions.

This specification is critical for ensuring accurate pressure differential measurement, reliable long-term performance, and safe operation in demanding environments. We achieve this low leakage through tight machining tolerances, careful material pairing, rigorous assembly procedures, and individual product testing. Every gauge that leaves our facility meets this standard, giving our customers confidence in the data they rely on.

Our commitment to this level of precision is just one example of how we build trust into every instrument we produce. For customers who need dependable, certified performance, we are ready to provide product documentation, technical support, and application guidance.

Since 1958, Mid-West Instrument has been a leading provider of premium differential pressure gauges. Need help finding the right pressure gauge and equipment for your business? Reach out to us today to speak with one of our experienced professionals.