How Accurate Are Your HVAC System’s Pressure Instruments?

HVAC systems are one of the major costs in operating a building and are a critical part of your building’s operational efficiencies. The Building Automation System ensures that a building’s environment is adequate for occupants by controlling humidification, temperature, air quality, air cleanliness and so on; while also attempting to meet the owner’s needs to minimize energy costs. Inefficiencies in an HVAC system increase building management costs and the sensors that you rely on to optimize performance may not be as accurate as you would think.

One way to ensure an efficient HVAC system is to verify the accuracy of the pressure instruments used in the system (e.g., pressure sensors and transducers). Pressure sensors in Building Automation Systems are used to control the flow of air through ducts, hot water through heating coils and cold water through chillers, and dehumidification coils. All of these processes use energy and cost money. The more efficiently they run, the less they cost to operate. Owners want to optimize their HVAC performance and not heat or cool air that is not required by the occupant.

In this blog, I will discuss why accuracy is important in low-pressure instruments for HVAC systems and how to interpret accuracy statements from HVAC sensor manufacturers.

Why Is Accuracy Important?

Accuracy refers to how far the measured value of a specific pressure is from the accepted allowable error for that measurement. The accuracy of a pressure instrument is determined by the maximum positive and negative difference between the measured value and the true value. This allowable error is expressed as a percentage of the ideal output span (% of span).

Pressure instruments with greater accuracy provide a tighter indication of the actual pressure conditions than pressure instruments with lower accuracy, which allows HVAC Controllers to better manage the heating and cooling needs of the building. However, the accuracy statement alone does not fully define an instrument’s ability to measure pressure, which is why pressure instrument manufacturers incorporate accuracy as part of a larger performance statement.

Not All Accuracy Statements Are Equal

Unfortunately, the HVAC industry has not established a set of standards for calculating the performance of pressure instruments. As a result, accuracy statements vary from one pressure instrument manufacturer to the next, requiring a closer look at what’s really in that accuracy statement.

Many manufacturers use a root of the sum squared (RSS) or best fit straight line (BFSL) methodology of expressing accuracy.

If you further review RSS or BFSL accuracy statements, they may not be all that accurate. What may not be included in that accuracy statement will make that error band much greater. These specifications are calculated using the least conservative methods (algebraically combining two or three specifications), meaning they do not offer a guarantee that all the measured values shown by the pressure instrument fall inside the stated error band.

Additionally, this method may not consider both the zero set point (the lower end of the measurement) or span set point (the expected output at the full range of the instrument). This generally means the zero and span set points of the instrument introduce additional errors that must be considered by the installer, which could require on-site resetting/calibration by the installation team to ensure it operates within the desired accuracy.

As a result, the HVAC technician installing the instrument may need to utilize a secondary calibration standard to adjust both the zero and span set point, which immediately increases startup costs, eliminates out-of-box interchangeability, and compounds the cumulative overall system performance. These “tweaks” to verify product add costs to the building owner, who has already paid the sensor suppliers to provide calibrated product but are falling short on performance!

What’s in an Accurate Accuracy Statement?

In determining the accuracy of pressure instruments, you need to consider all factors relevant to the specific application that could lead to measurement errors. Some of the most common sources of error include:

• Non-linearity Error
  • Deviation between the instrument’s linear trend line and its ideal output line.
• Hysteresis Error
  • Deviation between the instrument’s output in response to increasing vs. decreasing pressure. It is commonly expressed as a percentage of full span or output span.
• Non-repeatability Error
  • This is calculated by testing an instrument three times in a row over its range under the same conditions and determining the maximum difference between the measurement results. It is generally expressed as a percentage of full span.

Zero Offset Error and Span Setting Error

The errors in which the instrument does not read zero when the pressure to be measured is zero. Span setting errors are the differences between the optimal full-scale output and the actual full-scale value from the transducer.

Zero and Span Temperature Coefficient Error

The error band of the maximum deviation in zero and offset output as the temperature is varied from its factory calibrated temperature setting to any other temperature within the specified range.

The Best Method of Determining Accuracy

There are many methods of calculating accuracy statements for pressure instruments, but not all of them result in accurate accuracy statements. Ideally, an accuracy statement should include a summation of all the error sources – which is uncommon in the HVAC industry. At a minimum, accuracy statements should include non-linearity, hysteresis, non-repeatability, zero offset and span setting, which contributes significantly to how well the instrument performs.

While not all summation methods consider all of these errors, the terminal point method does by including both the zero and span settings as part of the accuracy statement. Ashcroft chooses to use the terminal point method of calibration. This methodology truly determines the actual error for a pressure instrument, allowing manufacturers to better indicate the accuracy of their products and contractors to better manage the heating, cooling and subsequent low air flow in these building.

Why? Energy Efficiency = Savings = Happy Owners!

TruAccuracy™ at Ashcroft

At Ashcroft, many of our pressure instruments are manufactured to our TruAccuracy™ standard, which is based on the terminal point method. Some of the products with this specification include our CXLdp differential pressure sensor, DXLdp differential pressure transmitter, and newly introduced GXLdp differential indicating pressure transmitter. These products meet their rated accuracies out of the box without the need for field calibration adjustments.

If a pressure sensor is assumed to perform at +/- 0.5% of span accuracy, but really adds up to +/- 2% or +/-3% of span accuracy, this correlates to inefficiency and higher operating costs for the owner. Ashcroft can help you achieve greater HVAC efficiency and save money with the TruAccuracy™ Performance Speciation on all low-pressure products, as well as many gauge and wet/wet offerings.

To learn more about the importance of accuracy in pressure instruments or how TruAccuracy™ can benefit your applications and operations, download our eBook, “How Accurate Is Your Accuracy Statement?”

We don’t like to pressure you, but we have more information.

I hope that this article helped you better understand the need for accurate pressure instruments in your building’s HVAC system, and what you should expect to find in a quality accuracy statement. If you need more help with your HVAC system needs, feel free to ask our experts a question. They’ll help you ensure your pressure instruments are running smoothly and efficiently to keep your HVAC system at peak performance.

I also recommend you visit the American Society of Heating, Refrigerating and Air-Conditioning Engineers website to learn more about HVAC systems.

To explore various options available for pressure transducers or to learn how accurate an accuracy statement is, you can visit our website where we offer product information pages, white papers, specification sheets, material selection guides, webinars, and many other tools.