In general, improper calibration and use are the main causes of errors. Careful operation in this correct way can be carried out with as little error as possible and the highest accuracy can be obtained.
1. The temperature of the measuring device
The capacity of the gauge changes with temperature. The temperature at which the gauge is measured in or out of its nominal capacity is the standard temperature. The coefficient of bulk thermal expansion of the glass used to make the measuring instrument is in the range of about 10 10-6 30 10-5k-1. A gauge with a body thermal expansion system of 30 10-6k-1 (made of soda-lime glass) is calibrated at 20 and only shows 0 when used at 2702% additional error, which is less than the limit error of most gauges, it can be seen that the standard temperature is not important in practical use, but in order to provide a good calibration benchmark, it is important to specify a standard temperature and equilibrium at that temperature before calibration.
2. The temperature of the liquid
The accuracy of measuring the temperature of the water used in the calibration gauge should be 01. When using the measuring device, it should be ensured that all its liquids should be at the same room temperature when measuring their volume.
3. Cleanliness of the glass surface
When measuring the volume of liquid in or out, the gauge is related to the cleanliness of its inner surface, and poor cleanliness will cause errors due to meniscus deformity, and there are two kinds of defects in this meniscus. The glass surface is not completely wet, i.e. the liquid surface is in contact with the glass at a distinct angle, rather than forming a curve tangent to the glass surface. The surface tension is reduced due to contamination of the liquid surface, resulting in an increase in the radius of curvature. If the inner wall of the measuring instrument used to measure the liquid is not clean, the liquid film on the inner wall will be distributed irregularly or incompletely, causing errors. If there is chemical pollution, even if it does not affect the capacity, it may cause errors due to the change of concentration due to chemical reactions. Containers equipped with grinding plugs should pay special attention to cleaning the grinding area.
To determine whether the gauge is thoroughly cleaned, it should be observed during filling (the dispenser should preferably be filled from below the liquid level, i.e. from the lower part of the valve of the burette, or from the flow port of the pipette), and the meniscus should be raised without deformation (i.e. without wrinkling at its edges). After filling the liquid beyond the nominal volume, the excess liquid should be drained (the dispensing meter should be drained through the flow port, and the metering meter should be sucked out using a straw). The upper glass surface should be kept evenly moistened, and the meniscus should not wrinkle and deform at the edges.
4. Adjust the concave liquid level
The concave level refers to the interface between the liquid and the air of the volume to be measured.
Most gauges can be used to set and read the concave level using a reference line or an index line. The concave level should be adjusted in such a way that the lowest point of the concave level should be tangent to the horizontal plane of the edge of the index line, and the line of sight should be on the same level as the edge of the index line. However, the mercury concave level should be tangent to the edge of the index line at its highest point, and when using opaque liquids, the horizontal line of sight should pass through the upper edge of the concave surface, and appropriate calibration should be done if necessary (see figure).
Proper arrangement of light can make the meniscus dull and well-defined, so it should be lined with a white background and unwanted stray light should be blocked. For example, a black paper tape or a thick black hose cut in a short section can be placed on the wall of the measuring instrument at a distance of no more than 1 mm below the liquid level of the measuring instrument.
When the index line is long enough to be viewed from the front and back of the gauge at the same time, the line of sight should be in a position where the front and back of the upper edge partially coincide to avoid parallax. When the gauge only has an index line in front, use a black shade band to adjust the edge of the index line, the parallax is negligible, but it should also be noted that the eye should read in the same level plane as the edge of the index line.
5. Outflow time
In the case of a dispensing meter, the measured capacity is always less than the measured capacity due to the liquid film left on the inner wall of the measuring instrument. The capacity of the liquid film is related to the time when the liquid flows out. When the outflow time exceeds a certain value, the capacity of the remaining liquid film is extremely small and unchanged, so the influence of the error in measuring the liquid volume is negligible.
The same effect can be achieved by dividing the outflow time into several segments with a certain waiting time, and reading errors can occur when the flow port is broken or blocked, and any change in the size of the flow port in order to increase the flow rate. This error reduces the accuracy of the readings and is not estimateable.
The outflow time is applicable to the meter-out glass gauge with water as the liquid. There will be no unreasonable capacity difference if the actual egress time varies within this range, but for safety reasons, the egress time range should be specified. The outflow time can still be marked on Class A burets and pipettes, and the user can check if the outflow is blocked or damaged by measuring the outflow time.
6. Summary
These influencing factors of the accuracy of the laboratory glass measuring instrument are really not a trivial matter, and at some point, these influencing factors will become the main contribution to the total uncertainty, and must be paid attention to.
*: Huankai** in "Food Microbiological Testing"**
Note: All content such as articles, **, etc., is only used for learning and Xi exchanges, if there is infringing content and other law-related content, please contact us to delete or modify it in time, hereby declare!