Instructions

TECHNICAL NOTE
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Flexible discharge tubing
When dispensing media with bottle-top dispensers, the sample
cup is usually moved into position at the tip of the discharge
cannula. In the case of small-volume sample containers, such
as test tubes or cuvettes, which are placed in stands for better
handling, it is advisable to do the opposite. This is where flexi-
ble discharge tubing comes in handy, as it can be moved to the
sample container.
It should be noted that for small dispensing volumes, the
piston must be depressed gently. Otherwise, there is a risk of
over-dispensing due to the spit eect. The spit eect originates
from the principle of mass inertia: the moving mass of the
liquid to be dispensed has an inherent momentum which is
proportional to the dispensing speed. To slow this momentum,
the speed of the liquid must be reduced to zero, which occurs
at both the upper and lower stopping points of the piston
during the dispensing process. We describe the change of the
momentum over time as a force whose magnitude depends on
the acceleration – a negative sign is used when braking acceler-
ation. If the liquid is traveling at a high speed at the reversal
point of movement, and this speed is also abruptly reduced to
zero, relatively high forces are produced in the system. In other
words, you can say that the moved liquid wants to pull more
liquid from the bottle until the forces are balanced again.
On one hand, these forces opposing the moving force are the
friction forces between the liquid and the surfaces as well as
the viscosity of the medium; on the other hand, they are the
restoring forces of the valve spring in the discharge valve. If
the movement force of the liquid is too high at the movement
reversal point (minus the friction forces and viscosity) in com-
parison to the restoring force of the valve spring, the discharge
valve opens for a fraction of a second and additional liquid
flows through which is discharged as a “spit drop.” In prac-
tice, this drop increases the dispensed volume as the sample
container is already under the tip of the discharge cannula
or discharge tube at this time. Although the spit drop only
amounts to a few microliters, it must be closely observed such
as during calibration.
Important information: the described spit eect is universal
and applicable to all dispensers on the market.
Bottle stand
It is always good practice to secure bottle-top instruments
such as the Dispensette®, seripettor®, and Titrette® from falling
over, especially when small or light bottles are being used.
The existing metal laboratory rods that are fixed to the wall
or furniture in many laboratories can be used to secure the
bottle-top instrument in addition to sleeves and clamps.
However, this solution is quite inflexible and quick reposi-
tioning of instruments requires a bit of practice – especially
if clamps are not yet installed at the new position. The bottle
stand from BRAND assists you with quick location changes.
Instruments are securely connected to a base plate via a stand
with clamp. The center of gravity is shied downwards by
the weight of the bottle holder and the base plate increases
the standing surface.
Work eiciently with flexible discharge tubing
Bottle stand to protect against tipping