Ohm's Law Without the Tears

The words “Ohm’s Law” triggers the fight or flight survival response in many air conditioning students. The thought of manipulating algebraic formulas with arcane symbols can cause enough anxiety that it is difficult for the simplicity of the underlying concept to break through. In truth Ohm’s Law is relatively simple.

I start by explaining why the letters E, I, and R are used. They stand for electrical properties, not the units we use to measure those properties. E stands for electromotive force, the force that causes electrons to move. We measure electromotive force in volts, that is why people often say that E stands for volts. Students often logically ask, why not use V? Explaining that E stands for Electromotive force will help them to remember E. I stands for the Intensity of the current flow. We measure the current intensity in amps. Using I to represent current is probably the most baffling of the three letters. I must confess that I personally had been working and teaching Ohm’s Law for several years before learning the I stands for current Intensity. When students asked why I was used to represent amps I simply said that is just the way it is. However, knowing what the I stands for helps people remember it. Finally, R represents resistance. Most people don’t have a problem with this. We measure resistance in ohms.

Of course Ohm’s Law is a mathematical relationship, but you don’t have to use mathematics to discuss the relationship of volts, amps, and ohms. For example, if you think about resistance as the opposition to current flow, it is easy to see that resistance and current will generally move opposite of each other. I like to use the analogy of a balance with voltage E being the pivot and resistance R and current I on opposite sides of the balance. This helps students to visualize that if resistance decreases, current must increase. Similarly, if resistance increases, current must decrease. This simple relationship is far easier for most students to understand than a mathematical equation.

The balance analogy assumes that voltage stays the same. Similar analogies can be used to discuss what happens if resistance stays the same and voltage changes. Draw the same balance but add an anchor or pivot on the end with resistance. Then it can be seen that if voltage moves up, current will have to move up with it. Likewise, if voltage moves down, current will have to move down with it. The same type of relationship can be shown by anchoring current. Then resistance must move up and down with voltage.

After graphically demonstrating these relationships, you can distill it all down to 3 simple rules:

• If voltage stays the same, resistance and current move opposite each other.
• If resistance stays the same, voltage and current move up and down together.
• If current stays the same, voltage and resistance move up and down together.
  

Do a few class “shout outs” just to drill in the concepts and build confidence. After the class understands the relationship of voltage, resistance, and current, then you can introduce the formula and explain that it is simply a mathematical way of showing the same thing. Work a few simple problems that they can easily solve in their head for reinforcement and confidence building. I would wait until the next lecture to dive into calculations for loads in series and parallel. Let them enjoy their technical competence for a day.

Take a look at Unit 28 Basic Electricity in Fundamentals of HVAC/R to see a full treatment of circuits and Ohm’s Law using the balance analogy. A relatively simple treatment of Ohm’s Law series and parallel calculations is also shown, including detailed examples that are worked out.

Teaching Airflow Measurement

Would you hire an electrician who does not own a voltmeter? Would you trust the circuit diagnosis of a technician who does not use a voltmeter? So why would you trust an air conditioning mechanic who does not own or use any air measurement tools? After all, the first word in air conditioning is air. Getting the airflow right is the first thing we should be doing. If the airflow is not correct, nothing else will be. It is a waste of time to check the charge on a system with poor airflow. Many mechanics test airflow by simply holding their hand over a supply register. While it is not wrong to feel the air, this does not replace a real measurement. Feeling the airflow is no more accurate than charging a unit by feeling the suction line.

I believe the reasons many mechanics do not use airflow measurement tools include not wanting to spend the money for the tools, not wanting to spend the time to take the measurements, and not really knowing how to use the tools. Education can address all three of these concerns. Many of the most accurate air measurement tools are expensive, but relatively inexpensive tools that are accurate enough for general service work are available. We should be teaching our students how to use a range of airflow tools, including less expensive ones that technicians might be more apt to own and use. Digital rotating vane anemometers are available for $150 that can be used to take fairly accurate readings. Very accurate digital manometers are available for under $200. A magnehelic and pitot tube can be purchased for around $100 and they can be used to take accurate readings as well. Most airflow measurements really do not take very long if you know how to take them, especially using the newer digital tools that will perform real time averaging. The trick is showing technicians how to take airflow readings using commonly available tools. I believe offering a series of labs that let students get a feel for using the different tools available will go a long way towards encouraging more use of airflow measurement tools in the field.

Labs that will familiarize students with airflow measurement include measuring:

• Total external static pressure across a blower using a magnehelic
• Static pressure drop across a filter using a magnehelic
• Total pressure, static pressure, and velocity pressure in a section of duct using a magnehelic and a pitot tube
• Total external static pressure across a blower using a digital manometer
• Static pressure drop across a filter using a digital manometer
• Total pressure, static pressure, and velocity pressure in a section of duct using a digital manometer and a pitot tube
• Average air velocity leaving a supply register using a rotating vane anemometer
• Average airflow velocity entering a return air grille using a rotating vane anemometer
• Airflow volume out of a supply register using a flowhood
• Airflow volume entering a return grille using a flow hood

I realize that every program’s budget might not be able to afford access to all of these tools, but I urge you to consider at a minimum a magnehelic gauge, a pitot tube, and a rotating vane anemometer. Another approach is to ask a technician that owns and uses airflow measuring tools to demonstrate their use to your class. This gives your class exposure to instruments that you might not be able to afford. Outside experts always liven a class up. No matter how clever you are, your students see you all the time. If the visiting technician is a graduate of your program, so much the better. Nothing builds program credibility like having successful graduates come around.

Because we believe the measuring airflow is important, airflow measurement is addressed in several units of Fundamentals of HVAC/R. Unit 56 Fans and Airflow gives a thorough explanation of air movement and duct pressures. Unit 57 Duct Systems and Duct Design discusses the role of duct pressures in designing a duct system. Unit 60 Testing and Balancing Air Systems discusses how airflow is measured and discusses how system performance is adjusted to meet system design. Since measuring system airflow is an integral part of troubleshooting an HVAC/R system, airflow plays a prominent role in several troubleshooting units including Unit 36 Troubleshooting Air Conditioning, Unit 40 Gas Furnace Installation, Startup, Checkout, and Operation, and Unit 48 Troubleshooting Electric Heat. Do your students a favor and introduce them to the art of airflow measurement.

Tools for Success

HVAC/R mechanics rely on their tools for survival. It is not possible to install, service, or troubleshoot HVAC/R systems without tools. Having the correct tools for the job and knowing how to use them can be the difference between success and failure. That is why Fundamentals of HVAC/R devotes an entire section of the book to tools. The units in the Tools and Equipment section are:

Unit 9 Hand and Power Tools
Unit 10 Screws, Rivets, Staples, and Other Fasteners
Unit 11 Electrical Measuring and Testing Meters
Unit 12 Refrigerant System Servicing and Testing Equipment
Unit 13 Heating system Servicing and Testing Equipment
Unit 14 Calibration of Meters and Instruments

These units provide an overview of the tools used by technicians who perform HVAC/R work. The units are filled with high quality, full color photographs of the tools and equipment. These units not only show the tools, but also provide illustrations and descriptions showing how the tools are used. This is particularly important for more unusual tools. For example, the use of a duct stretcher is shown in Unit 9. Yes, there really is a tool called a duct stretcher!

In later units of Fundamentals of HVAC/R, anytime specific tools and equipment are discussed, their use is described and diagrammed in detail. For example, there are 69 figures in Unit 26 Refrigerant Management and the EPA. Many of these photographs and illustrations show in detail how to connect recovery devices to HVAC/R equipment, including both system dependent and self-contained recovery devices. The connection differences between liquid recovery, vapor recovery, and push-pull recovery are shown in detail. The first quarter I used Fundamentals of HVAC/R, one of my students took his text into the lab to do his first split system refrigerant recovery. He managed to get all the connections between the recovery unit, the system, and the recovery cylinder right the very first time. I complimented him and asked if he had performed refrigerant recovery before. He smiled, pointed to a diagram in his book and said, “No, I just hooked it up like the book!”

Teaching tool use is more critical now than ever before. I have noticed an increasing number of students who are quite intelligent, do well in class, show genuine interest in our field, but struggle to perform fairly basic operations with hand tools. One student confessed to me after he graduated that he had never held a wrench until he took Air Conditioning. What he didn’t know was that it was fairly obvious. The good news is that his innate intelligence and strong work ethic allowed him to overcome this and go on to be successful. He took a job as a helper for one of the best refrigeration mechanics in town, who also happens to be a patient man. You see, the mechanic had cancer, and his ability to perform the physical part of the job was declining. The student became the hands and arms for a gifted mechanic, and in the course of a summer the student became proficient at using tools. He has now been with that company for two years and loves his job and the people he works with. If you have bright eyed, eager students from the “virtual generation” show them how to hold a wrench, give them lots of shop work to practice their tool use, and be patient with them. They could end up working with the best mechanic in town.

R22 Phase Out

If you are at all involved in the HVAC/R trade, you know that new equipment may not be manufactured or imported beginning January 1, 2010. This imminent date has most of us at least a little apprehensive about the future. Right now, all the details and rules have not been firmly established. In most people’s minds, the date a unit is “manufactured” is when it is made in the factory. The EPA has proposed a rule change that would set the date of “manufacture” as the date of final charge. For split systems, this would be when the unit is installed. Adopting this rule change would effectively mean that R-22 split systems could not be installed beginning January 1, 2010. This would make inventory of existing R-22 systems essentially worthless on January 1, 2010.

AHRI has launched a site dedicated to monitoring the issues regarding the HCFC phase out and passing on information to the industry: www.phaseoutfacts.org.

Another good web site to keep an eye on is the EPA site devoted to the HCFC phase out: www.epa.gov/ozone/title6/phaseout/hcfcfaqs.html

Although R-22 will continue to be available for servicing existing systems, the amount of new R-22 available will be substantially less than is available this year, see EPA web site for details
www.epa.gov/ozone/title6/phaseout/hcfc.html

R-22 systems will still be with us for some time since the majority of the current installed base of air conditioning systems use R-22. However, R-22 systems represent the past; R-410A air conditioning systems represent the future. All HVAC/R training labs should have a full complement of R-410A systems. We need to prepare our students for the future, not the past. The training materials you use should support training using the refrigerants of the future, including R-410A. Fundamentals of HVAC/R includes extensive coverage of R-410A. We don’t just talk about R-410A in Unit 23 Refrigerants and Their Properties, but throughout the book whenever specific refrigerant pressures and temperatures are mentioned in examples. In Unit 17 Refrigeration System Components and Operation, the refrigeration cycle diagram uses R-410A as the refrigerant. Unit 27 Refrigerant System Evacuation and Charging uses R-410A for many specific examples of charging charts and operating specifications. Specific details of handling zeotropic refrigerants like R-410A are given in Unit 26 Refrigerant Management and the EPA. In all, 14 units have specific examples of working with and using R-410A refrigerant. R-22 has certainly not been left out. There are still plenty of examples and details using R-22. After all, we will be working on R-22 systems for several years to come. If you have not already moved towards incorporating R-410 your curriculum, now is the time to take the first step. If you are looking for materials that will help your students meet the challenges of the future, please take a look at Fundamentals of HVAC/R.

The Language of HVAC/R

Many students find the large number of technical terms used in HVAC/R confusing. To the uninitiated, HVAC/R has its own language of technical jargon that can become a barrier to learning. Confusion over terminology can lead to not clearly understanding crucial concepts. This problem is compounded by the use of acronyms and abbreviations that are frequently used when discussing common topics. For example, one major valve manufacturer uses the acronym TEV to represent “thermostatic expansion valve,” while another uses the acronym TXV for the same thing. In Fundamentals of HVAC/R, we keep technospeak to a minimum, preferring to use common, easily understood language whenever possible. Making a relatively simple concept seem highly technical by using an overabundance of jargon does not help students.

Of course, students must still learn the HVAC/R language. Having a solid grasp of the terminology is necessary to make use of essential technical literature produced by equipment manufacturers. To encourage students to learn the language, we always use the complete word or phrase before introducing an acronym. It helps to explain concepts plainly, and then introduce the technical terminology that is used to refer to the concept. The students are more likely to remember the terminology if it is logically connected to something they understand. For example, I have found that students just starting to learn the operation of the refrigeration cycle often can recite the order of components, but they have not made the connection between the component names and what they do. It is much easier to remember “TEV” if you know that “TEV” is an acronym for “thermostatic expansion valve” and you know that the refrigerant expands as it goes through the thermostatic expansion valve.

We have three resources in the back of the text deigned specifically to help students learn the language of HVAC/R: an acronym dictionary, an English language glossary, and a Spanish language glossary. When a student wonders exactly what is meant by a particular acronym or abbreviation, they can find it in Appendix C which lists the definition of common acronyms and abbreviations used in HVAC/R. The acronym dictionary is also very useful when students are reading industry literature and need help with a particular abbreviation or acronym. Students can use either the English language or Spanish language glossary to look up the definition of common HVAC/R terminology. Fundamentals of HVAC/R has a very complete glossary to help students learn the HVAC/R language. It has the largest number of defined terms of any major HVAC/R text and the terms are fully explained, including multiple definitions of the same term.

I sincerely believe that students and instructors alike will find that Fundamentals of HVAC/R is an invaluable aid in learning the language of HVAC/R.