Cut A Hole About So Big..

Can you imagine how frustrating it would be to try and take measurements for a transition if you could not read a ruler? How many times have you seen students struggle to take measurements or accurately describe the size of a part or piece of equipment? For a mechanic, the inability to accurately describe what you need is a major barrier to performing your job. Make sure your students know how to intelligently discuss basic dimensions and measurements they will encounter in the field. You cannot assume that students are familiar with common dimensional concepts like area and volume. Many literally cannot read a ruler, and most do not understand the difference between the inch-pound measurement system and the SI measurement system. Although most schools use some type of entrance exam, they usually are not covering mundane practical application. I have had students who could solve algebra problems but could not read a ruler.

Find out what they know by asking them to measure something for you. Do not make it seem like a test, which will make them nervous. Just ask “John, could you do me a favor and please measure the diameter of that pipe for me?” One of the best ways for them to improve is simply to practice. First, they may need some coaching so they are not floundering. Many people have never noticed that the divisions on most tapes and rulers are arranged by height according to the units with the line for half inches being higher than quarters, which is higher than eighths, which is higher than sixteenths. Also, point out that each unit is half of the next larger unit. Although it seems obvious, many people have never made the connection that rulers just continually break things into two. I have found that students who struggle the most have never really learned to deal with fractions. Write down a scrambled list of fractional sizes less than 1 inch and ask them to order them from smallest to largest. If they can’t do it, they don’t understand fractions. A way for them to start using the ruler and learn the fractional relationships is to count the sixteenths and start out by using measurements like 12/16. Then teach them to reduce the fraction to a proper fraction. Eventually they will start to see the relationship of the different fractional values.

Many of these same students have no trouble learning to use a meter stick because it is base 10 and there are no fractions. They can easily read 45 centimeters and “three little marks” to 45.3 centimeters or 453 millimeters. When teaching SI (metric) units I purposefully try to avoid conversions from inch-pound units to SI units. The factors are invariably strange and confuse students. If your first exposure to a centimeter is that there are 2.53 of them in an inch, you wonder what demented mind came up with a system like that. I believe that the best way to teach SI measurement is simply to have the students use SI measurements. They will get a mental concept of the units by using them. Most people know what a liter is because they buy drinks in 2 liter bottles. If you asked them to tell you how many ounces equaled 2 liters they could not tell you.

There is a ruler reading game on the web that is very helpful at http://www.rsinnovative.com/rulergame/

There are many resources available to students on the web for students that need to improve their ability to deal with fractions. Below are a few links that can help.

http://amby.com/educate/math/

http://amby.com/educate/math/1-2_prob.html

http://www.khake.com/page47.html

Teaching Alternating Current Concepts

My students sometimes must think that I am a sadist. Many of them make it clear that working math formulas is not a favorite activity – so I give them ohm’s law, temperature conversions, and gas law formulas to get them started out right! I always get a rise out of my students when after a steady diet of Ohm’s Law formulas I reveal that Ohm’s Law does not really work for most HVAC/R circuits. They calm down a bit when I tell them to substitute impedance for ohms, it sounds like an easy fix. The trick is explaining properties like impedance, inductive reactance, and capacitive reactance in a way that makes sense without resorting to LCR formulas (inductive capacitive resistive). People who find series-parallel Ohm’s Law calculations challenging will not find LCR formulas any help in understanding alternating current principles. Instead, I rely on explaining the general concepts, analogies and demonstrations. Alternating current characteristics are discussed in Unit 29 Electrical Power and Circuits in Fundamentals of HVAC/R.

Most students can understand the concept of alternating current generating a voltage in the conductor. I start with explaining that inductive reactance is an opposition to changes in current flow produced by magnetic devices. I explain that instead of the wires moving inside a magnetic field the magnetic field is moving around the wires. The fact that this induced voltage is counter the original can be explained using an analogy to jumping out of a boat: you go one way and the boat goes the other. Finally, discuss the current characteristics of all AC inductive devices – the current surge when they are first energized. Since there is no magnetic field when they are first energized, there is also no inductive reactance and a very high current rushes through the device. A magnetic field is established, which creates inductive reactance, and the current drops. This is a very important concept for the students to grasp.

The effects of inductive reactance are easy to demonstrate. Have the students measure the resistance of an inductive alternating current motor and calculate the expected amp draw using Ohm’s Law. Then run the motor and measure the amp draw, noting that it is much lower than Ohm’s Law predicted. If you have a clamp on meter with peak hold you can also show how much higher the inrush current is than the operating current. Run a multispeed motor and measure the voltage generated across the unused speed taps to demonstrate the counter electromotive force being generated in that part of the winding. Wire a 115 volt solenoid coil and a 60 watt 115 volt light in series. Leave the core out of the solenoid coil. You may have to experiment to find a paring of light and coil that allows the light to burn dimly and keeps the solenoid from overheating with the core out. After the circuit is energized insert the core into the solenoid coil. The light will get dimmer or even appear to go out. You can discuss how the iron core increases the magnetic field and therefore increases the inductive reactance. Finally, wire a small 115 volt shaded pole motor in series with a 115 volt light. Operate the circuit and measure the voltage across both devices. The students will see that the sum of the effective voltage measured across the motor and the light is higher than 115 volts. Then stop the motor and observe the light – it gets brighter. This is because the motor’s inductive reactance is lower when it is not spinning. I find demonstrations like these are more helpful to the students than anything else I have tried in trying to explain inductive reactance. With a little imagination and time you can come up with your own demonstrations using materials already available in your shop. Next week I will discuss capacitive reactance demonstrations.

Are Your Students Thirsty?

We are all familiar with the saying “You can lead a horse to water but you can’t make him drink!” Well maybe not, but you can make him thirsty and he will search out the water. The same can be said for students and knowledge. Some just have more thirst for learning than others. It can be frustrating to spend time constructing lessons and then have people not pay attention. Those same folks will be lost souls in the lab because they did not prepare. They don’t exhibit a thirst for knowledge. It would be easy to just write these students off and deal with the people who want to learn. The trouble is, they are not always the same students. Because students are people, they come to class with distractions that can get in the way of learning. You cannot cure their sick child, balance their bank statement, or get them a date with the cute girl across the hall, but you can create a thirst for air conditioning knowledge that can replace these worries for an hour or so.

There are many methods employed to get students focused on the material. They are as individual as instructors. One overlooked technique for teaching is testing. There is nothing like an impending test to create a thirst for knowledge. They want to know what is on the test. So right before a test, give a quick review of things you really want them to know. Keep the number of items covered to a minimum so they can remember them. Of course the time is limited as well because you still have a test to give. What you cover should be helpful to the students when they take the test, otherwise they will not listen the next time. Try not to just give away questions, but cover the material in a more general fashion. For example you can briefly discuss the relationship of voltage, resistance, and current by offering a few shout outs like “when resistance increases the current” … class responds “decreases.” Then the test can have some word problems that call for understanding the relationship of voltage, resistance, and current.

For most students, the test just makes them thirstier. They want to know how they did right after they complete the test. If time permits, grade the test immediately upon completion. My favorite technique for this is to have the students grade their own tests. I have them put away their pens and pencils and I hand out markers. Then I go through the test, answer each question, and discuss any issues. Students will ask questions at this point that they leave unasked in class. The discussion becomes a powerful lesson. Of course you still have to grade them. Students occasionally neglect to mark a few wrong answers when grading their own papers. The point is really not to have them do the grading, but to see how they did and learn from the experience. A word of caution: do not have them grade each other’s test. I know teachers may have used that technique on you in the past, but it is a violation of FERPA. FERPA is a relatively new federal law that guarantees students the right to privacy regarding their educational records. Since the point is for the students to learn how they did, it would not make sense for them to be grading someone else’s test.

What about cheating? Certainly some students will view this as an opportunity to “answer” the questions while they are being discussed. That is why they must put away their pens and pencils – to avoid temptation. But in the end, who are they cheating – themselves! Cheating in a class you are not compelled to take where you are preparing for your future just really does not make sense. When a student turns in a test with a low grade they gave themselves I have learned two valuable things: I did a poor job teaching that particular student and that student is honest.

I have a one question test for all of you: Are you making your students thirsty?

Free Online HVAC/R Training!

Many of our students are accustomed to 24/7 access to electronic information. I know that most instructors simply don’t have the time to create online content. If you are like most of us, you are already working at home to prepare for and manage your classes, especially now that enrollments have swelled. But I love for my students to do research on their own time after class; it shows real interest in HVAC/R and commitment to our trade. One way to provide access to online training material is to point students towards information offered by component and equipment manufacturers. There are some well done power point presentations, videos, and articles that manufacturers post for free on their web sites. Forward thinking manufacturers have figured out that showing technicians how their products work, how they should be installed, and how they should be services saves them time and money. Time spent educating technicians in advance saves time trying to do it over the phone in an emergency. Money spent on training saves money on warranty returns. Companies who take the time and resources to make training materials freely available to the HVAC/R community should be supported, applauded, and encouraged because educational resources freely offered are a gift to the HVAC/R community.

Our students can benefit from these offerings. It takes some time to dig around the internet for them, but much less time than it would take to produce comparable materials yourself. I will share a few of my favorites. I am sure there are resources that I don’t know about. If you know of one, please share it with the rest of us by posting it on a comment to this article.

I believe my favorite site contains three very well done online training courses for electric motors by US Motors, a division of Emerson. The modules are Motor Cross Referencing, Motor Mechanical Terminology, and Motor Efficiency. After the students complete the training, they can receive a certificate of completion in the mail. These are not 10 minute time wasters with glossy promo photos and little information. It will take the students a while to work through each course, but they will learn a great deal about motors in the process. Click on US Motors to go there.

Another favorite is the Yellow Jacket site. They have videos on Refrigerant Recovery, Evacuation, and Tubing Tools & Techniques. They are very well done and cover the basics of these important subjects. They are available in a small format that will stream on the web, or in a larger file size that can be downloaded for larger screens and better resolution. They do use their products in the videos and the videos do promote their products a little. I am fine with that. Click on Yellow Jacket to go there.The address is

Fluke has a boatload of articles, demos, videos, and simulations. The materials are very well done. I especially like the electrical safety materials. Fluke will send you training videos on DVDs for free. All these materials are obviously designed to show how to use Fluke products, but most of the information is applicable to testing in general. And again, I don’t mind the materials talking about Fluke products since they are providing them for free. Some of the more interesting features they have on the web site are virtual meter demonstrations. You can “use” a meter online by going to the page with information on a meter and choosing “Virtual Demo.” The Fluke site has so much that it is easy to get lost navigating the site. But the search is worthwhile, they have so many HVAC/R products and do a great job explaining their use that just looking at what is available and what it does is helpful for the students. Their HVAC/R specific materials can be found at Fluke

Several compressor manufacturers have great technical specs available online including

Tecumseh, Copeland, Bristol and Carlyle

The Carlyle site has a program that you can download called Compressor Selection software It is a complete refrigeration system simulation! You select the compressor, the refrigerant, choose the condensing and evaporating temperatures, and the program will show the capacity, energy use, temperatures, and pressures throughout the system. Some time spent playing with this can really explain how changes in system variables affect the whole system.

If you are using Fundamentals of HVAC/R, consider using the on line MyHVACLab online course management and content that is designed to work with the book. There is no cost to the school to use MyHVACLab; students must purchase an access code. If purchased packaged with the book, the cost can be as little as $7 more than the book alone. MyHVACLab provides a complete HVAC/R course that ties in with the book. Of course you can edit, delete, or add to the default course that is provided, but you start out with a complete course.