There are a number of things that need to be considered when choosing between whether or not to use a copper or aluminum bendable averaging sensor. These factors include the sensors response time, corrosion resistance, overall cost, environmental and health concerns as well as the overall weight of the finished products. Each of these factors will be briefly discussed in order to show why the bendable copper averaging sensors should be the overwhelming sensor of choice for use in your typical HVAC application’s today. These applications include the monitoring of the mixed and supply or discharge air temperatures in larger ducts or air handling units. If you have ever stopped to think about it, copper can be found in a number of areas within your commercial HVAC system including heating and cooling coils or fin and tube heat exchangers.
One of the major differences between copper and aluminum averaging sensors is the overall thermal response time of the sensors due to the material properties of both copper and aluminum. Heat is transferred from a higher temperature object to that of a lower temperature object. Copper has a thermal conductivity that is approximately 1.75 to 2 times higher than that of aluminum. Therefore, the copper averaging sensors will respond faster to the actual change in temperature within the ducts and can aid the overall control of your HVAC system by reducing the amount of time that it takes your building management system to begin opening or closing your valves and actuators to control your heating and cooling valves or dampers. This increase in response time will help to reduce the “overshoot” and “undershoot” otherwise known as hunting for your valves and actuators when used in conjunction with the PID control within your building management system. The tighter control will then allow you to reduce the energy consumption of your building management system as well as to reduce the number of complaints from the building occupants.
Another major difference between the copper and aluminum averaging sensors has to do with the overall corrosion resistance of the two materials. Aluminum is used in a large number of applications due to its low maintenance and its relatively slow corrosion rate. Under most atmospheric conditions, the oxide layer on the outside of the aluminum will help to reduce the overall corrosion, with one of the major exceptions to this rule being when used in a seaside or salty atmosphere. When aluminum comes in contact with a salty atmosphere it becomes highly active, meaning that the salty atmosphere will destabilize the normally protective oxide layer on the outside of the aluminum and cause a form of corrosion called “pitting”. Keep in mind that not all aluminum alloys will react so strongly to the salt air. We have all seen how copper corrodes over time when you look at the green “patina” or film which is a naturally occurring process that occurs on copper over an extended period of time or when it comes in contact with an acid. This corrosion is made up of both calcium carbonates and calcium sulfates. It is the presence of the copper sulfates, like that of lead that makes copper hold up extremely well in heavy industrial applications. Please note that like aluminum, there are different alloys of copper that will react differently to different materials or environments. You have also seen how copper is also used in the architectural design of buildings from roofing materials to the decorative details on the outside of a building to create an aesthetically pleasing appearance. Due to these differences, my opinion is that the copper averaging will be able to withstand most of the typical atmospheres and corrosive environments seen in the HVAC industry today.
The last difference and probably one of the least known reasons is due to the fact that copper is known to have an inhibiting effect on the growth of fungi’s and bacteria within the HVAC duct according to the article entitled “Germ Warfare” by Harold T. Michels. A number of fungi’s, bacteria, and other microbes can be found within the dark, damp spaces within your typical HVAC ducts. Once these fungi’s, bacteria or other pathogens are established within the duct, it is very difficult to totally remove them since many of them will become much more resistant over time. Today, a number of different technologies are being used to improve the overall air quality of your building through the use of particulate filters, air cleaners, and ultraviolet light. Even through the use of all of the different technologies, there are still ways in which the microbes and bacteria can be spread throughout the entire building through the use of your HVAC system. Yes, it is true that you can clean and sanitize your HVAC ducts but according to the EPA, there are also a number of other risks that may be associated with a number of these chemicals as well. While many of the bacteria found inside of your HVAC systems ducts do not cause serious harm, there are still a number of microbes and organisms that may cause serious health problems that could lead to sick building syndrome or other building related illnesses. According to the article “Germ Warfare” there are a number of pathogenic risks in HVAC systems that include Legionellae, Mycobacterium, and molds such as Aspergillus Niger. It has been shown that a number of these bacteria and fungi’s can be reduced and even eliminated through the antimicrobial effect of the copper and copper salts. According to the article a test was completed by Dr. C.W. Keevil of the School of Biological Sciences at the U.K.’s University of South Hampton. In this test they placed Aspergillus Niger mold spores on both aluminum and copper coupons and kept them at a temperature of 68oF for a period of six hours. At the end of the six hours it was determined that there was no difference in the overall amount of A. Niger spores on the aluminum while the spores on the copper had been completely eradicated. Existing data on copper surfaces at room and chilled temperatures suggest that the use of copper is an effective means in which to suppress the microorganisms. For this reason the use of copper components should be a conscientious decision, made to improve the overall air quality of your building by reduce the overall amount of growth and buildup of bacteria within today’s HVAC systems.
The two main advantages of using an aluminum averaging sensor over that of a copper averaging sensor is due to the overall cost of each of the sensors as well as the freight cost of the sensors due to the difference in weight of each of the sensors. As of today, the cost of a pound of aluminum is approximately one-third the cost of a pound of copper whereas a 24 foot Aluminum averaging sensor will weigh approximately one third the total weight of a typical 24 foot copper averaging sensor. Even though the sensors are cheaper from an initial cost perspective, the benefits of using a copper averaging sensor over that of an aluminum averaging sensor from the indoor air quality or health perspective far outweigh that of the initial cost of the sensors and savings in the freight costs. With the prevention or reduction of fungi’s, microbes or other pathogens you should be able to see an increase in productivity within your business as well as to hopefully reduce your future healthcare costs due to the reduction of many of the building related illnesses.
“Germ Warfare” article written by Harold T. Michels, Ph. D., P.E. Vice President of Technical and Information Services for the Copper Development Association. www.copper.org
Thanks to Barry “Flash” Gordon and our friends at ACI for this great post.