When you are installing or inspecting a mechanical pressure gauge, one of the most important things to verify is that the pointer reads zero when there is no applied pressure or vacuum. If it doesn't, the indication could be misleading. That's where pressure gauge features like a zero box and dial pin can help.
At Ashcroft, a leading authority in pressure gauges since 1852, we ensure customers have the options they need to maintain accurate measurements and keep their systems running efficiently.
Read this article to become well-versed about the difference between a zero box and a dial pin (also known as a stop pin). You will also learn about Ashcroft's TrueZero™ feature and how it takes pressure gauge safety and reliability to the next level. Finally, you can deepen your knowledge further with links to other resources to help you find the right pressure gauge and options designed to address known challenges in your specific application.
What is the difference between a zero box and a dial pin on a pressure gauge?
Both the zero box and dial pin are pressure gauge components offering operators an immediate visual indicator to verify proper gauge function or alert to potential issues. However, they each serve distinct purposes and have different implications for gauge accuracy and safety. Here's a closer look at both:
Zero Box
The zero box is a printed box on the dial face surrounding the zero position. It allows the operator to visually confirm whether the pointer rests within the acceptable tolerance range for zero, without needing to apply vacuum or pressure. This makes it possible to verify the gauge’s integrity right out of the box. Instruments like the Ashcroft® 1009 Pressure Gauge and the Ashcroft® 1279 Duragauge™ Pressure Gauge, as well as the Ashcroft® 8008 and 8009 models, all offer a zero-box feature.
Dial pin (also known as a stop pin)
A dial pin, or stop pin, is a fixed mechanical stop located at the zero or 6 o’clock position to prevent the pointer from moving below zero. It also blocks pointers that would indicate negative pressure or overpressure conditions.
The purpose of the dial pin is to protect the internal components of the gauge in the event of rapid pressure drop or negative pressure. However, because it physically blocks the pointer, it can hide whether the pointer is returning to a true zero position or not. If there's a discrepancy due to internal pressure build-up or shipping conditions, the pointer might appear to be at zero, but in reality, it could be off.
Figure 1. Zero Box vs. Dial Pin Example
What are the added benefits of a zero box?
While the dial pin is a safe choice for keeping the pressure pointer from dropping below zero, in practice, it introduces uncertainty. If the pointer is pressed against a stop pin, you can’t tell if it’s truly at zero or being held there by internal pressure, a broken tube, or a disengaged pointer.
The zero box allows the pointer to float freely and provides a clear visual reference for whether the gauge is in spec. Eliminating the dial pin also prevents calibration drift that can occur when the pointer rests against the pin with force, which may shift the pointer's position over time. Instead of forcing the needle into position, it shows you whether the gauge is naturally resting at the correct point, making it a more transparent and trustworthy solution.
How a zero box ensures measurement accuracy in real-world applications:
1. Detects Internal Pressure or Shipping Effects
Pressure gauges often experience pressure differentials during shipping or storage. If a gauge arrives with the pointer outside the zero box, it may need to be vented—a process commonly referred to as "burping the gauge"—to equalize internal and external pressures. Once vented, a healthy gauge should return to within the zero box range.
2. Highlights Permanent Damage or Overpressure
If venting does not return the pointer to zero when the system is shut down, the gauge may have experienced permanent damage, such as a ruptured bourdon tube or mechanical failure. The zero box provides a clear visual indicator of these issues, helping you identify problems before restarting your system.
3. Prevents Misleading Readings
Without a zero box, it can be easy to overlook pointer deviation, especially if the dial pin masks the pointer’s resting position at zero. This can hide the presence of vacuum cycles, creating potential risks for operators, especially in vacuum applications or sensitive systems.
4. Improves Safety and Reliability
The ability to visually confirm gauge integrity at a glance enhances safety in high-risk environments. Operators can quickly detect out-of-tolerance gauges before putting equipment under pressure.
5. Simplifies Routine Inspections
During periodic inspections or audits, a zero box makes it easier for technicians to validate that gauges are still functioning within specification, saving time and reducing guesswork.
Ashcroft's TrueZero™ indication
Ashcroft designed a tolerance-based zero box we call TrueZero™ because it reflects the full-scale accuracy of the gauge. For example, a 1% accuracy gauge will have a smaller zero box than a 2% gauge. This ensures that the zero indication is always aligned with the gauge’s design specifications.
Figure 2. Benefits of TrueZero™ Indication
Where to use a pressure gauge with a zero box
Zero box features, especially TrueZero™, are valuable across a wide range of applications:
- Chemical and Petrochemical Processing
Where harsh environments and high stakes require absolute measurement accuracy. - Oil & Gas
For pressure monitoring in hazardous and high-pressure conditions. - HVAC Systems
For confirming vacuum and low-pressure readings with confidence. - Safety-Critical Environments
Like manufacturing lines or process plants, where one false reading could shut down operations. - High-Vibration or High-Pressure Systems
Where gauge damage is more likely, the zero box helps catch problems early.
Even in less demanding environments, a zero box is a smart choice because it takes the guesswork out of pressure verification and helps avoid misreads during setup or inspections.
Ready to learn more?
Whether you choose to use a zero box or a dial pin feature will largely depend on your specific application. For more information about pressure gauge options, here are a few articles that may interest you.
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How To Identify Your Pressure Gauge and Get the Proper Replacement
- How Do I Select the Right Pressure Gauge Range?
- How Often Should I Check the Calibration of My Pressure Gauge?
Contact us to speak with one of our industry experts who can answer other pressure gauge questions. In the meantime, use our Pressure Gauge Selection tool to find the best gauge for your needs. 