4 density routine maintenance guide EN

Maintenance Guide

Cleaning
Sampling
Tips and Hints

Day-to-Day Routine Maintenance
of Density Meters


Contents

Day-to-Day Routine Maintenance
of Density Meters
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 document explains what precautions should be taken to avoid errors when
measuring the density of liquids.

Contents

Instrument Test

4

Sampling8
Cleaning9

3

Instrument Test

Test
A fast, simple and effective measure to ensure accurate results is a regular 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 adjustments which change each time the internal instrument settings and thus,
can give rise to result shifts.
How often?

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. Measurement 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.


Which substance?

4

The most frequently used test substance is deionized water as it is available 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.

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…)
Which tolerance
should be applied?

The following guidelines may help to define reasonable tolerances avoiding 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 resolution 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 repeatability.
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.

5


Instrument Test

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

Density [g/cm3]

15 °C

1.25477 ± 0.00003

20 °C

1.24954 ± 0.00003

25 °C

1.24432 ± 0.00003

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 specifically for each combination of standard and instrument. The tolerance 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.
What to do
if the test fails?

6

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 deionized 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 instrument is completely clean and dry
4. Repeat the Test


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.

7


Sampling

Air Bubbles
Avoid bubbles

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/cm3]

2

0.000838

1

0.000052

0.5

0.000003

The problem is that most small bubbles or air cushions (due to bad wetting 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

8

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.


Cleaning

Cleaning
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.

Rinse

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 enable quick drying afterwards.
Some general recommendations for the rinsing solutions:
Sample

Solvent 1

Solvent 2

Water based

Water

Acetone or ethanol (puriss)

Acids

Lots of water

Acetone or ethanol (puriss)

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 lowboiling petrol ether mixture
or acetone.

Conc. Sugar
­solutions / syrup

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

Acetone (puriss)

*Deconex dissolves well in water, acetone, and ethanol! Available
from METTLER TOLEDO

9

Cleaning

Rinsing by
oversampling
(“analytical 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.

10

Dry

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 completely 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/cm3]

10

0.001500

1

0.000150

0.1

0.000015

0.01

0.000001

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.
Fully automatic
­cleaning

With the METTLER TOLEDO SC1 and SC30 automation units, the measuring cell is automatically cleaned and dried. The two rinsing 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.

11


Good Measuring Practices
Five Steps to Improved Measuring Results
The five steps of all Good Measuring Practices guidelines start with an
evaluation of the measuring needs of your processes and their associated risks.
With this information, Good Measuring Practices provide straight forward
­recommendations for selecting, installing, calibrating and operating
laboratory equipment and devices.
• Guaranteed quality
• Compliance with regulations, secure audits
• Increased productivity, reduced costs
• Professional qualification and training

5
Routine
Operation

4

Calibration /
Qualification

1
Evaluation

Good
Measuring
Practices

2
Selection

Good Density and Refractometry
Practice™
Reliable density and refractive index
values – optimized by GDRP™
www.mt.com/GDRP

3
Installation /
Training

Learn more about Good Measuring Practices program
www.mt.com/gp

www.density.com
Mettler-Toledo International Inc
Laboratory Division
CH-8606 Greifensee, Switzerland

Subject to technical changes
© 05/2015 Mettler-Toledo AG
Global MarCom Switzerland / MC

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