Extech EX330 temperature calibration

I noticed that the temperature readings on my Extech EX330 multimeter didn't match the readings on two different thermostats, so I decided to try calibrating it. Someone on the EEVblog forums contacted Extech support about calibration, who provided these directions:

4.1.0 mVDC Adjustment
4.1.1. Set the EX330 to mVDC.
4.1.2. Apply 100.0mVDC to the input sockets of the EX330.
NOTE: The display should indicate "100.0 mV."
4.1.3. Adjust VR3 until you have a reading of 100.0mVDC.

4.2.0 VAC Adjustment
4.2.1. Set the EX330 to VAC.
4.2.2. Apply 1.000VAC/ 60Hz into the input sockets of the EX330.
NOTE: The display should indicate "1.000 VAC".
4.2.3. Adjust VR2 until the UUT displays a reading of 1.000VAC.

4.3.0 Capacitance Adjustment
4.3.1. Set the EX330 to CAP.
4.3.2. Apply 100.0nF into the input sockets of the EX330.
NOTE: The display should indicate "100.0 nF".
4.3.3. Adjust VR1 until the UUT displays a reading of 100.0nF.

4.4.0 Temperature Adjustment
4.4.1. Set the EX330 to Deg C.
4.4.2. Apply 25°C into the input sockets of the EX330 using a type K thermocouple wire.
NOTE: The display should indicate "25°C".
4.4.3. Adjust VR5 until the UUT displays a reading of 25°C.

4.4.4. Apply 750°C into the input sockets of the EX330.
NOTE: The display should indicate "750°C".
4.4.5. Adjust VR6 until the UUT displays a reading of 750°C.

4.4.6. Set the EX330 to Deg F.
4.4.7. Apply 77°F into the input sockets of the EX330.
NOTE: The display should indicate "77°F".
4.4.8. Adjust VR7 until the UUT displays a reading of 77°F.

I don't have a stable 25°C or 750°C reference, so I adapted the calibration procedure to use an ice bath and boiling water.

Calculate boiling point

Due to lower air pressure, water boils at a lower temperature at higher altitudes. The barometric formula can be used to calculate the pressure at a given altitude:

$\begin{aligned} P &= P_0 \exp\left(\frac{-gMh}{RT}\right),\\ \small{\text{where }} & \small{P_0 = \text{air pressure at sea level} \approx 760 \,\mathrm{mmHg}}\\ & \small{g = \text{acceleration due to gravity} \approx 9.807 \,\mathrm{m}/\mathrm{s}^2}\\ & \small{M = \text{molar mass of air} \approx 0.02896 \,\mathrm{kg}/\mathrm{mol}}\\ & \small{h = \text{altitude above sea level}}\\ & \small{R = \text{ideal gas constant} \approx 8.315 \,\mathrm{J}/(\mathrm{mol}\cdot\mathrm{K})}\\ & \small{T = \text{outdoor air temperature}}. \end{aligned}$

The final calculation is not very sensitive to air temperature, so you can fix it at 15°C (288.15 K).

The vapor pressure of a liquid at a given temperature is the pressure at which it evaporates. At lower pressure, a liquid evaporates at a lower temperature. The relation between vapor pressure and temperature can be approximated with the Antoine equation, shown here with the empirical constants for water:

$T_\mathrm{°C} \approx \frac{1730.63}{8.07131 - \log_{10} P_\mathrm{mmHg}} - 233.426.$

All together, the boiling point in Celsius at a given altitude $h$ in meters is:

$T_\mathrm{boil,°C} \approx \frac{1730.63}{5.1905 + 0.00005146 \cdot h} - 233.426.$

Here's a table, with altitudes rounded to the nearest 100 m:

Altitude above sea level	Boiling point of water
0 m	100°C (212°F)
300 m	99°C (210°F)
600 m	98°C (208°F)
900 m	97°C (207°F)
1200 m	96°C (205°F)
1500 m	95°C (203°F)
1800 m	94°C (201°F)
2200 m	93°C (199°F)
2500 m	92°C (198°F)

Calibrate

Open the multimeter

Remove the silicone case, battery cover, and the 3 screws for the back cover. Find the potentiometers labeled VR5, VR6, and VR7 located on the back (battery side) of the circuit board.

Ice bath calibration, °C

Put a couple dozen ice cubes in a large bowl and add just enough cold water to cover the ice. After a few minutes, the ice bath will be 0°C. Attach the temperature probe to the multimeter, set it to °C, and submerse the probe in the water. Take care that it is not touching the sides of the bowl. Now adjust VR5 with a screwdriver until the display reads 0°C.

Boiling water calibration, °C

Put a pot of water on a burner and heat it until it begins boiling. Set the multimeter to °C and submerse the probe in the water, taking care not to let it touch the sides of the pot. Adjust VR6 until the display reads the boiling point you calculated above (e.g., 98°C at 600 m altitude).

Boiling water calibration, °F

Switch the multimeter to °F. Adjust VR7 until the display reads the proper boiling point (e.g., 208°F for 600 m altitude).