An SG type thermostat for Cecilware fryersLast week, I was finishing up a delicious sandwich at Pete’s Carryout in Bethesda, Maryland, when Ned, the perpetually gregarious owner and grill cook, mentioned to me that his fryer oil wasn’t keeping its temperature well. This got me thinking about simple DIY activities for commercial natural gas fryers, the easiest of which is calibrating their GS style thermostats.Firstly, what is a GS style thermostat? Robertshaw, the company founded by gas-regulating thermostat inventor Frederick W. Robertshaw in 1899, set the industry standard for these types of hydraulic thermostats and they are ubiquitous in food service heating applications. The GS style is a closed hydraulic system consisting of a sensing bulb, bellows and piston, lever and spring, snap mechanism, and valve. The bulb contains a liquid whose volume expands greatly when subjected to heat. This heated liquid expands against a bellows, which then expands the bellows until the piston in the bellows actuates the lever. The lever then finally snaps the valve to a closed position until the liquid cools enough to release the pressure against the lever. In this way the flow of gas to the burners can be turned on and off.Â
A cutaway schematic of a hydraulic thermostatic valve
 Because of the snap action of the thermostat, there is no modulation to the gas flow; either the burner is receiving gas or it isn’t. How then, does the thermostat alter its action to conform to a specific desired temperature? The answer lies in a screw that provides resistance against the expansion of the bellows, meaning the hydraulic liquid needs to get hotter to expand it fully. And it is the simple tightening or loosening of this screw that we are going to cover in this week’s DIY. What you will need:                                        Â
Either a small flathead screwdriver or hex key, depending on what type of fryer you have
A pair of pliers
A deep fryer thermometer (digital is fine, but make sure the probe has a physical attachment. Infrared thermometers can be less accurate)
 The process:                                               1. Turn on the main burner and immerse your thermometer in the oil.2. Adjust the thermostat’s knob to 350°F, and pay attention to the burner.3. When the burner goes out, compare the temperature of your thermometer to the thermostat. Is it 350°F?4. If it’s not within 15° of that on your thermometer, you’re going to have to adjust the thermostat. Turn the thermostat to 375°F. Remove the knob of the thermostat, making sure not to disturb its current setting.5. A small screw will be in the center of the thermostat shaft. This screw adjusts tension against the bellows.6. Adjustments to GS thermostats are typically in increments of 25°, and the general thought is that’s better to be 5° over than 22° under. So, when the temperature of the thermometer approaches 375°F, turn the screw clockwise until the flame on the burners go out.7. Replace the knob and compare the temperatures of the thermometer and thermostat again. If there’s unacceptable variance, remove the knob again and adjust as necessary, remembering that clockwise turns on the screw lower the thermostat’s temperature, counterclockwise turns raise it.It’s that simple. With this knowledge, you’ll have no trouble keeping the temperature of your fry oil perfect for ensuring the most consistent fried food. Just don’t expect your french fries to be quite as good as Ned’s are (Old Bay FTW)!Â
A commercial deep fryer
Stay tuned for upcoming blog posts, including more DIY topics and food service industry-specific content!By Tristan
