Why Should I Replace all of My SiC Elements at the Same Time?
In what is a common theme on this blog, this question is one of those ones that should seem easy, but is deceivingly complex. On the surface, this seems elementary – if a SiC heating element in your dental sintering oven burns out, you just replace it, right?
Sure, if you want to literally burn money.
Let me explain.
SiC Elements are Weird
On a molecular level, Silicon Carbide elements are a crystalline matrix of carbon and silica. As explained in a previous post, the Silica sacrifices itself as a protective oxide layer on the surface of the element, protecting it and allowing the element to enjoy a long, hot, and productive life.
As the elements age and the Silica is steadily consumed, the crystalline matrix of the element decomposes, causing the resistance of the element to increase. We consider an element to be at end-of-life when its resistance is approximately 300% of its factory-rated resistance (which is marked using chalk on the element cold-end).
It should be noted that the factory resistance of the heating element is measured at 1000C. The cold resistance of the element is not a useful figure – for SiC elements, resistance has to be measured when the element is hot, otherwise, the value is meaningless.
I know, I know, more engineering and science stuff. Bear with me, I am getting there.
Depending on the design of your dental furnace, not all of your heating elements may be seeing the same power. Also, there is a natural variation in SiC element resistance – usually 15% or so. The end result is that some elements are going to see more power than others, and as a result, are going to age faster and increase in resistance faster.
In short, some elements are not going to last as long as others. You will see this in your furnace when one of the elements is not glowing as brightly as the others (or it just stops working altogether).
When that happens, you change the one, failed element, right?
So what’s the big deal?
Electricity is Like Water
Similar to water, electricity follows the path of least resistance. That shiny new element you just put into the furnace has a lower resistance than the other elements in the furnace. As a result, more electricity (current) flows through the new element than flows through the old elements.
This. Is. Bad.
Since the new element has a disproportionately larger amount of current flowing through it, it will run much hotter than the elements. At best, this causes uneven heating in your sintering furnace. At worst, the element is running quite a bit hotter than its design specification, and as such, it starts aging prematurely.
Due to Ohm’s law, that new element is going to continue to run at a higher power until its resistance has become the same as the older elements in the furnace.
The end result being that your brand new element – in a matter of hours – has just been prematurely aged to the “age” of the old elements.
Is It the Same for MoSi2 Elements?
Thankfully, no. MoSi2 elements do not change in resistance over their lifespan, so you can change out single MoSi2 elements from your sintering oven with no loss in life from either the new element or the old elements.
As much as it hurts the pocket book, the correct procedure for replacing SiC elements in your dental sintering oven is to change all of the elements at the same time. Doing so will ensure that all elements reach their full life, and ultimately, save you and your lab money.