Analytical Reporter May/ June 2020

general

11 Issue 3 2020

Best practices - Electrical conductivity (EC/TDS) Proper probe submersion Why? Incomplete submersion can cause erratic and/or inaccurate readings. Fix: Check that the vent hole is completely submerged on a four-ring probe. Use a plastic beaker Why? Glass and metals can cause EMC interferences (electromagnetic interferences) with your EC probe readings. Fix: Use a plastic beaker and keep probes at least an inch away from all sides of the beaker.

Always rinse your probe Why? Salt build-up and other residues can contaminate samples and cause erratic readings. Fix: Always rinse the probe with purified (de-ionised) water before storage, after removing from storage, and in between readings. Always properly store your probe Why? Some EC probes are combination probes and can therefore dry out if stored improperly. Fix: You can “revive” a dried-out combination probe in pH storage solution for at least one hour. Clean your probe regularly Why? Deposits on the probe can build up over time on the electrode portion and the vent hole. This can cause erratic readings and issues whilst calibrating. Fix: Use warm water with soapy surfactants and then rinse the probe thoroughly. Do not use cleaning solution or solvents as those can be chemically incompatible with the body of the probe.

Fix: A single or multi-point calibration can be used when calibrating an EC probe. If the probe is used daily, calibrate it daily. If not, calibrate the probe prior to use. Always use fresh calibration standards Why? Calibration standards have no buffering capacity and are easily contaminated. Fix: Always use fresh calibration standards and rinse your probe with purified water (de-ionised) before calibrating. Have a separate beaker with some calibration standard or sample in it, in order to quickly rinse the probe with before taking a measurement. This will help prevent contamination of the standard or sample.

Pick the right probe for your sample Why? Not all EC probes can measure in every EC range, and some probes may not be ideal for all samples. Fix: Based on your sample, choose a probe that can cover your measurement range. Check if the probe you want is compatible with your measurement style: do you need a stationary meter or something more portable? Check for bubbles Why? Air can become trapped inside the probes or stay on the surface of the probes when they are submerged. This can cause erratic or erroneous readings. Fix: Submerge the probe past the sleeve hole to fill the space around the probe. Carefully move the probe up and down in the sample and then gently tap the probe on the bottom of the beaker to dislodge bubbles Take the time for stability Why? it takes time for the probe to reach thermal equilibrium, even if it has integrated temperature compensation. Fix: Take an extra moment to ensure the temperature and the EC reading has fully stabilised. This will help to ensure accurate readings.

Calibrate often Why? All EC probes need to be calibrated for the greatest accuracy.

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Calibration systems for vacuum gauges by Pfeiffer Vacuum I

n view of increasingly stringent quality assurance requirements where the accuracy and reproducibility of the vacuum measured during production and service work is essential, as well as the vacuum gauges required for this, the demand for calibration systems is steadily increasing in the market place. In critical processes, the accuracy of vacuum measurements is important for process stability, quality and for monitoring these processes. The pharmaceutical industry requires precise vacuum measurements for safeguarding its high-quality products, which is particularly the case when freeze drying expensive drugs. For this reason, regular calibration of vacuum gauges is required. This also applies for final vacuum measurements on vacuum systems in service applications in line with factory or international standards. To cover the majority of applications, Pfeiffer Vacuum has developed two calibration systems – Basic and Pro system. Calibration system Basic • For static and dynamic calibration • Calibration range 1.013 – 10-4 hPa • Easy to use • Chamber ISO/TS 3567-compliant

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Up to three reference gauges for medium accuracy • DAkkS-certified reference gauges • Flexible • We also deliver customised solutions Calibration system Pro • For static and dynamic calibration • Calibration range 1.013 – 10-4 hPa • Easy to use • Chamber ISO/TS 3567-compliant • Up to five reference gauges for high accuracy • DAkkS-certified reference gauges • Flexible • We also deliver customised solutions Applications • Freeze-drying in the pharmaceutical and food industry. • Coating systems for optics, ophthalmics, decorative coatings, diffusion barrier layers for PET bottles and packaging films, wear protection coatings for tools, etc., incorporating the use of multiple measuring instruments. • Final vacuum measurement in pump production and service workshops. • Process monitoring and process control for determining the end of the process, in order to produce consistent quality and minimise rejection rates.

Calibration methods Calibration is based on a comparison of the measurement values of the gauge to be calibrated and the reference gauge according to ISO/TS 3567. The gauge to be calibrated and the calibrated reference gauges are connected to a standardised vacuum chamber (according to ISO/TS 3567, form b) and are exposed to the same vacuum. There are two basic methods of calibration – static and dynamic calibration. With the calibration systems supplied by Pfeiffer Vacuum, transmitters can be calibrated statically with a measuring range of >1hPa as well as dynamically with a measuring range of <1hPa.

The reference gauges values and the gauges to be calibrated must be determined for every defined pressure. According to ISO/TS 3567, at least three measuring points must be taken into account (e.g. 1, 2 and 7 in table). The calibration results must be recorded in a calibration certificate in compliance with DKD 6-2 of the German Calibration Service (DKD) and be kept as proof of this. The user decides whether the deviations from the reference values are admissible. The deviations shown in the table are sufficiently accurate.

Example of accuracy level of reference and calibrated gauges Standard

Unit under calibration

p (hPa)

p (hPa) -4

1,003 · 10

1,004 · 10-4

2,001 · 10-4

2,002 · 10-4

4,004 · 10-4

4,006 · 10-4

7,003 · 10-4

7,005 · 10-4