THIS IS A VERY SELECTED AND IN PARTS EDITED EXTRACT FROM ISO 8655 PROVIDED AS A MINIMUM GUIDELINE TO THE PIPETTE CHECKING PROCEDURE : To ensure a full understanding of the Standard please consult your personal copy of this Standard.
4.1 Analytical balance or equivalent weighing device, with a resolution appropriate to the selected volume of the apparatus under test (see Table 1).
The test shall be carried out in a draught-free room with a stable environment. The test room shall have a relative humidity above 50% and a constant ( +/- 0.5oC ) temperature between 15oC and 30oC. Prior to the test, the apparatus to be tested and the test water shall have stood in the room for a sufficient time, at least , to reach equilibrium with the room conditions.
Especially for small volumes below , errors due to evaporation of the test liquid during weighing shall be taken into consideration. Apart from the design of the weighing vessel (4.3), the test cycle time is important.
6.4 Test cycle time
The test cycle time (time required to complete the weighing of one dispensed volume) shall be kept to a minimum. It should ideally not exceed 60 secs. It is important that it is regular, both within each cycle and as far as possible from cycle to cycle.
7.2.4 Prepare the piston pipette and the test cycle as follows:
a) Fit the selected tip to the piston pipette.
b) Fill the tip with test liquid and expel to waste five times to reach a humidity equilibrium in the dead air volume of the air-displacement piston pipette.
c) Place the weighing vessel with its added water on the balance pan.
7.2.5 Perform the following test cycle (see Figure 1 and Figure 2):
a) Replace the disposable tip of the piston pipette.
b) Fill the piston pipette with test liquid, immersing its delivery orifice to below the surface of the water.
Release the operating button slowly, if hand operated, and withdraw the pipette vertically and carefully from the surface of the water. Touch the delivery orifice against the side wall of the container with the test liquid.
c) Expel the water to waste in order to pre-wet the tip and refill the piston pipette as described in b).
d) Record the mass of the weighing vessel to the nearest readable graduation as in Table 1, or tare the balance to zero ( ).
e) If the weighing vessel has a lid, remove it. Deliver
the contents of the pipette into the weighing vessel, touching the delivery end
of the pipette tip against the inside wall of the vessel just above the liquid
surface at an angle of approximately to and draw it approximately to along the inner wall of the weighing vessel to remove
any droplets at or around the tip orifice. Replace the lid if applicable.
Where applicable, use the blow-out feature of the piston pipette to expel the last drop of liquid before drawing the delivery end of the tip along the inner wall of the weighing vessel.
If it is necessary to remove the weighing vessel from the balance pan to permit delivery of the dispensed volume, avoid excessive handling and possible contamination by the use of lint-free gloves. Return the weighing vessel to the balance pan after delivery.
f) Record the mass m1 of the weighing vessel.
7.2.6 Repeat the test cycle described in 7.2.5 until 10 measurements have been recorded as a series of masses m1 to m10
Calculate the systematic
error es of the
piston-operated volumetric apparatus in microlitres
using the equation
or in percent using the equation
where Vs is the selected test volume and Vbar is the mean volume delivered at this testing.
Calculate the random error of piston-operated volumetric apparatus as the repeatability standard deviation. This random error can also be expressed as a percentage, by the coefficient of variation.