This page sheds light on a few details of my forge control set-up. The PID and two-stage burner design is based off of one created by Stacy Apelt. It allows for a low stage to help maintain ignition (almost as a pilot does) and a high stage to raise temperature as needed.
This is the wiring diagram I drew up to get my brain around it. In use, you can actually get by without the high fan control, as the forge needs only minor adjustments to increase in temperature. If you balance your atmosphere in the low stage, when the high stage kicks in, it will be just slightly rich (reducing). As it is, there is so little difference between the fan speeds that I just keep the high-fan control in the "off" position most of the time. This also allows me to get around some feedback/interference problems I've been having between the two controllers and the gas solenoid.
With this setup, you set your target temperature on the PID. If the temperature coming in from the K-type thermocouple is below the target, the PID sends a signal to the SSR. This causes the SSR to switch on the gas solenoid and the high fan control (if the high fan control is even being used). This allows more gas / air into the forge to increase the temperature. As the target temp is reached, the PID turns the SSR off and the gas solenoid shuts down, reverting the burner back to the low stage.
In order for the PID to control the gas, you have to create a two-stage propane control. Here's the parts and how I put the control together. A needle valve for each stage allows complete control of atmosphere in either high or low stages. When using propane, make sure and make about 3 wraps around the pipe threads with teflon tape to seal them up, and do not use compression fittings. Only use teflon'd pipe fittings or flare fittings. The side with the gas solenoid is the high stage.
HERE is a short video of the burner running in the open just to test the system. You can see the two stages as I manually kick switch from low to high a few times. This is running very rich in the open in order to maintain ignition. Normally, the swirl and heat of the flame in the forge will maintain ignition at a much more balanced (and significantly lower pressure) atmosphere.
And here is the forge with the blower installed (not running...I hadn't lined the forge yet in this picture).
This is the forge in action, holding a range around 1750F-1780F. As the PID learns, it will be able to tighten up the temperature swings. Also, some fine tuning to the needle valves and blower gait valve should allow me to tighten it even more.