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Guiden om att mäta densitet (PDF) |
Denna guiden för densitetsmätning förklarar vilka försiktighetsåtgärder som bör vidtas för att undvika fel vid mätning av densitet, gravitation eller koncentration av vätskor. Genom att följa dessa rekommendationer kan du uppnå bästa möjliga resultat vid mätning av densitet, speciellt med digitala densitetsmätare. Följade ämnen omfattas i guiden:
- Test och justeringar: Vad är bäst? Regelbundna tester eller regelbundna justeringar? Hur ofta bör densitetsmätaren testas eller justeras? Vilken tolerans bör tillämpas?
- Prover: Vilka typer av prover kan mätas med en digital densitetsmätare? Vilka prover är svåra och hur mäter man dem? Hur kan detta påverka mätningen?
- Provtagning: Är det skillnad på provtagning med spruta och med ett automatiskt påfyllningssystem? Hur undviker man luftbubblor?
- Rengöring: Hur kan cellen rengöras beroende på provet? Vilka lösningsmedel bör användas? Vilken effekt har felaktig rengöring på mätningen?
- Resultatsverifiering: Hur man ser till att mätningen inte påverkats av luftbubblor eller kvarvarande lösningsmedel? Hur konverterar du resultatet i andra enheter eller koncentrationer? Hur kontrollerar du automatiskt om resultatet ligger inom produktspecifikationen (Quality Control)?
1. Test and Adjustments
The commonly held opinion that frequent adjustment of the instrument guarantees accurate results is not true. Any adjustment operation results in changes being made to the internal instrument settings.
If the adjustment is not properly performed, all the measurements performed afterwards will be wrong.
Instead of frequent adjustment, it is better to regularly verify the measurement accuracy of the system by measuring a sample of accurately known density (e.g. distilled water or a standard) which is called test, calibration or check. Then the measured density is compared to the known nominal value of the sample.
Get more information in the Density Measurement Guide
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One Click Test with Standards - Video
See how to run a test with a standard to check if your density meter or refractometer is still working properly.
Test (Calibration)
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Adjustment
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2. Samples
Viscous samples
Be careful when measuring viscous samples and make sure that the samples do not contain air bubbles when you introduce them into the measuring cell. It often helps to warm the sample in a closed vessel and to allow it to stand for a few minutes before the measurement. The viscosity of the sample decreases on warming and air bubbles can more easily escape.
The shear forces that arise when viscous samples are measured in the measuring cell can lead to inaccurate results. The value of the density displayed by the meter is then usually too high. All METTLER TOLEDO DM Density Meters are able to automatically correct the sample viscosity measurement error. If your density meter has such viscosity correction, you should always switch it on if you want to obtain the best possible accuracy for samples with a viscosity greater than 25 mPa·s.
METTLER TOLEDO SC1 and SC30 automation units are able to handle samples with a viscosity of up to 30’000 mPa·s (which equals approximately liquid honey). For samples which have a higher viscosity or which are even solid at room temperature (e.g. paraffine) the heated versions SC1H and SC30H can be used.
Get more information in the Density Measurement Guide
Aggressive samples
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Volatile samples
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Samples with dissolved gases
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Non-homogeneous samples/suspensions
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3. Sampling
With a syringe
Manual sample handling with a syringe is always operator-dependent, which means that it is error-prone, difficult to reproduce, and time-consuming. Make sure that your operator is properly trained and injects the sample in a reproducible manner.
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Do NOT fill the cell just barely! Possible contamination would remain in the cell and yield a false measurement:
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Make sure that the sample comes out of the cell at least an extra 10 cm, so that contamination can be pushed out and only new sample is left in the cell.
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Fill the measuring cell at a slow speed and with a laminar flow (5–10 cm per second) to ensure complete wetting of the cell walls (no trapped bubbles along the walls). Make sure that no air is entrapped in the syringe. The plunger has to be pressed slowly and continuously without stopping.
Get more information in the Density Measurement Guide
Automatic filling
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Avoid bubbles
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Automated Density Determination - Video
Switch from syringe to SC1 automation to improve your density measurements. Increase accuracy and avoid operator dependent results.
4. Cleaning
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Deposits of previously measured products are not always visible. For example, if products containing oil or fat are measured, the measuring cell may be coated with a very thin film of oil. T o prevent this, the cell should be cleaned with suitable rinsing solutions (as described later) and dried - preferably after each measurement.
If all measured samples are of a similar kind and able to dissolve the residues in the measuring cell (e.g. when the density meter is used to measure different juices), it is also possible to do a large over-sampling with the new sample to ensures a complete removal of the old one (“analytical rinse”).
Restrictions:
- Use at least a sampling pump (e.g. METTLER TOLEDO FillPal™), a proper over -sampling is difficult to achieve with a syringe
- Immerse the sampling tube of the pump in the sample, then remove it so that air is sucked in the tube (~2–3 cm air in the tube) and immerse it again in the sample. Repeat this procedure approx. 5 times before the cell is filled for the measurement. This ensures that the old sample is properly flushed out of the cell.
- Verify the procedure to make sure that the required repeatability and limit of error are maintained. Measure the most critical sample first (for instance, the one with the highest sugar content) followed by deionized water then repeat this step a couple of times.
- If for instance sugar containing products are measured, make sure that the cell remains filled with either sample or with water between measurements to avoid the sample drying out and crystallizes sugar on the cell walls.
- Completely clean and dry (as described below) the measuring cell at least once af ter each working day.
Get more information in the Density Measurement Guide
Rinse
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Dry
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Fully automatic cleaning
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Automated Density Determination - Video
Switch from syringe to SC1 automation to improve your density measurements. Increase accuracy and avoid operator dependent results.
5. Result verification and documentation
Automatic result conversion
Often the result has to be converted using a table. Looking up in or interpolating from a table is error-prone and time-consuming. Automatic conversion using built-in tables (e.g. alcohol, Brix, temperature compensation according to API) prevents reading or calculation errors and saves time. A digital density meter of the latest generation allows the use of built-in conversion tables to show the result directly in the desired unit. METTLER TOLEDO DM Density Meters have to following built-in result units / concentration tables:
- Specific gravity, light and heavy Baumé, T waddell
- Sugar: Plato, Brix (Emmerich, NBS 113), HFCS 42/55, Invert Sugar, KMW , Oechsle, Babo
- Alcohol: OIML, AOAC, Proof Degree, HM C&E, Gay Lussac
- Petro: API degrees and gravity tables for crude oils, refined products and lubricants
- Up to 30 user -defined concentration tables (can be entered as tables or formulas)
Get more information in the Density Measurement Guide
Error detection
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Result limits
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Proper Documentation
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