These factors can affect the accuracy of the glass gauge in the laboratory

1.The temperature of the gauge

The capacity of the gauge varies with temperature. The temperature at which a gauge is measured in or out of its nominal capacity is the standard temperature. The body thermal expansion coefficient of glass used in the manufacture of gauges is in the range of 10×10-6~30×10-5 K-1. A personal thermal expansion system of 30×10-6 K-1 (made of sodium-calcium glass) gauges calibrated at 20 ° C show an additional error of 0.02% when used at 27 ° C, which is less than the limit error of most gauges. Thus, the standard temperature is not important in practical use, but in order to provide a good calibration reference, It is important to specify a standard temperature and to equalize the gauge at that temperature prior to calibration.

2.Liquid temperature

The accuracy of measuring the temperature of the water used in the calibration gauge shall be ±0.1℃. When using the gauge, all the liquid in the gauge shall be at the same room temperature when measuring its volume.

3.Cleanliness of the glass surface

The measuring device is related to the cleanliness of its inner surface when it measures the capacity of the liquid. The cleanliness difference will cause errors due to the deformity of the bending liquid surface. There are two kinds of defects of the bending liquid surface. The surface of the glass is not completely wet, that is, the liquid surface is in contact with the glass at a distinct Angle, rather than forming curves tangent to the surface of the glass. The radius of curvature increases as surface tension is reduced due to surface contamination of the liquid. If the inner wall of the measuring device is not clean, the distribution of the liquid film on the inner wall is irregular or incomplete, which causes errors. If there is chemical contamination, even if it does not affect the capacity, it may also be due to chemical reaction caused by concentration changes and errors. Containers with grinding stoppers should pay special attention to cleaning the grinding area.

To determine whether the gauge is thoroughly cleaned, should be observed when filling the liquid (the quantity of the gauge is best from below the liquid level, that is, from the lower part of the plug valve of the burette, or from the flow mouth of the pipette), the bending liquid level is increased without deformation (that is, in its edge without wrinkling). When the filling liquid exceeds the nominal capacity, the excess liquid should be discharged (the discharge meter should be discharged through the flow port, the inlet meter should be sucked out by a pipette). The upper glass surface should be kept moist and uniform, and the bending surface at the edge should not be wrinkled and deformed.

4.Bending level setting

Bending surface refers to the interface between the liquid and the air with the capacity to be measured.

Most gauges can be set and read with reference lines or indexing lines. The bending level should be set as follows:

The lowest point of the bending liquid surface should be tangent to the horizontal plane of the edge of the dividing line, and the line of sight should be on the same horizontal plane with the edge of the dividing line. However, the mercury bending surface shall be horizontally tangent to the edge of the dividing line at its highest point. When using an opaque liquid, the horizontal line of the line of sight shall pass through the upper edge of the bending surface, and appropriate calibration shall be made if necessary.

The proper lighting arrangement can make the bending surface dim and clear. For this purpose, a white background should be used and unwanted stray light should be blocked out. For example, a piece of black paper tape or a small piece of black thick rubber hoop cut into the gauge wall can be placed no more than 1mm below the gauge positioning liquid level.

When the length of the indexing line is long enough for simultaneous observation from the front and back of the gauge, the line of sight should be at a position where the front and back of the upper edge partially coincide to avoid parallax. When the meter only has a dividing line in front, use a black shading belt to set the dividing line edge, parallax can be ignored, but it should be noted that the eyes should read in the same horizontal plane with the dividing line edge.

5.Outflow time

For a measuring gauge, the output capacity is always less than the input capacity due to the remaining liquid film on the inner wall of the measuring gauge. The volume of the liquid film is related to the time of the liquid outflow. When the effluent time exceeds a certain value, the capacity of the remaining liquid film is minimal and constant, so the effect of the error in measuring the liquid capacity is negligible.

The same effect can be achieved by dividing the outflow time into several segments with a waiting time, when the flow orifice is damaged or blocked, and any attempt to change the size of the flow orifice in order to increase the flow rate will cause reading errors. This error reduces the accuracy of readings and is immeasurable.

The effluent time is suitable for glass measuring devices with water as liquid. The actual outflow time varies within this range without an unreasonable capacity difference, but for safety, the outflow time range should be specified. The drain time can still be marked on the Class A burette and pipette and the user can measure the drain time to check for blockage or damage of the flow port.

Reminder: the laboratory glass measuring accuracy of these factors, really is not a storm in a teacup, at some point, these factors will become the total uncertainty of the main contribution, must pay attention to.