LimsPulse
COD digester · Calibration procedure

Block temperature check

Confirms that the COD reactor block holds 150 °C in every well used for digestion.

Illustration of COD digester
PURPOSE

Why this is checked

Dichromate oxidation is strongly temperature dependent. If the block runs cool, organic matter is not fully oxidised and COD results are low; if it runs hot, tubes can leak. The check shows that the displayed temperature and the actual well temperature agree.

Frequency

Typically monthly and after repair or relocation, with a full multi-well check at least annually, as defined in your SOP.

Learn how the COD digester works
YOU WILL NEED

Standards & tools

  • Calibrated digital thermometer with a thin probe, or a calibrated thermometer, traceable to national standards (resolution 0.1 °C)
  • Digestion tube filled with silicone oil or glycerol to hold the probe (a 'thermometer tube')
BEFORE YOU START

Prerequisites

  • Reactor switched on at 150 °C and stable for at least 30 minutes
  • Thermometer calibration certificate valid, with correction known
  • Safety shield in place; heat-resistant gloves available
PROCEDURE

Step by step

  1. 1Fill a clean digestion tube with oil to the same level as a normal sample, insert the thermometer probe and cap or seal it loosely.
  2. 2Place the thermometer tube in the first test well (for example a corner well) and wait until the reading is steady (often 10–15 minutes).
  3. 3Record the reference reading, apply the certificate correction, and record the reactor display.
  4. 4Repeat in other wells, covering corners and centre (for example 5 wells, or all wells at the annual check).
  5. 5Calculate the deviation of each well from 150 °C and the spread between wells.
RECORD

Example observation table

Well positionDisplay (°C)Reference thermometer, corrected (°C)Deviation from 150 °C
Front left150.0149.2−0.8
Front right150.0150.4+0.4
Centre150.0151.1+1.1
Back left150.0150.6+0.6
Back right150.0149.8−0.2
CALCULATION

How to calculate the result

Formula
Deviation = corrected reference reading − 150 °C; Spread = highest − lowest well reading
Worked example
  1. 1

    Centre well: 151.1 − 150.0 = +1.1 °C

  2. 2
    Mean of 5 wells = (149.2 + 150.4 + 151.1 + 150.6 + 149.8) ÷ 5 = 150.22 °C
  3. 3

    Spread = 151.1 − 149.2 = 1.9 °C

  4. 4

    All wells within 150 ± 2 °C, so the block passes.

ACCEPTANCE CRITERIA

Pass if

  • Every well checked within 150 ± 2 °C (typical limit in standard COD methods; use your SOP limit)
  • Display agrees with the reference within the tolerance in your SOP (for example ±2 °C)
  • Timer runs 120 minutes within about ±1 minute when checked against a stopwatch
IF IT FAILS

What to do

  • Label the reactor 'Out of calibration' and stop using it for COD.
  • Check that the thermometer tube was seated fully and the reading was steady, then repeat once.
  • If the failure is confirmed, request service or adjust the controller offset as allowed by the manufacturer, then repeat the check.
  • Assess COD results since the last passing check, using KHP check standard records to judge impact.
MORE

Other COD digester 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.

See how