Foresight review
In the last issue of this series, Xiaobian mainly focused on the weighing principle of electronic balances, the definition and classification of calibration, the basic knowledge of weights and the relationship with the accuracy of balances. In the classification of the calibration, I spent a lot of detail on the elaboration of the calibration. I must have been deeply impressed by the rigorous and sophisticated balance calibration technology. I don’t know if Xiaobian should explain the complicated mathematical principles as thoroughly as possible. Has the method allowed everyone to unravel the doubts in their hearts? In fact, in the weighing of the balance, there is also an invisible big hand firmly controlling the weighing result, which is the temperature. This issue of Xiaobian will show you the wonderful magic of this big hand!
Legacy issues of weighing principle
In the last sharing of calibration, Xiaobian gave a brief introduction to the weighing principle of electronic balances. At the same time, it also mentioned that environmental factors such as temperature and humidity also affect the sensor of the electronic balance, but as for how it affects, it only sells. It’s a pass. So today we will come inside the sensor of the electronic balance to explore how temperature affects weighing.
Electronic balances generally use electromagnetic force balance sensors, the weighing principle is shown in the following figure:
The electromagnetic balance sensor is in an initial equilibrium state before the electronic balance is loaded. When the object to be tested is placed on the weighing plate, the column and the visor move downward under the action of the gravity of the object to be measured, and the photosensitive diode D2 detects the light emitted by the light-emitting diode D1, and generates a current signal, which is subjected to I/V conversion. The circuit and the PID regulator are converted into a current corresponding to the weight of the measured object and drive the moving coil. Under the action of the magnetic field of the permanent magnet, the moving coil generates an upward electromagnetic force to move the light shielding plate upward, and the current signal of the D2 output is reduced. Small, until the visor returns to the initial equilibrium position, the output current of D2 drops to zero. At this time, the electromagnetic force F generated by the moving coil is equivalent to the gravity of the object to be measured , that is, F=G=mg , where m is the mass of the object to be measured, and g is the acceleration of gravity. ã€1】
At the same time, according to the electromagnetic force formula F=BLI sin θ, where B is the magnetic induction intensity of the air gap magnetic field, L is the effective length of the moving coil (forced wire), I is the moving coil current, and θ is the angle between the energized conductor and the magnetic field. Since the size of the moving coil in the sensor is fixed, both B and L are no longer changed, and θ is 90°, so sin θ=1, so the size of F corresponds to I. Combining the previous description, we get m=BLI / g . ã€2】
When the temperature is constant, B and L are constant values, and g is also a constant value. Then m is proportional to I. By detecting the moving coil current, the mass of the measured object can be obtained indirectly. When the ambient temperature changes or the overcurrent element heats up, both B and L will change, causing m and I to no longer be proportional, causing the electronic balance to produce large nonlinear measurement errors.
It is worth mentioning that when the electronic balance is in the warm-up phase, as the internal temperature increases, the magnetic induction B will gradually decrease, and I will also decrease, which will cause the electromagnetic force F to become smaller and the balance to lose balance. The indication will exhibit a positive unidirectional drift. The balance is only fully preheated to bring the magnetic steel to thermal equilibrium. At the end of this process, the balance is balanced, and the peeling function is used to zero the display. The balance is now in a real usable state. ã€2】
The invisible hand of manipulating the balance
Electronic balances vary according to the environment in which they are located. Under normal circumstances, balances of different accuracy levels have different requirements for temperature range and temperature fluctuation. The higher the accuracy level, the more stringent the requirements for ambient temperature. According to the relevant provisions of national standards, the normal working conditions of electronic balances need to meet the specific requirements of the following table:
The main effect of temperature is that the change will bring about thermal expansion and contraction. The electronic balance is reflected in the change of the gap between the small and precise components in the sensor. These changes will be recorded by the sensitive balance, thus affecting the reading. accuracy. If there is no specific operating temperature range, the normal temperature condition of the electronic balance is 10 °C ~ 30 °C, the measurement performance should meet the indication error of the single weighing result according to the national standard, and the weighing is repeated or weighed at different positions. Relevant provisions for indication error (repetition and eccentricity).
Temperature change is one of the important factors affecting the accuracy of weighing results of electronic balances. The laboratory has a certain temperature difference between morning and noon, as well as the electronic balance equipment heating, personnel flow, etc., between the highest temperature and the lowest temperature of the day. It is often able to reach 10 °C. The impact of this balance is obvious, so what can we do to eliminate the effects of temperature symmetry results? First of all, during the use of the balance, it should be in a relatively stable environment as much as possible. When the ambient temperature of the balance changes greatly, the weighing result of the balance will drift, for example, from the low temperature warehouse. In a warm laboratory, the balance needs to be energized for a certain period of time in the use environment; secondly, when the temperature changes beyond a certain range, we can eliminate this drift by calibration.
The length of the energization time can effectively avoid the influence of temperature changes on the balance. In general, the higher the accuracy of the balance, the longer it takes to warm up. Xiao Bian here suggests that one hundred thousandth of a day's warm-up time is more than four hours, and one-tenth of a tenth of a day's warm-up time is more than one hour.
Fun temperature compensation, all in Ohaus electronic balance
For electronic balances, a good structural design should take full account of the effects of the temperature symmetry system and take measures to reduce or eliminate the effects of temperature changes. In the design, Ohaus electronic balance carefully evaluates the influence of the temperature symmetric weight system, and optimizes the mechanical design, component selection, and intelligent algorithms to eliminate the influence of temperature and ensure that the balance is within the range of rated temperature. The metering performance meets the requirements of international regulations such as OIML.
From the entry-level Pioneer CP series and the Adventurer AR series, to the advanced Adventure AX series, to the most advanced Explorer EX series, and finally to the Explorer quasi-micro balance (EX5) series, all with dynamic temperature compensation, real-time correction of the environment The effect of temperature symmetry results. In particular, the AutoCalTM fully automatic calibration system in the full range of Explorer and some AX series balances automatically reacts to the most real-time response to temperature drift and time drift, when the temperature drift value exceeds ±1.5 ° C or between 3 and 11 hours. When the user can customize the internal calibration time, the balance calibration is automatically triggered to completely eliminate the adverse factors caused by the external environment.
How, Xiaobian's professional and comprehensive explanation has made your understanding of the complex and profound temperature "magic" clear and thorough? If you have more questions about the impact of temperature on the balance, or are looking for more professional and detailed selection guide, please call 4008-217-188, or click to enter the "request information" and leave relevant information, our professional engineers We will contact you in the first time!
references:
[1] Sun Penglong, He Kaiyu, Bu Xiaoxue, Li Pengfei, Shi Lei. Effect of Ambient Temperature on the Accuracy of High Precision Electronic Balances[J]. Metrology & Testing Technology, 2016, 43(10): 34-35.
[2] Tang Hui, Shang Hongtao, Liu Xiangbing. How to improve the accuracy of electronic balance weighing [J]. Medical equipment
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