I recently purchased a MES-40 Model 20070512 from Amazon.com I mainly smoke pork butt, ribs, and brisket at 225F. My experience has been the smoker runs 25 Deg F less than the setpoint.
I use an iGrill with two probes. I checked the probes in boiling water. I live at 3000'. Probe 1 read 206. Probe 2 read 209. Water boiling point at 3000' is 209 so the probes don't seem to a large source of error. To prevent emf possibilities and to keep the probe in air, not touching metal, I put it through 2 sets of 4 corks. This is blurry, but should give you an idea what it looks like.
On Sept 19 I smoked a prime rib. Being in a bit of a hurry, I set the factory controller to 270, put a probe in the rib, and one in the chamber next to the rib. Around 7:12 PM I gave up and put the rib in the oven. The white line is the meat temp and the red line is the chamber temp. The three major dips occurred when I opened the door to spray the rib.
I understand the on/off control creates the oscillating temperature.
Then I took some out of town work and didn't have time to smoke or worry about this.
Knowing I would be back in town I ordered an Auber Instrument WSD-1503CPH so I could replace the on/off control with PID control. The WSD is rated to 1800 watts and the MES-40 pulls 1200 based on the face plate information.
Today I bought the additional parts (replacement cord to plug into the controller for controlling the heating element, new drill bits [never miss the opportunity to get new toys ]) opened the access panel and looked at the wiring.
I had to cut the heat shrink off to see I should have also purchased spade connectors. I disconnected both connectors to the heating element and measured the element resistance. 12.5 ohms. Using E/R = I that computes to 9.6 amps. Exhausting the rest of my electrical knowledge I used P = EI getting a nominal value of 1152 watts. Close enough to 1200 so this shouldn't cause the 25 degree offset.
Tomorrow is back to Lowe's to get the wire connectors, wire in the WSD controller, and mount the wall mounted temperature probe. Then an empty chamber cycle to see what happens to the chamber temperature.