• The mass/volume of the temperature probe is crucial. Smaller probes are always better. At Hach, catalog number 2630600 is used by the service technicians when calibrating the DRB200. It is NIST traceable and has a small probe. The probe is approximately 1/16 inches in diameter and is barely longer than a COD vial. When inserting the probe, it goes all the way to the bottom of the vial. Larger mass/volume probes will read lower temperatures.
• The volume of Glycerol in the vial is also important. The manual says that with the temperature probe inserted, the vial must be filled to a height of 56 mm +/- 0.5 mm. Less volume will result in higher temperature readings and greater volume will cause lower readings. It is probably as critical to your measurement as the mass of the temperature probe.
• A temperature overshoot occurs as the DRB200 heats a block to target temperature. One should wait 30 minutes into the heating cycle before recording the temperature. By this time the overshoot has passed and the block temperature has leveled off.
• Air currents can also affect temperature measurements. Because the thermal coupling of the block to the glass vials is so small, many factors influence the dissipation of the heat of the vial contents. Select a location where there are minimal air currents.
Here is one last suggestion.
The DRB200 has 3 user defined programs: PRG1, PRG2, and PRG3. One may create their own program for a new temperature and time that corrects for temperature inaccuracy in either block. These are described in the user manual on page 8, "User Programs".
For example, if the right block measures 4 degrees °C low, you may create a user defined program that heats 4 °C greater than the program you had been using. So if your temperature measurement on COD test was 146 °C at 30 minutes into the heating cycle (using a low mass probe and correct vial volume, etc), you may define PRG1 for 154 C for 2 hours. Using this new program you can then measure again to see that this resulted in the necessary correction. If not, you may adjust your programmed temperature value upward or downward until you can achieve the desired value.
The user defined programs are always available and may be re-defined as often as needed. This process can circumvent the need for calibration and will allow you to use the instrument until the next time repairs are required.