Last month we talked about how elements produce heat, how they age, and a quick way to estimate when your elements need to be changed. Frequent visual inspections, keeping good firing notes, and performing an element glow test are fantastic ways to stay on top of your kiln maintenance. There is a more accurate way to determine element life however; the resistance test. This month’s Tech Tip will explore how to perform this test, and go over how to properly use the multimeter.
If you remember last month’s Tech Tip, we talked about an element produces heat due to resistance. The resistance is based mainly on what the element is made from. We measure the amount of resistance a material has in a unit called OHMs, much like how we measure physical distances in units like inches or kilometers. When measuring physical distances we use tools like a tape measure or pedometer, but when measuring electricity (voltage, amperage, or resistance) we use a tool called a multimeter. Before we talk about the tool, we should discuss what we are measuring in more detail, and how this information applies to elements.
In 1827 Georg Ohm, a German Physicist, published a paper describing the mathematical relationship between electrical voltage, amperage, wattage, and resistance. This relationship is now referred to Ohm’s Law, and has the following equation:
In this equation, I stands for amperage, V stands for voltage, and R stands for resistance. In real life, we can measure the electrical voltage at the outlet, and we can measure the resistance of the elements, and then determine what the amperage draw of the circuit should be. This matters because there is a direct relationship between amperage draw, wattage use over time, and BTUs (British Thermal Units) – or heat output. Its a bit of an oversimplification, but for our purposes here, a kiln drawing fewer amps will produce fewer BTUs. It is quite important, at this point, to understand that amperage (I) is an equational output. What that means is that, amperage draw only changes as a result of a change in either the input voltage or the resistance. So what does that mean? Mathematically, we can see that a lower amperage draw (lower heat output) will result from either a DECREASE in input voltage or an INCREASE in element resistance. OOH WAIT….does that sound familiar? If you remember from last month’s Tech Tip, as elements age, their resistance INCREASES.
AH HA!! So what does that mean?
That means that we can determine when an element is ready to be changed by its measured resistance, thereby avoiding inefficient and costly firings or down-time due to tired elements. We can also rule-in or rule-out elements as a causal factor in a failed firing based on their resistance.
So how do we measure element resistance?
The first thing we need to have is a digital multimeter. Analog multimeters are simply not accurate enough for our purposes, and can be rather hard to read. Quite simply put, a multimeter is an electronic device that is designed to measure many different facets of an electrical circuit, or electrical flow all in one device; hence the name multimeter.
They come in many different styles and brands, and some are better than others. Personally, I am prefer the brands Fluke and Klein, but there are many others out there. Just remember that you get what you pay for when you buy a multimeter. My Fluke is a VERY expensive model, but it has lasted me over 10 years, and has a LIFETIME warranty. Plan to spend at least $100 if you want to buy one, and I recommend talking to YOUR electrician about which brand is right for you. Look for a clamp-style (easier to measure amperage), and one that is auto-ranging with a digital output.
Using a multimeter
Every multimeter is different (so you should read the manual), but testing resistance is pretty simple:
Step 1: UNPLUG THE KILN – I can’t stress this enough.
Step 2: Put the indicator probes in the multimeter (typically they are color-coded), and set the meter to read ohms. (its the setting marked with a Ω)
Step 3: Open the kiln panel for the circuit that you are trying to measure. It is usually easiest to measure kiln section circuits instead of measuring individual elements, but each kiln is different. Check with your kiln manufacturer for specific details for your kiln, or you can always email me here at Bracker’s for more information.
Step 4: put one probe on one side of the circuit, and the other probe on the other side of the circuit. The multimeter will display a number. This is the circuit’s measured resistance. Determining where the “sides” of the circuit are can be tricky. Compare the diagrams below to what you have:
Determining if your elements are worn out is as simple as comparing the resistance you measured to the resistance for your kiln’s elements. Your number will be higher, and each manufacturer has a ‘range’ for when an element is no longer good. Once your resistance is higher than the ‘dead element’ number, its time to change them.
One special note, if the resistance you measure is the letter I, that means there is a broken element or wire. The I stands for the word “Infinite”, and sometimes looks like a number 1 depending on the model of the multimeter.
Not all elements have the same resistance, and elements are NOT interchangeable between most kiln models. You may find the correct resistance reading for your elements either in your kiln’s manual, or by contacting either your kiln’s manufacturer. You may also feel free to contact us here at Bracker’s Good Earth Clays. I will be happy to look up the numbers for you.