Kjeldahl · Calibration procedure
Digestion block temperature
Checks that the digestion block reaches its set temperature and that all tube positions are heated evenly.
PURPOSE
Why this is checked
Digestion that runs too cool is incomplete and gives low nitrogen. A block that runs too hot can lose nitrogen and evaporate acid. Positions at the edges of the block often run cooler, so uniformity matters as much as accuracy.
Frequency
Typically every 6–12 months and after repair, as defined in your SOP.
Learn how the Kjeldahl worksYOU WILL NEED
Standards & tools
- Calibrated thermocouple (type K) with a traceable certificate, suitable for at least 450 °C
- Digestion tube filled with a heat-transfer medium (for example sulphuric acid or a high-temperature oil) or the supplier's temperature probe adaptor
BEFORE YOU START
Prerequisites
- Block empty, clean and dry
- Fume extraction running if acid is used in the test tube
- Thermocouple certificate in date
PROCEDURE
Step by step
- 1Set the block to the routine digestion temperature (for example 420 °C).
- 2Place the probe tube in the first position to be tested (include centre and edge positions).
- 3Wait until the reading is stable (for example 15–20 min after the block reaches set point).
- 4Record the reading, then move the probe to the next position and repeat.
- 5Test at least the corner, edge and centre positions, or every position if your SOP requires.
- 6Calculate the deviation of each reading from set point and the spread (maximum − minimum).
RECORD
Example observation table
| Position | Set point (°C) | Reading (°C) | Deviation (°C) |
|---|---|---|---|
| 1 (corner) | 420 | 418.6 | −1.4 |
| 4 (edge) | 420 | 421.2 | +1.2 |
| 8 (centre) | 420 | 419.4 | −0.6 |
| 12 (centre) | 420 | 422.1 | +2.1 |
| 16 (edge) | 420 | 420.3 | +0.3 |
| 20 (corner) | 420 | 417.9 | −2.1 |
CALCULATION
How to calculate the result
Formula
Deviation = reading − set point; Uniformity (spread) = highest reading − lowest reading
Worked example
- 1
Mean of six readings = 419.9 °C
- 2
Largest deviations: +2.1 °C (position 12) and −2.1 °C (position 20)
- 3
Spread = 422.1 − 417.9 = 4.2 °C
ACCEPTANCE CRITERIA
Pass if
- Each position typically within ±5 °C of set point (or the manufacturer's specification)
- Spread across positions within the limit in your SOP
IF IT FAILS
What to do
- Stop using the affected positions or the whole block.
- Adjust the controller offset if the manual allows, then repeat the check.
- Call the supplier if heaters or sensors are faulty.
- Check nitrogen recovery on samples digested since the last pass.
MORE
Other Kjeldahl calibrations
In LimsPulse · Calibration Management
LimsPulse schedules this calibration, records the result against limits, and blocks the instrument everywhere if it fails or goes overdue.