by Marc Ward
Clay Times May 2004
Are You in Control?
Say, you’re a gear-head… or think you want to make your life simpler. You’ve been looking through magazines and catalogs and you find some cool stuff to help run your kiln. There’s all these fancy smancy gizmos that turn your kiln on, turn your kiln up, turn your kiln off, there’s even thinga-ma-jigs that will page you or call your cell phone when certain things happen with your kiln. There are alarms, flashing lights, even voice generated prompts that you could make sound like the voice of HAL in the movie 2001. Here’s a brief look at a few of the less complicated controllers that you could put on a kiln.
Most of the older electric kilns out there had high limit shutoffs. They came in the form of a kiln sitter with a cone placed between two rods that held up a lever. The cone melted/deformed and the lever dropped which shut the power off to the kiln. Pretty simple…. Simple until you shoved a kiln post up next to this dumb beast and went to bed. Next morning, the kiln’s still on and really cooking. Freak out!! It’s like your own little Three Mile Island. The lever didn’t fall. Make those pots again… after all it was a custom order.
You want a shutoff control for your gas kiln, but don’t really trust the ‘ole kiln sitters with the cone in them. Whatcha’ goina’ do? You need a simple controller with a non-latching relay. Great, what’s that? This is a pyrometer or temperature measurement device that has a way for you to set a desired temperature (like the dial on your oven). Once the temperature is reached, the controller switches things off. But, because we have a non-latching relay (a button that pops out and breaks the contact) the controller can’t come back on until you manually reset the button. In your oven, when the temperature shuts off as it reaches temperature, it comes back on again to hold the temperature in a constant range. Basically, with your oven, you have what the kiln world refers to as a soak controller. This is our same simple controller, just minus our button thing (the non-latching relay).
But, you want more heh? Most new electric kilns now come with ramp controllers. You plug in values of how fast you want the kiln to climb…. Maybe hold at a certain temperature for X hours then cool down at a certain pace. The controllers that do all this for electric kilns can easily be adapted to gas-fired kilns. With the electric kiln, it was simply turning the kiln on and off to keep the kiln on its prescribed path of temperature gain, hold, or drop. With a gas kiln you can do the same thing by turning the burners on and off at prescribed intervals. And just how is that accomplished? You can take the same kind of controller found on electric kilns and have it turn a solenoid on and off. A solenoid is a gas valve that will open and close on demand from the controller. Need more heat, burner on. Need less, burner off. This is called pulse firing. The main burner pulses on and off. This all depends on having a reliable pilot light with this simple on/off system. The gas shouldn’t be coming on if there is no way to light it. That’s what the pilot light does; it makes sure there is a source of ignition present. There are different kinds of pilot lights and the safety systems that monitor them… I’ll get into that next issue.
With electric kilns, there’s not as much to control in regards to the firing as there is with gas fired kilns. Because of this, I’m hesitant to recommend controllers to those new to gas firing. It basically boils down to this; If you want to put controllers of any kind on a kiln because, after all these years, you have grown tired of having to monitor a kiln, then some form of controller might be right for you. If, on the other hand, you want to put a controller on a newly built kiln because you are unsure of how to fire or you are inexperienced, this is an unwise way to proceed. You need to be in control of this controller and truly understands what and how it does what it does. You really need to be a good driver before you soup up your car and take it to the local drag strip. One final word of caution; Many controllers are designed as switches. This means they may not actually be able handle the load (amperage) of what you are trying to turn on and off. They may need a magnetic contactor or relay installed to handle the actual load. Check with the controller manufacturer and/or the solenoid valve manufacturer if you are unsure about this. Next issue, I’ll talk about the different kinds of pilots and how they are monitored.