Working principle and production of brick moisture rapid tester

In the process of firing bricks and tiles, the moisture content of the bricks is a parameter that needs to be strictly controlled. According to the process requirements, the moisture content of the bricks should be controlled between 14% and 14.5%. At present, it is usually tested according to the drying method, that is, the bricks on site are sampled and weighed, and the weight of the sample after removing the moisture is weighed after drying, and the moisture content of the mud bricks is calculated. This detection method has high accuracy, but the cycle is too long, and it usually takes several hours to get the test results. This article introduces a brick moisture content measuring instrument with simple circuit, easy installation and adjustment, convenient use, rapid testing and high accuracy, which is particularly suitable for on-site measurement and sampling detection.

1. Working Principle

This tester is based on the principle that the resistance value of the brick changes when the moisture content changes. It can distinguish a moisture content change of 0.1%. At the same time, because the measuring section is controlled to be small, its accuracy can reach a high degree. The electrical principle is shown in Figure 1.

The moisture content sensing signal is obtained by the detection probe inserted into the brick. The greater the moisture content, the smaller the resistance at both ends of the probe; the smaller the moisture content, the greater the resistance of the probe. The change in the probe resistance corresponds to the change in the moisture content of the brick. The core component in Figure 1 uses a monolithic integrated circuit LM3914 (domestic model SF3914, a product of Shanghai Radio Factory No. 7) that can drive 10-bit LEDs for linear analog display. The LM3914 has the characteristic that the number of LED display digits is in a good linear relationship with its input voltage. Its internal circuit is shown in Figure 2. There are 10 voltage comparators inside the LM3914, which are connected to a floating ten-level high-precision voltage divider between pins ④ and ⑥. It also has a floating DC voltage reference source, input buffer, and display mode selection amplifier circuit. The voltage between the reference voltage pins ⑦ and ⑧ is 1.25V, so a voltage divider of 125mV, 250mV...1250mV can be obtained at the in-phase end of the comparator respectively. The reference voltage source can provide a maximum current of 3 to 4 mA, and the drive current of each LED is basically 10 times the output current of the reference voltage source, that is, the drive current of each LED in Figure 1 is ILED = 10×1.25/R5 (mA).

LM3914 also has a dot/line display selection function. When its 9th pin is connected to the 11th pin, the dot graph display is selected. At this time, only a certain number of LEDs are displayed, and the number value corresponds to the input voltage. When the 9th pin of LM3914 is connected to the Vcc terminal, the line graph display function is selected. At this time, several LEDs are displayed, and the number of LEDs displayed corresponds to the input voltage.

In order to improve the measurement resolution, the circuit in Figure 1 adopts the method of graded testing. Since the moisture content of the brick is not in strict proportional relationship with its resistance change, but has a large nonlinearity, the use of graded testing is conducive to improving linearity and accuracy. In the figure, the internal reference voltage source UREF (1.25V) of LM3914 is divided by R1-R4 as the in-phase input of op amp A1. According to the value of its voltage division, its moisture content measurement area can be determined, so K1 can be adjusted to select the measurement area. For example, when switch K1 points to "14%", the selected moisture content measurement area is 14% to 15%. According to the actual needs of brick making equipment, the circuit in Figure 1 selects a total moisture content measurement area of ​​13%-16%.

The measurement process can be analyzed as follows: Assume that K1 points to 14%, then the output voltage U01 of A1 in the circuit will be zero when the moisture content of the brick is 14%. When the moisture content of the brick changes to 14% + 0.1x%, A1 and A2 are proportionally converted and amplified, so that LM3914 drives x LED displays (or the xth LED display), so the moisture content of the measured brick is equal to the thousandth of the LED display value driven by LM3914 plus 14%. Similarly, it can be deduced that when K1 points to other gears, the measured value is equal to the gear indication number plus the thousandth of the LED display value. In this way, through gear conversion, the circuit can be adjusted when the moisture content is

In the measurement range of 13%-16%, it is always possible to distinguish a 0.1% change in the indicated value. Switch K2 in Figure 1 is a dot/line conversion switch, which can enable the LED to realize dot graph display or line graph display as needed.

2. Debugging and production

During debugging, the results measured by the drying experiment method can be used as the standard value, and the values ​​of the voltage-dividing resistors (R1~R4) can be adjusted to make the circuit measurement accuracy meet the requirements. Here, taking the "13%" gear as an example, the debugging steps are as follows: first, set the switch K1 knife to the "13%" gear, insert the sensor probe into the mud brick with a moisture content of 13%, adjust the R1 value in a small range, so that the output voltage of A1 is zero, and then insert the probe into the mud brick with a moisture content of 14%, and adjust the R6 or R7 value until all the 10-bit LEDs of LM3914 are just displayed. At this time, it means that the measured value is 14%. After R6 or R7 is adjusted, the value does not need to be adjusted in the subsequent debugging of each gear.

Finally, follow the above steps to adjust each gear in turn and adjust the value of each voltage divider resistor to make the moisture measurement accuracy meet the requirements.

To achieve high-accuracy measurement, each resistor element should be a metal film resistor or a wirewound resistor with good stability. The circuit in Figure 1 uses the precision op amp OP07. If the requirements are not very high, an LM358 dual op amp can be selected.

In order to make the circuit work properly, ±5V dual power supply should be used. The brightness of LED display is determined by R5, and the value of R5 is generally selected to be 1~2.7kΩ. The sensor probe is made of bakelite and stainless steel, and the dimensions are shown in Figure 3. Because the insertion depth of the sensor probe has a certain influence on the test results (the maximum influence exceeds 0.5%), the depth of the probe inserted into the mud brick should be kept consistent during each test.

Since the temperature, texture and impurities of mud bricks have a certain influence on the measurement results (for example, according to the author's test, the absolute indication error increases by about 0.15% for every 10°C change in temperature), a small range indication correction should be performed when using this brick moisture tester. In order to eliminate the influence of the composition differences of various parts of the mud bricks on the test results, the method of taking the average value of multiple measurements should be adopted.