Calibração Micropipetas

Método 1

1)  Ajustar o ‘dial’ da micropipeta para o volume mais baixo (1µl). Pipetar água

destilada para um balão/copo, previamente tarado. Pesar e registar o volume. Repetir

o processo cinco vezes.

2)  Ajustar o ‘dial’ da micropipeta para o volume mais alto (10µl). Pipetar água

destilada para um balão/copo, previamente tarado. Pesar e registar o volume. Repetir

o processo cinco vezes.

3) 

Ajustar o ‘dial’ da micropipeta para o volume médio (5µl). Pipetar água destilada para

um balão/copo, previamente tarado. Pesar e registar o volume. Repetir o

processo cinco vezes.

4) 

Decidir se as discrepâncias nas medidas são sistemáticas ou aleatórias. Se forem

sistemáticas será necessário desmontar a micropipeta e limpar ou repetir as pesagens.

Caso seja aleatória, é necessário melhorar a técnica de pipetagem.

5) 

Realizar este processo cada vez que seja necessário ou uma vez por mês.

Método 2

1)  Ajustar o ‘dial’  da micropipeta para o volume de 2µl. Tarar previamente a ‘ponta’.

Pipetar água destilada e retirar a ‘ponta’ da micropipeta e registar o seu peso. Repetir

o processo cinco vezes.

2)  Ajustar o ‘dial’  da micropipeta para o volume de 4µl. Tarar previamente a ‘ponta’.

Pipetar água destilada e retirar a ‘ponta’ da micropipeta e registar o seu peso. Repetir

o processo cinco vezes.

3)  Ajustar o ‘dial’  da micropipeta para o volume de 6µl. Tarar previamente a ‘ponta’.

Pipetar água destilada e retirar a ‘ponta’ da micropipeta e registar o seu peso. Repetir

o processo cinco vezes.

4)  Ajustar o ‘dial’  da micropipeta para o volume de 8µl. Tarar previamente a ‘ponta’.

Pipetar água destilada e retirar a ‘ponta’ da micropipeta e registar o seu peso. Repetir

o processo cinco vezes.

5)  Ajustar o ‘dial’  da micropipeta para o volume de 10µl. Tarar previamente a ‘ponta’.

Pipetar água destilada e retirar a ‘ponta’ da micropipeta e registar o seu peso. Repetir

o processo cinco vezes.

6)  Verificar se as medidas efectuadas dentro de cada um dos volumes são semelhantes e

se não existe muita discrepância entre os seus valores. Caso não haja esta semelhança,

repetir o processo após limpeza geral de toda a micropipeta.

 

The following comments are from my experience along with tips from

the folks at Ranin. YMMV. If you doubt my comments or your repair

skills, then pay someone else to do it: I recommend Ranin (in the US)

or any Gilson-approved repair facility.

Calibration is relatively simple. The tool needed depends on the age

of pipetmen. For older models you will need an allen (hex) wrench to

loosen the upper lock nuts, and also a slotted screwdirver for the

calibration disc. For the newer models you need something to turn the

calibrating disk. I'm sure there is a special tool for this that is

used by the Pros, but I found I can get by with a pair of very fine

needle nose pliers.

I should warn you, though, that calibration should almost never be

needed unless a unit is damaged. Normal wear should not result in the

need for calibration. This is because the calibration only affects the

upper point of travel of the plunger (hence the "zero" position). This

corresponds to the intercept on the plot of volume delivered versus

setting. The slope of such a plot is completely dependent on the

micrometer thread pitch and the piston diameter, which never

change. It is far more common that the delivery is off as a result of

a worn shaft seal or even possibly an old and inflexible shaft O-ring.

Also possible is a small crack in the shaft near the end. Or, when set

to the maximum volume, the seal sometimes falls off the plunger inside

the pipettman and the poor seal causes mis-delivery (this is fixed by

placing the seal back onto the plunger). Also possible is dirt

obstructing the shaft oriface or preventing a good seal at the O-ring.

Damaged internal parts (springs, couplings) also can result in

a poor seal at the O-ring. All of these possibilties should be

eliminated before concluding that recalibration is needed.

The basic idea of calibration is that the upper travel point of the

plunger is set by the vertical position of the orifice that the top of

the plunger (the part with the button is attached) last passes through

when it emerges from the botdy of the pipetman. This oriface is in the

center of the rotating knurled wheel that is used to set the volume.

If you have ever dissassembled a unit to replace the friction ring,

you know that the diameter of the bottom of the plunger is bigger than

the top (where the button attaches) and the junction of these two

regions acts as a "stop" at the calibration disc. If you remove the

micrometer/knob/plunger assembly and then remove the button (it snaps

off and on) you will see what I mean. To remove the old style assembly

(which does not include the micrometer), loosen the three lower hex

nuts in the knob and pull the knob+plunger out of the body. For

reassembly is easier if the piston and springs first are removed from

the bottom of the unit to relieve the pressure, as it is essential to

completely reseat the knob onto the top of the micrometer before

tightening the hex nuts. To dissassemble the newer style units

(lacking hex nuts in the knob), simply turn the knob until it is about

1.2 times the maximum rated volume, at which point the volume dial

stops changing and the entire assembly can be pulled out of the body.

This is obvious if the friction ring has disintegrated but is

 

difficult with a new ring. You should be able to disassemble and

reassemble a unit without altering its delivery vs setting function.

If it has changed, you have not reassembled it correctly (or possibly

the original problem was related to the shaft seal near the bottom of

the pipette and this was altered by the disassembly/reassembly

process).

To calibrate, you must have an idea of how much and which direction

the volume is off. It should be off by a constant no matter where it

is set. If not it probably does not need calibration but rather

it probably needs a new plastic shaft seal, or much less comonly, a

new o-ring. If it is delivering too little, you will need to move

the shaft set point UP (out of the pipette), else it must move it

down (into the pipette).

The set point can be changed with the unit assembled (though the

upper (brass) hex nuts must be loosened on the old-style units). It is

tricky to do this unles you have the right tool however, and it is

easier to change the setting if it is disassembled and the plunger

removed. However, the process of disassembly and reassembly for each

calibration change is time consuming and increases the chance that the

unit can be damaged. The set point is changed by turning the inner

disk (with the orifice for the plunger) that is threaded into the top

part of the knob. This disc is black plastic and has six equally

spaced holes on the newer units and is metalic and has two

slots on the older units. Initially, turn this disc one half turn

in the desired direction, then check the accuracy again. This tells

you how much change results from a half turn, and allows estimating

how much additional change is needed. Once the setting is correct, the

upper hex nuts on the old style units must be tightened. Since all the

units of a given size and model are threaded the same, once you know

how much volume corresponds to one turn, you should be able to set the

calibration on a similar model unit without guesswork. I did not write

down these numbers so I can't tell you what they are.