First of all, we must know that the method of calibration (calibration) of the pipette and pipette according to their respective measurement standards (JJG 196 and JJG 646) is exactly the same, that is, the weighing method is used to calibrate the pipette volume.

The calibration operation is roughly described as follows:

1. Under constant temperature and humidity environment, peel the weighing container on the analytical balance;

2. Absorb the distilled water of nominal range (ie the maximum range), inject into a weighing container and weigh to obtain the weight of discharged distilled water Δm;

3. At the corresponding water temperature, convert the weight of distilled water Δm to the volume using the Kt value of water, that is, V=Kt*Δm. The formula of Kt value can check the corresponding measurement standard. It can be known from the formula that the main influencing factor of the Kt value is the density of water.

Secondly, from the measurement method point of view, the method of calibrating the volume of the pipette and pipette is almost the same. If the nominal accuracy is the same, is the accuracy of the actual operation of liquids other than distilled water exactly the same? the answer is negative.

In order to understand this, we need to know the difference between the pipette and pipette structure and the principle of pipetting.

The pipette is a simple physical structure, generally made of glass, and the volume is determined by the scale. When calibrating the corresponding scale of the corresponding volume, the method used is the above calibration method. The corresponding quality of the corresponding volume of distilled water is fixed. If the quality of the distilled water discharged after absorption is "correct", it means that the distilled water discharged after absorption The volume is "correct (within the allowable error range of the nominal value)", then the suction volume represented by the lowest point of the concave liquid surface where the distilled water is located when suctioning is "correct", this point is the corresponding volume Where the scale is. Once this point is determined, the volume inside the pipette is physically determined, and then this volume hardly changes with external factors such as liquid density, air pressure and other factors.

The pipette in the general sense is an air piston pipette. When the pipetting button is moved, the position of the internal piston changes, which in turn generates negative pressure, thereby drawing liquid into the suction head. The determination of the suction volume is determined by the internal and external pressure difference and a series of other factors, but the main determinant is the internal and external pressure difference. When calibrating, if the quality of the distilled water sucked out is "correct" (weighing method), it means that the volume of distilled water drained after sucking is "correct (within the allowable error range of the nominal value)" , Then it means that the volume of pipetting is "correct". Therefore, when determining the pipetting volume of a pipette with distilled water, what is actually determined is the combined effect of the pressure difference between the inside and outside of the pipette and a series of other factors that can exactly absorb the correct volume of distilled water.

At this time, it is easy to understand that the pipette will perform differently when it sucks different types of liquids and under different environmental conditions. For example, when there is no change in the environmental conditions, that is, the pressure difference is constant, if the density of the liquid is different from distilled water, that is, the mass of the same volume of liquid is different from distilled water, in order to balance the internal and external pressures, the volume of liquid absorption will be different.