4 density routine maintenance guide EN

distilled water or a standard is measured and compared with the nominal density value of the test sample.. So there are four components which normally have to be summed up to form the to

Day-to-Day Routine Maintenance

of Density Meters

Cleaning Sampling Tips and Hints

Contents Modern digital instruments are easy to use and allow the density of

liquids to be determined with a high degree of accuracy High-resolution instruments are however no guarantee for accurate results This docu-ment explains what precautions should be taken to avoid errors when measuring the density of liquids

Contents

Sampling 8 Cleaning 9

Day-to-Day Routine Maintenance

of Density Meters

How often?

Which substance?

Test

A fast, simple and effective measure to ensure accurate results is a regu-lar and frequent instrument test A sample of accurately known density (e.g distilled water or a standard) is measured and compared with the nominal density value of the test sample

Such a test can be executed by an experienced user at any time and verifies the measurement accuracy of the meter It avoids frequent adjust-ments which change each time the internal instrument settings and thus, can give rise to result shifts

Tests should be done routinely in relatively short intervals (days, weeks) Often a test with water is done every day, as it is done quickly and ensures that the instrument works accurately

METTLER TOLEDO DM Density Meters offer the possibility to define fixed intervals for test sets with an automatic reminder for the operator Mea-surement Methods can be set up in way that the operator gets warned again or the instrument is blocked from use if the defined test interval is expired

The most frequently used test substance is deionized water as it is avail-able in almost every laboratory and in a high and reproducible purity It might be necessary to degas it by boiling

A different test can be defined separately with larger intervals (months, a year), using certified and traceable standards for quality assurance and traceability purposes

Which tolerance

should be applied?

METTLER TOLEDO offers combined (density and refractive index) certified standards in different ranges:

• Water (0.99 g/cm3; nD 1.33…)

• Dodecane (0.75 g/cm3; nD 1.42…)

• 2,4-dichlorotoluene (1.25 g/cm3; nD 1.55…)

• 1-bromonaphthalene (1.48 g/cm3; nD 1.66…) The following guidelines may help to define reasonable tolerances avoid-ing frequent error messages caused by too strict tolerances

• For test samples of unknown uncertainty (e.g deionized water from the lab) the tolerance should be defined at 2 times the instrument resolu-tion plus the operator repeatability

➔ Never go below that value range, but keep it in general as narrow

as possible according the instrument resolution and operator re-peatability

Example: DM40 Density Meter with a resolution of 0.0001 g/cm3 Operator repeatability (as example) = 0.00005 g/cm3 (standard deviation when the operator measures the same sample 3 times in

a row If an operator works properly, he should not get a S.D more than that of the instrument’s rounding capability)

Tolerance = 2 x instrument resolution + operator repeatability = 0.0002 g/cm3 + 0.00005 g/cm3

➞ round up to a tolerance of ± 0.0003 g/cm3

• When using certified organic standards which usually have a relatively high temperature coefficient (density change with temperature change), please also allow for the specified temperature error of the instrument

So there are four components which normally have to be summed up

to form the tolerance, in order to avoid establishing tolerances which are too strict:

Uncertainty of the standard, limit of error instrument, temperature error and repeatability

Example: certified standard 2,4-dichlorotoluene with the following stated values:

DM50 Density Meter with a resolution of 0.00001 g/cm3, limit of error of 0.00003 g/cm3 (for the density range of the standard) and limit of error for the temperature of 0.02°C

(a) Uncertainty of the standard: ± 0.00003 (b) Limit of error instrument: ± 0.00003 (c) Temperature error: ± 0.00002

➞ 0.02 °C (limit of error for the temperature) * 0.00105 g/cm3/°C (α calculated from given densities at different temperatures of the standard = 1.24432 – 1.25477 g/cm3 / 25 – 15°C)

(d) Operator Repeatability: ± 0.00001 (example, has to be determined)

Tolerance = sum of the 4 components = ± 0.00009 g/cm3

This is an example and the tolerance has to be calculated specifi-cally for each combination of standard and instrument The toler-ance for a certified standard may become larger than the 2 to 5 times instrument resolution as it is the case for a normal test with local deionized water

If the value obtained deviates from the expected (true) value more than the defined tolerance, proceed as follows:

1 Check if the correct substance has been used, e.g pure fresh deion-ized water

2 Clean the cell thoroughly and completely dry it in the end (see chapter cleaning)

3 Measure the density air value and verify if it is correct, i.e if the instru-ment is completely clean and dry

4 Repeat the Test

What to do

if the test fails?

5 If the test continues to fail with varying results from each test, then the cleaning should be continued with more care, possibly using more powerful cleaning solvents and longer cleaning cycles, until the test plus the air measurements in between show repeatable behavior (within a few hours the oscillation value of DM Density Meters varies not more than ±1) Only when the tests fail with repeatable results should a new adjustment be performed

With LiquiPhysics density meters and refractometers special test methods can be setup When assigned to a shortkey, the test is executed with one click

Ask METTLER TOLEDO’s LiquiPhysics support for more details

Avoid bubbles

Automatic filling

After the filling, check if the cell is bubble-free Air bubbles (or a thin air film) are a serious problem Even very small quantities of air cause big errors when doing density determinations:

Diameter of the air bubble

[mm] Max Measuring error caused [g/cm 3 ]

The problem is that most small bubbles or air cushions (due to bad wet-ting of the cell) are not or hardly visible by eye Also in dark samples bubbles are hardly visible

METTLER TOLEDO DM Density Meters have a built-in Bubble Check™

to detect bubbles The most secure way for reliable results is a Multiple Measurement though

Automatic filling systems ensure that the cell is filled with the correct speed and in a reproducible manner, independent of operators and samples

With the METTLER TOLEDO SC1 and SC30 automation units it’s even possible to fill very highly viscous or sticky samples into measuring cell

Air Bubbles

Inappropriate cleaning is the most common source of erroneous results! Make sure that the measuring cell does not contain any

residue from previously measured samples or rinsing solutions

Deposits of previously measured samples are not always visible, but can cause erroneous results

Before the beginning of rinsing, remove all sample from cell and tubes For each kind of sample 2 appropriate rinsing solvents have to be defined

Purpose of the 1st rinse: Must be able to completely and quickly

dissolve all the sample, so that no contamination is left in the cell This solvent is often not very volatile

Purpose of the 2nd rinse: Must completely dissolve the 1st solvent

above, and it must evaporate quickly without leaving any residue to en-able quick drying afterwards

Some general recommendations for the rinsing solutions:

Fats and oils Deconex*

(0.3 to 0.5% in water)

Acetone or ethanol (puriss)

Petrochemicals Toluene or petrol ether Hexane or similar if temp

is > 30 °C

At room temp use low-boiling petrol ether mixture

or acetone

Conc Sugar solutions / syrup

Water (use enough water before rinse with acetone ➞ risk of poly merization)

Acetone (puriss)

* Deconex dissolves well in water, acetone, and ethanol! Available Rinse

It is also possible to do a large over-sampling with the new sample to ensure a complete removal of the old one However, this is admissible only 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 fruit juices)

Procedure:

• Use a sampling pump Over-sampling is difficult to achieve with a syringe only

-> Recommended pump: METTLER TOLEDO FillPal™

• 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 This ensures that the old sample is properly flushed out of the cell Then fill the cell with the new sample

• Verify the procedure to make sure that the required repeatability and limit of error are maintained

• If 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 sugar crystallizing on the cell walls

• Completely clean and dry (as described in Rinse) the measuring cell at least once at the end of each working day

Cleaning Rinsing by

oversampling

(“analytical rinse”)

Even very small quantities of rinsing liquids (or residues of previously measured samples) in the cell may cause substantial measuring errors Example: If the density meter cell was rinsed with ethanol and not com-pletely dried, and then a measurement of water is performed, the error due to remaining rinsing liquid is as follows:

Remaining Ethanol in Cell [µL] Measuring error caused [g/cm 3 ]

Dry the cell completely with dry air by using a drying pump

Usually, ambient air is aspired through a bed of silica gel drying agent

To check if the cell has been dried completely, i.e an appropriate drying time has been chosen, measure the air density and compare it to the known value at that temperature

With the METTLER TOLEDO SC1 and SC30 automation units, the measur-ing cell is automatically cleaned and dried The two rinsmeasur-ing liquids for cleaning (e.g water and acetone) are mixed with lots of air and pumped through system at high speed This results in a pulsating flow which provides very efficient near-mechanical cleaning and also reduces solvent consumption

As the inside and outside of the SC1/SC30 sampling nozzle is thoroughly cleaned and dried after each measurement, sample carryover is not possible

With an automatic Cell Test the cleanness of the cell can be verified

Dry

Fully automatic

cleaning

For more information

www.density.com

Mettler-Toledo International Inc

Laboratory Division

CH-8606 Greifensee, Switzerland

Subject to technical changes

© 05/2015 Mettler-Toledo AG

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Good Measuring Practices

Five Steps to Improved Measuring Results

Good

Measuring

Practices

1 Evaluation

2 Selection 3

Installation /

Training

5

Routine

Operation

4

Calibration /

Qualification