The Calibration OFFSET’s need a smaller increment amount to provide more accurate calibration.
In Setup Menu, both Left and Right TC OFFSET parameter only adjusts in 0.25C increments.

Suggest that both OFFSET parameters provide for increments of 0.10C.

Also, when OFFSET values are greater than 3 significant digits, the firmware rounds down the parameter value to only 3 digits ( e.g., 9.50, 9.75, 10.0, 10.2, 10.5, 10.7, 11.0, etc. ). So, decreasing the increment to 0.1C would also fix this problem.

Code snippet from file setup.c, lines 26 thru 33:

static setupMenuStruct setupmenu[] = {
	{"Min fan speed    %4.0f", REFLOW_MIN_FAN_SPEED, 0, 254, 0, 1.0f},
	{"Cycle done beep %4.1fs", REFLOW_BEEP_DONE_LEN, 0, 254, 0, 0.1f},
	{"Left TC gain     %1.2f", TC_LEFT_GAIN, 10, 190, 0, 0.01f}, 
	{"Left TC offset  %+1.2f", TC_LEFT_OFFSET, 0, 200, -100, 0.25f},            < ------
	{"Right TC gain    %1.2f", TC_RIGHT_GAIN, 10, 190, 0, 0.01f},
	{"Right TC offset %+1.2f", TC_RIGHT_OFFSET, 0, 200, -100, 0.25f},           < ------
};

Some Background:

I installed the latest firmware, so that calibration settings are at the defaults.

However the mainboard’s 27L2C chip, a dual operational amplifier, has a significant amount of inherent offset on the oven’s left thermocouple.

The K-type thermocouples that this oven uses are quite linear in the range that the oven operates at. This is as shown in the plot below

Thermocouple table data is from: https://www.tc-inc.com/thermocouple-information/type-k-thermocouple-output-table.html

Pre-calibration, using the Manual/Bake Mode, I recorded data of set temperatures vs. displayed temperatures at a series of set temperatures. This data when plotted shows that the ovens displayed temperatures are quite linear. This confirms that the electronics (operational amplifier) seems to in acceptable condition before being calibrated to more precise values.

Calibration for my T-962 oven was as follows:
For the calibration procedure, I used a TM-902C thermocouple temperature meter with K-type thermocouple probe. I mounted the probe tip using copper wire, such that probe tip, was at the same height and within 2mm of touching T-962 oven’s thermocouples.

The oven’s thermocouples are located at a height just above the tray sides. I only have one TM-902C meter, so I did this once each, for left and right oven thermocouples.

I then used the Manual/Bake Mode to record steady state data of both displayed temperatures on the oven’s LCD and the TM-902C meter.

I then used an online linear regression calculator to find a best fit line for both left and right thermocouples. This provided a linear equation (shown above) which I used to calculate the slope and X-axis intercept. Using these values, then translating the line to intercept the X axis at zero degrees C ( also shown above ). The slopes (gains) and intercepts (offsets) amounts are then used to calibrate the oven in the Setup Menu.

Unlike the GAINs settings, the T-962 oven’s circuit board has no provisions for adjusting operational amp’s two output OFFSET’s, which are operated in non-inverted arrangement, it only has fixed 1K ohm resistors at the op amp outputs on each channel.

I have also ordered a pin compatible precision operational amplifier chip (Analog Devices AD8552 ) which I plan to use to replace the 27L2C dual op amp chip. The AD8552 chip has sampling with very low offset and zero drift.