By Hylan Joseph
The object of this document is to present you with certain concepts of electricity that, as an espresso machine tech, you would have to know. You will then be presented with information on the use and function of a multi-meeter that you will also need to know. Finally, you will be given written instructions, a multi-meeter, and equipment to test. With this, you will be given a chance to demonstrate your grasp of the information presented, and your ability to use the multi-meeter as you would need to be able to do in the field as an espresso machine tech.
Concepts of Electricity
The basic concepts of electricity you will be tested on are
Electricity can be a complicated subject. To make it easier to grasp, the metaphor of the similarity of electricity flowing throw wires and water flowing through a pipe is frequently used.
Voltage in a wire can be compared to the pressure of water in a pipe. Pressure is what pushes water through a pipe. Voltage is what “pushes” electricity through a wire. No pressure, no water movement. No voltage, no electrical movement.
Current in electricity can be compared to the volume of water moving through a pipe (generally related to the size of a pipe). The bigger the pipe, the more water for a given pressure that will move through the pipe. The more current in a wire, the more electricity that moves through the wire.
Power is a measure of how much energy is available to do something useful. In our case, we will be considering the electrical power used to heat water to make espresso.
In our metaphor of water through pipes, water power is thecombination of the water pressure and the size of the pipe. With electricity, it is a combination of the voltage (pressure) and the pipe size (current).
To understand power, let's consider the power needed to wash away a pile of dirt that was carelessly dumped on your sidewalk. Intuitively, you might think that a fire hose – having a large hose size – would be a better choice to wash away the dirt than your garden hose with it's smaller size.
In the same way, you might think that using a 240 volt heater would heat our coffee water faster than a 120 volt heater.
In both cases, the answer would be maybe. A fire hose with extremely low pressure, with its large volume of water just dribbling out, would just not move much dirt. However, the garden hose, if given really high pressure (like with one of those gas powered pressure washers), could really move some dirt.
In both water and electricity, it takes both voltage (pressure) and current (pipe size) to get work done. It is how those are combined that tells you how much, or how little, work can be done.
With electricity, power is measured in watts, and the formula for figuring out the amount of power you have is power in watts equals the voltage in volts times the current in Amps.
Power = Volts x Amps
From this we can see if we want 1000 watts of power heating our water, and our supply voltage is 240 volts then we need 4.17 Amps [1000 watts = 240 voltsx 4.17 Amps (rounded off)].
If we want the same 1000 watts and have only 120 volts, then we need 8.3 Amps [1000 = 120 x 8.3 (rounded off)].
As you can see, by halving the voltage, we had to double the Amperage to stay even. Again, it takes both voltage (pressure) and current (pipe size) to get the work done.
What you may not see is what causes the amount of current to change in an electrical wire. With water it is the pressure and the size of the pipe. With electricity, while we do use larger wires to carry more current, it is not the wire size that limits the current. It is the resistance of the wire.
Resistance is the measure of how well – or how poorly – something carries electricity. The higher the resistance, the less current flows through it. Resistance is measured in Ohms. Resistance is mostly due to what a material is made out of. Rubber and plastic have high resistance, so electricity does not flow through them well. That is why they are used to insulate electrical wires. Copper has very little resistance to the flow of electricity, which is why wire is frequently made of copper.
The material that heater elements are made of has a kind of medium value of resistance. It restricts the flow of electricity, but does not stop it. One of the side effects of restricting current flow with resistance – if you don't completely stop the flow – is that the energy that is being resisted by that resistance, and can't go anywhere else, tends to turn to heat. Inside the boiler of an espresso machine we can use that heat to heat water, just like the resistance of a similar heating element makes the red glow you see in electric room heaters.
So, the heater circuit of an espresso boiler consists of a voltage supply (the plug), the heater element that has resistance to current (and gets hot), wires to connect the voltage supply to the heater element, and a thermostat that completely shuts off the power to the heater element when the water is already hot enough.
It is a simple system, but things can go wrong. You will need to be able to use a multi-meeter to test for voltage, current (Amps) and resistance (Ohms) in order to check for proper operation of the circuit and to find faults when they exist.
The multi-meeter has two controllers, two test leads and probes, and one slot for testing current through a wire placed in that slot.
The two controls on our meeters are a “hold” button. Pressed once, it freezes the display when pressed, then unfreezes it when pressed again. The display says “hold” when it is frozen. Make sure it is not frozen.
The other control is the selector switch. With the selector switch you can choose to test Volts, Ohms, or Amps.
To test the voltage going into a circuit, you would select “V” on the selector switch, touch the test probes each to one of the two power wires coming in, and read the voltage on the display. If there is no voltage coming in, nothing will heat.
To test for current through a wire, either going into or out of the heater element, you would set the selector to “A” for amps (current) and slide the slot of the meeter over the wire to be tested. Then read the current in amps on the display.
To test the heater element, you need to know if it has the correct resistance. Remembering that resistance is measured in Ohms, you would set the meeter to Ohms. Unlike voltage and amperage, resistance is a property of the material itself. As such, it is measured without electricity connected to the circuit. So you unplug, or shut off the power before testing. With the power off, and your meeter set to Ohms, you touch the probes of the meeter, one each, to each side of the connectors on the heater elements. Different heater elements have different values of resistance, but in most cases a reading of between 12and 30 Ohms will be good. Below or above that is bad. A reading of “OL” indicates that there is no connection at all (like a light switch that is turned off).