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Sensors, Measurements, and Physics
Sensors, Measurements, and Physics
As HVAC/R Technicians, we use tools and instruments to make measurements every day. In fact, 90% of our job could not be done efficiently without some kind of measurement.
“How do we measure?”
“With what instruments?”
“How accurate are these measurements?”
These are all questions a thoughtful technician should ask before spending money on a tool or implementing solutions to solve a problem.
Tools are our primary resource for measurement. Measuring tapes, scales, pressure transducers, thermocouples; the list goes on. But how exact are these measurements? How precise must our measurements be for us to use them to make decisions regarding the mechanical operation, occupant comfort, and occupant health? To answer these questions, we need a crash course in a little bit of physics.
Don’t worry; we aren’t going into the rabbit hole too deep. I simply want to introduce you to a concept called the Heisenberg Uncertainty Principle in quantum mechanics. The Uncertainty Principle states that the more precisely you determine a particle’s position, the less precisely you can determine that particle’s momentum. Basically, the Uncertainty Principle limits our ability to measure things exactly. In modern physics, there simply is no such thing. There is only the agreed-upon accuracy and precision everyone is satisfied with to make practical decisions. For example, if you asked me how tall I was, my answer might be 5’11”. But is that 5ft. 11in. exactly? The line on the measuring tape with which I measured my height has a thickness, doesn’t it? Where within the thickness of that line do I fall? This principle is, of course, much more noticeable in the quantum (atomic) scale than on the macro scale. However, when measuring airflow and trying to solve occupant health concerns by measuring indoor air quality, accuracy, and precision matters. Our margin of uncertainty matters.
A technician doesn’t have to lose sleep over the fact nothing can be measured exactly. In our trade, and most of life, it’s not necessary. We can use this knowledge, however, to be more critical about the types of tools to choose to use. A duct traverse with a rotating vane anemometer can provide a quick and dirty idea for system airflow. Still, it’s not accurate nor precise enough to make complex troubleshooting decisions based on its results. The margin of uncertainty is too high. A flow hood or The Energy Conservatory’s TrueFlow Grid would be required for more accurate measurements upon which to base airflow balancing solutions. So how do we quality check for accuracy and precision? First, here’s a diagram showing the difference between the two and various combinations of accuracy and precision:
It’s important to understand the differences between these different combinations of accuracy and precision. Take any three brands of micron gauges and do a vacuum pump test. It helps if you have a digital gauge on the pump itself. Pull only on the pump first, and record the level of vacuum achieved (a good vacuum with fresh oil should be able to pull below 50 microns). Next, add the gauges one at a time and pull a vacuum in three more separate tests. You will very likely record three different micron levels for each gauge. In this experiment, the pump was our reference. In reality, the pump gauge itself would also need to be scrutinized for accuracy and precision, but as a demonstration, this test works well. Looking at the recorded micron levels from the gauges relative to the pump, where on the graph would your test results lie? Most are either accurate, but not precise, or precise, but not accurate.
When thinking about buying a measurement tool, first consider what you are trying to accomplish with that measurement. If you aren’t planning on making accurate and precise capacity calculations, then you don’t really need the more accurate and precise hygrometers and airflow measurement tools. You may not need a digital manifold to charge a system to proper superheat and subcooling, because the margin of uncertainty is forgiving in that context. But a more accurate and precise sensor for a digital manifold or probe sure makes a pin-hole leak during a standing pressure test a lot easier notice.
Sensors are another factor to consider when choosing a measurement tool. Sensors are responsible for most of what makes a tool more expensive (not always, but most of the time). The product data for any instrument can be found either in the manuals or through a quick phone call to the manufacturer. In a recent podcast with Aeroqual’s Bernadette Shahin, we discuss some points to look for when researching the quality of a particular sensor.
- A sensor that can measure the target parameter only (humidity, pressure, CO, microns, VOCs, etc.)
- A quality sensor should be able to have low cross-sensitivity to other parameters and should be stable in its ability to measure the target parameter over time.
- A sensor has a published response time to change, but a manufacturer can confirm how long a sensor takes to reach an acceptable range of accuracy and precision expected from the sensor
Home automation is becoming very popular in the HVAC trade, and many are using Indoor Air Quality monitors to determine when and for long a mechanical system will operate, in order to maintain a comfortable and healthy home. To do this effectively, the accuracy and precision of the sensors in that monitor should be scrutinized. AQ-Spec is an excellent resource for this specific type of evaluation. Other manufacturers may have done their own third-party testing, and results can be released upon request.
Keep in mind we aren't referring to this concept as the “margin of error,” as many often call it. There is hardly ever any ill-intent when it comes to making a measurement, and manufacturers are not trying to create poor quality products. There are simply varying levels of uncertainty when we are using the tools at our disposal, and picking the right tool for the job is essential for quality solution implementation.
Another principle in physics, which all technicians should be aware of, is the Observer Effect. This theory states that the very act of observing a property's state of being will inevitably change that property's state of being altogether. In other words, by the time you have made a measurement, the state of whatever property you are measuring has changed, and no longer holds the same state of being it held before you started to measure it. A good example of this would be connecting hoses to a system. The mere act of attaching your hose has let out a little bit of pressure from the system; therefore, the pressure you will read is different from the pressure the system held the moment before you connected. This example might incentivize some to make the switch to probes, and ditch the hoses! Another example of the Observer Effect is checking electrical current on an indoor blower motor with the cabinet door removed.
One last point to make…accuracy and precision ≠ resolution. The resolution simply specifies to what decimal point the measurement is going to display. The resolution does not provide any direct information about accuracy, or precision. So next time you are comparing tools to see which is the best quality for the price, you may find a more accurate tool at a lower relative resolution. It is important to note, however, that precision and resolution are related. The higher the precision, the higher the resolution necessary to interpret the readings. It's all about what you need the measurement to do for the application in which you use it.
Here are some links, in case you want to gain more insight into sensors, the Observer Effect, and the Heisenberg Uncertainty Principle: