(This is the fourth of four posts detailing the mods that I have made to my CharGriller Smoker Pro.)
This mod describes using a Stoker (automatic temperature control device, website: http://www.rocksbarbque.com/index.html) configured with two 5 CFM blowers for a single pit.
The Stoker Controller
The controller is the white box beneath the front shelf.
The cables for the fans are grey and run to the right (toward the SFB).
The cables for the food and pit thermocouples run to the left (where they enter the cooker).
The idea of a dual fan in place of a single larger fan is to improve the startup and recovery times of the cooker but without creating a wind tunnel that may push loose ash through the SFB to the food.
This process works with the existing Stoker firmware by setting up two sensor blower relationships.
The primary blower is set to maintain the real target temperature, typically 225F.
The secondary blower which will operate only at start and when the temperature drops well out of normal range. This blower is set to maintain a temperature about 7 degrees less than the primary or about 218F.
Once that temperature is passed on the way up, the secondary blower is idle.
To make this work, John Jackson at Rock's Bar-b-que cut a unique blower adaptor that mounts both fans on the standard stainless steel dog bowl. (Painted black, here).
The Dual Blower Adapter
Configured with two 5 CFM fans and painted black. Installed over the air inlet daisy wheel of the SFB.
The temperature monitoring thermocouples can pass directly through the space between the two halves of the cooker when it is closed. While the gasket material is soft enough that it does not pinch the thermocouple cables badly, in time it might cause problems. I installed a port near the right hinge to allow entry of the thermocouples.
The port is placed at the hinge to minimize the motion of the wires and any bending as the cover is opened and closed, reducing the chance of wire breakage.
Thermocouple Entry Port
The port is an electrical aisle component intended to create a waterproof seal around Romex cable (much larger than thermocouple wire). It does leave enough room to pass the thermocouples through to the inside without creating a large heat leak.
Ideally, one would manage both fans from a single sensor. However, the Stoker software assumes a one to one relationship between sensors and fans. For this reason, two thermocouples are required to manage the two fans, but they must be close together so they can work in tandem properly.
Test Burn Thermocouple Configuration
Immediately in front of the aluminum pan (used as a water filled dummy load) are the two pit sensing thermocouples.
To the left and right are two other thermocouples that are being used to evaluate the consistency of temperature across the grill. With an inverted charcoal grill beneath them, these sensors at each end show higher temperatures than in the middle by as much as twenty degrees. When the cooker is level adjusted properly, they are nearly identical, left to right.
One more theremocouple is in use in this system (for testing purposes). It is located in a gap of the insulation of the SFB of the CG. It measures something like the outside surface temperature of the SFB. I place a scrap of insulating material over it to get the best results.
The 'Ambient' Thermocouple
This thermocouple is named 'Ambient' in the Stoker configuration.
The Stoker supports network access to its measurements and a third-party program called Stokerlog records them and displays them graphically. (it also performs other services).
The Primary Stokerlog Graph of an 11+ Hour Test Burn
The 'Ambient' thermocouple is the one that registers over 700F during the initial warm-up of the cooker.
The vertical marks at the bottom of the graph indicate when the fan is on or off. The dark areas are really times when the fan is going on then off (normal operation).
The red line in the graph indicates the pit temperature measured by the thermocouple controlling the fan.
The Secondary Stokerlog Graph of the Same Test Burn
Stokerlog assumes that the second sensor and blower are on another pit and creates a separate graph.
Like the primary graph, the blower on-off time is at the bottom of the graph. It shows that it operated only during startup and when the remaining fuel was gettiing too low for one fan to maintain the heat (about 11 hours later).
The outside temperature was a consistent 49F during the entire period. However, a number of wind bands came through during it. These correspond to the spikes in the SFB temperature (labeled ambient) and the blue areas the represent fan activity.
In addition, rain began to fall (about 5AM) some of which, with the help of the wind, found its way to the smoker surface.
These two factors mean that in more typical California weather, the burn time could be much longer.
Despite the variability of the elements, the Stoker maintained temperature in a good BBQ range for more than 11 hours. The second fan performed its function at startup and at the end.
Previous tests indicate that the recovery time (when fuel is available) from opening the cover to return to normal temperature is less than 5 minutes. So you can do a little lookin' without hurtin' your cookin'.
(It should be noted, that long burn times don't happen with just the Stoker. Building a larger basket, insulating the smoker and other tweaks are necessary to retrofit a factory CG to burn for a long time.)