Monitoring of Measuring Instruments

Which devices have to be monitored?

All measuring instruments which are used to confirm warranted product qualities are subject to monitoring.

Analytical laboratories have to verify and document the accuracy of all measuring devices used in order to achieve reliable results. This especially applies to laboratories which operate according to GLP guidelines, are accredited to DIN EN ISO/IEC 17 025, or are certified to DIN EN ISO 9001. All these standards and guidelines require the availability of written instructions describing the monitoring procedure in detail. The error limits or accuracy and coefficient of variation have also to be defined, and there must be instructions on how to proceed if the acceptable limits are exceeded.

Timing and frequency of monitoring

The instrument's accuracy and measuring uncertainty must be known and documented before its admission for use. In addition, the instrument has to be tested again at defined intervals (see DIN EN ISO 10 012).


The performance of measuring instruments may be affected by e.g., the use of aggressive chemicals, and the manner and frequency of cleaning. Since the required accuracy largely depends on the circumstances of each application, the user has to determine the intervals for routine testing.

Typical intervals for liquid handling instruments and plastic volumetric instruments are every 3-12 months, for glass volumetric instruments, every 1-3 years.

Testing procedures

Volumetric instruments are tested gravimetrically. Testing for liquid handling instruments is performed according to ISO 8655; for glass volumetric instruments, ISO 4787 is applied.

Many influences must be considered when carrying this out. Therefore, BRAND® provides Standard Operating Procedures (SOP), including detailed testing instructions, for every type of volumetric instrument. The testing procedure is outlined step by step. To make it even easier, BRAND® offers software  which can perform all calculations, store them in a database, and print out a detailed test report.

Time required for testing

The monitoring of measuring devices should not become the main occupation in the laboratory; it should be limited to a reasonable extent. The demand is for simple procedures which are quick and inexpensive to follow.

The combination of testing instructions (SOP), specially developed EASYCAL™ calibration software together with volumetric instruments delivered with individual or batch certificate are best suited to minimize the time needed for this procedure.

Monitoring of DE-M marking instruments

Volumetric instruments which are DE-M marking to "MessEV" are also subject to the monitoring procedure.

There is no clear directive whether such instruments have to undergo initial testing or not. The user is responsible for answering this question. However, to be on the safe side, the initial test of a representative random sample is recommended. In addition, such a test will document the initial state in relation to subsequent tests.

Another option would be the purchase of volumetric instruments with a certificate from the manufacturer.

Terms used in the monitoring of measuring devices


Calibration in the strictest sense consists of determining the actual volume delivered. The calibration procedure should be quick and simple, eliminating potential sources of error. Therefore, BRAND® provides detailed testing instructions for every type of volumetric instrument, free of charge.


Adjustment consists of correcting the deviation of the measured value from the nominal value.

Depending on the manufacturer, the adjustment of liquid handling instruments is generally accomplished by turning an adjustment screw. After setting, a new calibration check is required. This procedure must be repeated until the volume is within the error limits.

Procedure for volumetric testing, e.g., Microliter pipette Transferpette® adjustable volume, 20-200 μL

We recommend a calibration of the Transferpette®, as described below, once every 3-12 months. Depending on frequency of use and pipetted media, shorter testing intervals may be defined by the user.


  1. Verify instrument type and nominal capacity.
  2. Read serial number.
  3. If instrument is soiled, disassemble and clean if necessary (see operating manual).
  4. Check for damage (housing, shaft tip, ejector, piston, seal). Obtain spare parts as required.
  5. Place the Transferpette® pipette (unpacked) including accessories into the testing room for at least 2 hours to adjust to room temperature.

Gravimetric test

  1. Determine temperature of the liquid for testing.
  2. Mount a new pipette tip.
  3. Condition the instrument: aspirate and release testing liquid five times. This will improve the accuracy of the test.
  4. Mount a new pipette tip and pre-rinse it once.
  5. Place the weighing vessel on the balance and tare.
  6. Remove weighing vessel from the balance.
  7. Discharge testing liquid into weighing vessel, pressing the pipetting key to the second stop to empty tip completely.
  8. Place weighing vessel on the balance. Read and note value.
  9. Retare the balance.
  10. Repeat steps 2. to 9. ten times. Enter values obtained at 100%, 50% and 10% of nominal volume into the test record.

Gravimetric testing values at

21.5 °C (Z = 1.0032)

Tested volume (μl):      200.0000

Specified value (mg):   199.3620

x1                                200.2000

x2                                199.6000

x3                                199.4900

x4                                199.7000

x5                                199.7000

x6                                199.2900

x7                                199.3500

x8                                199.4100

x9                                199.2000

x10                               199.1900


Functional test

1. Mount a new pipette tip.

2. Pre-rinse the tip once with testing liquid (distilled/deionized water).

3. Hold the filled pipette vertically and observe for approx. 10 seconds whether a drop is forming at the tip. Make sure the tip is not being heated, e.g., by the sun. Discharge the liquid. In case of smaller volumes (approx. < 50 μL) no drop will form even if there is a leak, due to surface tension.

A tip for finding leaks in small volume pipettes:

Discharge a small drop from the filled tip so that a small air cushion (air bubble) is present below the liquid. If, during observation, the air cushion falls, there is a leak.

1. Calculation of the mean volume

A mean volume ( x ) of the weighing values is calculated by dividing the sum of the weighings by the number of weighings made. This mean mass is then multiplied by a correction factor (Z, units of μl/mg) to give the mean volume (V) delivered. The factor Z combines density of water at the testing temperature and effects of atmospheric pressure. For a typical temperature of 21.5 °C and air pressure of 1013 mbar (hPa), Z = 1.0032 μL/mg.


2. Calculation of accuracy:

Extract from the table 'Factor Z for

liquid handling Instruments*

Temperature            Factor z

°C                                mL/g

18                               1.00245

18.5                            1.00255

19                               1.00264

19.5                            1.00274

20                               1.00284

20.5                            1.00294

21                               1.00305

21.5                            1.00316

22                               1.00327

22.5                            1.00338

23                               1.00350

23.5                            1.00362

24                               1.00374

24.5                            1.00386

25                               1.00399

25.5                            1.00412

26                               1.00425

* table refers to 1013 hPa

3. Calculation of the standard deviation, necessary for the determination of coefficient of variation

4. Calculation of the coefficient of variation:

Tested volume (μl): 200.0000
Mean volume (μl): 200.1514
A [%] 0.153
CV [%] 0.153
A [%] specified* 0.600
CV [%] specified* 0.200

* Error limits in operating manual.

à This pipette meets specifications.

If the calculated values for Accuracy (A [%]) and Coefficient of Variation (CV [%]) are less than or equal to the error limits, the instrument is calibrated to operate within specifications.

What to do if the instrument exceeds the error limits

  1. Review the operating manual to ensure that the instrument was operated properly.
  2. Follow the troubleshooting guide in the operating manual for assistance.
  3. Recalibrate the instrument in accordance with the operating manual.

If despite these steps the instrument still does not meet the specifications, remove from service and contact the manufacturer for support.

Calibration software

The monitoring of measuring devices to GLP and DIN EN ISO 9001 is not exactly straightforward. Complex equations easily lead to calculating errors, and the documentation of results can be tiresome. To facilitate this tedious task, some manufacturers have developed special calibration software.

e.g., EASYCAL™ calibration software by BRAND®

EASYCAL™ calibration software by BRAND®

EASYCAL™ performs all calculations and generates the complete documentation, automatically. All you need is an analytical balance, a PC, a printer (optional) and EASYCAL™ software.

  • Suitable for instruments from all manufacturers
  • Specifications of many instruments preloaded
  • Testing according to ISO 4787, ISO 8655, etc.

A demo version of the software can be downloaded from the Internet ( or can be requested on CD-ROM without charge.

Monitoring made easy

EASYCAL™ calibration software facilitates the monitoring of measuring devices to GLP and DIN EN ISO 9001, both for liquid handling instruments and for volumetric instruments of glass and plastic. The software is user-friendly and intuitive to use. After determining the type of instrument to be tested, all necessary data are entered step by step on the 'Entry of measuring results' screen.

Two options are available for entering the weighing results:

manual entry, or direct import from the balance via cable, followed by automatic evaluation. After defining the error limits, EASYCAL™ performs all calculations automatically. By one push of a button, a comprehensive test record can be printed out. All results are stored in a database. The test history keeps track of all tested instruments, facilitating the monitoring over time. The test intervals determined in relation to the testing instructions (SOPs) can be defined individually.

Calibration service for liquid handling instruments

Calibration can be a time-consuming procedure. For this reason, BRAND® offers a full calibration service including instrument adjustment and, if required, repair.