Sartorius 2405 User guide

Digital Micro Balances
2405
Instructions for installation and operation

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1. UNPACKING
2. INSTALLATION OF THE BALANCE
2.1. Checking the voltage setting
2.2. Detaching the transport arrestment
2.3. Assembling the balance
2.4. Inserting the glass plates
2.5. Leveling the balance
3. DESCRIPTION OF THE OPERATING KNOBS
4. CHECKING AND ADJUSTING
4.1. Focusing the projection scale
4.2. Zero point
4.3. Sensitivity
5. WEIGHING PROCEDURES
6. SUPPLEMENT
6.1. Accuracy of the built-in set of weights
6.2. Correction of the weighing result
6.3. Determination of the air density
6.4. Conditions for installing micro balances
7. TECHNICAL DATA

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1. Unpacking
1.1. Carefully remove the balance from their cartons.
1.2. Carefully unpack accessories.
2. INSTALLATION
2.0.1. IMPORTANT: see supplement 6.1. to 6.4.
2.0.2. Remove outer and inner lid.
2.1. Checking the voltage setting:
2.1.1. Place balance on the rear on to the plastic dust cover. Push the cover plate in the base plate aside
and change connecting wires, if necessary. Close the cover plate and set up the balance. Remove the
adhesive tape carefully and wipe off the arrears with alcohol.

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2.2. Detaching the transport arrestment:
2.2.1. Weight application must indicate “29.99”.
2.2.2. Take off cover of the damping pot.
2.2.3. 3 Loosen knurled nuts, remove paper strips, swing aside springs and retighten nuts.
Remove cardboard strip.

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2.2.4. Take out balance beam. Take off 3 plastic discs (1). Loosen 3 knurled nuts (2), swing aside springs
and retighten nuts.
2.2.5. Remove cardboard strip.
Connect balance to the mains. Press gray push button (see 3.1.).
2.2.6. Remove brackets at the ends of the locking bars of the weight application.
2.2.7. Pull out bars carefully.

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2.3. Assembling the balance:
2.3.1. Check arrestment pins (1), damping (2) and bearing (3) and clean, if necessary.
2.3.2. Check knife-edges and bearing and clean, if necessary.
2.3.3. Set balance beam on to the arrestment pins (1). The knife-edge must not come into contact with the
bearing.

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2.3.4. Place cover plate on to the damping pot and turn anti-clockwise until the slit the cover plate runs paral-
lel with the balance beam. Turn clamp clockwise until it locks.
2.3.5. Check stirrup plate and clean, if necessary. Set stirrup plate on to the arrestment pins.
2.3.6.2. Unscrew 2 knurled nuts and take off bow.

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2.3.7. Attach stirrup and insert it.
2.3.8. Unscrew 2 knurled nuts and take down carefully the locking plate for the weights. Put back the perfo-
rated plate afterwards.
2.3.9. Attach lamp housing and plug in the cable.
2.3.10. Place the inside hood on to the balance.

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2.3.11. Swing out pan lifting device. Open upper sliding door and attach pan bow.
2.4. Inserting the glass plates:
2.4.1. Place glass plate into the balance and close upper sliding door. Put the balance pan on to the holder
and swing in the pan.
2.4.2. Place glass plate into the balance and close sliding door.

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2.4.3. Put locking screws for the front plate into position.
2.4.4. Fasten front plate.
2.4.5.Turn weight application to “0.00” and set micrometer to “00”.

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2.5. Leveling the balance:
Level balance by adjusting the front feet while observing the spirit level.
3. DESCRIPTION OF THE OPERATING KNOBS
3.1. Push buttons of the Auto-Arrest:
1: Balance arrested
2: Balance partly released. Switch weights are applied in this position
3: Balance fully released for the final weight indication.

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3.2. Weight application:
Knob c: Switch weights from 0.01 ....... 0.09 g
Knob b: Switch weights from 0.1 ......... 0.9 g
Knob a: Switch weights from 1 …........ 29 g
3.3. Knob for zero point adjustment.
3.4. Turning the knob clockwise: the lower sliding door is opened and the pan swings out.
Turning the knob anti-clockwise: the balance pan swings in and the lower sliding door is closed.

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3.5. The upper sliding door must be opened in order to place tubes and other large on to the hooks.
3.6. The left sliding door should not be opened unnecessarily.
3.7. Micrometer knob:
After standstill of the projection scale turn micrometer knob until reading mark coincides with one line of
the projection scale.

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4. CHECKING AND ADJUSTING
4.1. Focusing the projection scale:
4.1.1. Take off hood, release balance partly (press push button (2)) and adjust objective until projection scale
is in focus. Arrest balance and place hood on the balance.
4.1.2. Brightness of the projection scale: release balance partly (press push button (2) ), loosen knurled
screw and move lamp holder until Optimum brightness on the projection screen is achieved. Tighten
knurled screw again and arrest balance.
4.2. Zero point:
4.2.1. Release balance fully and set zero point.

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4.2.2. If zero point acc. to 4.2.1. cannot be set, arrest balance and take off outer and inside hood.
Direction of rotation at a - indication
Direction of rotation at a + indication
4.3. Sensitivity:
4.3.1. Checking:
Place a 10 mg weight on to the balance pan and turn weight application to "00.01". Release balance
fully and set zero point.
4.3.2. Turn weight application to "00.00". Projection
scale must indicate exactly "100".

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4.3.3. Adjusting the sensitivity: pull plug out of the hood.
if scale does not come up to “100”
if scale swings beyond “100”
4.3.4. Repeat zero point and sensitivity adjustment alternately until both readings are correct. The balance
must be checked again after approximately 1 hour.
5. WEIGHING PROCEDURES
5.1. WEIGHING PROCEDURE I:
5.1.1. Set zero point. Swing out balance pan, place sample on to the pan and swing in the pan.

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5.1.2. Release balance partly. Turn knob of the biggest weight decade until the projection scale swings into
the minus range. Then turn knob on step back.
5.1.3. Proceed in the same way with the following smaller weight decades.
Arrest balance and after a few seconds release the balance fully. After standstill of the projection scale
turn the micrometer knob until reading mark coincides with one line of the projection scale.
Weight indication: 1,732706g
5.1.4. Arrest balance. Take off sample and check the zero point. In case of a deviation it is recommended to
take the mean value of the two zero point readings as the basis for the weight determination.

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5.2. Weighing procedure II:
5.2.1. Set zero point. Open upper sliding door, place sample on to the pan and dose sliding door. The weight
is determined according to 5.1.2. ...5.1.4.
6. SUPPLEMENT
6.1. Accuracy of eight set
lt is practically impossible to adjust all weights to a degree of perfection that errors can be eliminated
completely.
For this reason the initial and final weighings should be carried out with the same weight combinations.
Weight discrepancies can be determined with the highest possible accuracy by means of the wide
range of the 10 mg scale.
6.2. Correction of the weighing result
The built-in weights above 0.1 g are of non-corrosive non-magnetic steel (density 7.88 g/cm3). The
weights below 0.1 g are made of a special aluminum alloy (density 2.7 g/cm3). For medium air density
of 0.0012 g/cm3they are adjusted to match brass weights of density 8.4 g/ml. Differential weighings of
the same material do not require a correction. In case of absolute weighings the corrected result in
consideration of the accuracy of the weight set (maximum error ± 21 µg) is determined as follows:
Mp= N1+ N2+ K
ML ML ML ML
K = N1(9.4 ∙10-6 - MG1+ MP ) + N2(3.0 ∙10-4 - MG2+ MP )
MpMass of sample material
N1Indication of mechanical weights 19.9 ... 0.0 g
N2Indication of mechanical weights 0.09 ... 0.00 g
ML Density of air
MP Density of sample material
MG1 Density of built-in steel weights = 7.88 g/cm3
MG2 Density of built-in aluminum weights = 2,7 g/cm3

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6.3. The determinationofatmospheric pressure andits influence onanalytical weighings
The demands in respect to the accuracy of analytical and especially of micro-analytical weighings are
often extreme. Even though the accuracy required for the end result is usually only 1 %, the require-
ment of a much higher accuracy is conditioned by the necessity of also having to weigh the mass of
the tare which often exceeds to a considerable degree that of the proper sample.
In Order to improve accuracy, it is necessary to systematically eliminate all sources of errors. One of
the most critical factors in this respect is the elimination of the influence of the air buoyancy. To what
extent it may be necessary to take this factor into account, depends — aside from the required toler-
ances - on the specific weight of the sample in relation to that of the weights used.
Felgentraeger1) in “Feine Waagen, Waegungen und Gewichte" draws attention to the fact that most
chemists either are not aware of the significance of air buoyancy or fall to do anything about it because
the procedure seems too complicated.
The tables provided by that author furnish the necessary information for all respective calculations up
to the most exacting requirements. Lately, H. Ulbricht has published two papers on the significance of
air buoyancy2). These publications give information only on the importance of air buoyancy without
furnishing data on how to determine it. It is exactly this point, however, which causes difficulties.
The tables found in Kohlrausch and D'Ans/Lax3) can be used only for a coarse reduction of mass rela-
tive to vacuum at normal air pressures. They cannot be used for determining changes in buoyancy
caused by variations of atmospheric pressure.
This paper presents an attempt at furnishing to the chemist information which will enable him in a sim-
ple manner to determine atmospheric pressure and to use this information for analytical weighings. lt is
hoped that in this way a contribution is being made towards the full utilization of the inherent accuracy
of modern analytical balances by those who need it.
One must first distinguish between the determination of the absolute mass i.e. the reduction of the
found values to vacuum atmosphere and the determination of the influence of changes of air buoy-
ancy. The latter are significant also in „relative" weighings.
Usually, only the second case will be of practical interest. Since the required corrections in this case
amount to only 1/10th of those required in the first instance, it may, sometimes, be unnecessary to ap-
ply these corrections at all. This does not mean that one should not be aware of the magnitude of cor-
rection and consider the questions of its application.
Calculating variations of atmospheric pressure
The atmospheric pressure is governed by the barometer reading, which in turn depends on tempera-
ture and absolute air humidity. The latter can be determined from the relative humidity and tempera-
ture. The following equation indicates the relation between the change of air pressure UD in % and
these factors. Herein, Ub is the Variation of the barometer reading in mm of mercury, Ut the change
in temperature, t the temperature in °C and Uf the change of the relative humidity in %.
(1) UD = 0,13 Ub - 0,35 Ut – 0,0005 t ∙Uf [%]
Thus, a change of air buoyancy of 0.1 % is caused at t = 20° by a change in air pressure of Ub = + 0.8
mm Hg or a temperature Variation of Ut = - 0.29° or a change in humidity of Uf = -10%.
This equation applies in the range from 5 to 30°C. The tables, fig. 1 facilitates the use of this equation
since the three factors can easily be found there.
One can see at first glance that the effect of humidity changes is relatively small for which reason it
can often be disregarded. The following example is indicative of the actually possible conditions:

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The temperature in a laboratory may be 17°C in the morning and 21° in the afternoon when a weighing
is repeated which was first made in the morning. lf one assumes that during the same time, the humid-
ity fell from 50% to 40% and the barometer from 758 to 755, which variations are not uncommon, then
using fig. 1, we obtain:
UD = -0,4 – 1,4 - +0,1 = -1,7%
Fig. 1 Diagram to ascertain variation of air buoyancy. To determine the influence of humidity read from percentage of Variation
of humidity to the left to temperature (not difference of temperature). Read Variation of air buoyancy below. Then add re-
sults of Variation of temperature,
The buoyancy of the air decreased by approx. 2%. The following example shows the effect of this
change on the evaluation of a weighing.
Calculating the buoyancy difference
Since the buoyancy is the weight of the replaced
ML
(2) A = MS
∙M
the change of the buoyancy is
UAUA
(3) M = 12 ∙MS10-6
wherein UD must be entered as % age. If M is given in grams, UA result from equation (2) by entering
for air buoyancy MLthe median value of 1,2 x 10 –3 g/ml.
Example:
For M = 100 g and MS= 1 the relative change of mass for the air buoyancy change of - 1,7 % is:
von 1
107112 6
−
−∙,∙
= 20 ∙10-6, i.e. for 1 g of
the sample weight the difference is 20 µg, for 100 g. 2.0 mg.