ameters. According to process requirements, brick water should be controlled between 14% to 14.5%. Drying method is usually detected by the current, soon-site sampling brick weighing, after drying the sample said that after removing the weight of water, mud by the moisture content calculated. This means of detection accuracy, but the cycle is too long, the general required several hours to get test results. This paper describes a simple circuit, alignment is easy, convenient, rapid and accurate test brick high water content measuring instrument, especially for field measurements and sampling.

 
First, the working principle
The tester is based on when the water content of the brick is relatively resistance measurement principle of change. It can identify changes in water content of 0.1%. Measurement control and because a smaller section, it can achieve a higher degree of accuracy. Electrical circuit shown in Figure 1.
Moisture sensing signal from the probe be inserted into the detection of brick. Water content the greater the resistance the smaller the probe ends; water content smaller, the greater the resistance of the probe, the probe resistance changes in water content corresponds to the brick. Figure 1, the core components used to drive 10-bit linear analog display LED monolithic integrated circuit LM3914 (domestic model SF3914, Shanghai radio seven plant products), LM3914 LED display digits with its input voltage into a good linear relationship characteristics, and its internal circuit shown in Figure 2. LM3914 internal voltage comparator 10, and ④, ⑥ foot floating between the ten high-precision voltage divider is connected, it has a floating DC voltage reference source, the input buffer and display mode selection amplifier circuit. Reference voltage ⑦, ⑧ pin voltage between 1.25V, respectively, so the comparator can be compared side with access to 125mV, 250mV ... 1250mV partial pressure. Voltage reference provides 3 ~ 4mA maximum current, and each LED drive current is basically a reference voltage source output current of 10 times that in Figure 1 for each LED drive current ILED = 10 × 1.25/R5 (mA .)
LM3914 also has a point / line display options. ⑨ feet when it is connected with the 11 foot figure shows the selection of points, then only a certain number of the LED display, which number corresponds to the input voltage. When the LM3914's ⑨ Vcc terminal connected to the feet and the selection of line graph display, this time a number of digital LED display, LED display of its number corresponds to the input voltage.
In order to improve the measurement resolution, Figure 1 circuit uses the sub file testing. Water content of the brick is not as strict with their change in resistance proportional relationship, but has a large non-linear, the use of sub-file tests will help to improve linearity and improve accuracy. Figure LM3914 internal reference UREF (1.25V) is R1-R4 divider was used as the op amp noninverting input of A1, according to the size of its partial pressure can be measured to determine its water content region, so tune K1 can choose to measure the area. For example, when switch K1 point to "14%", the selected area is a good measurement of moisture content 14% ~ 15%. Figure 1 circuit device according to the actual needs of brick, the total water content measurement of the selected region is 13% -16%.
Measurement process can be this way: Suppose K1 point to 14%, when the A1 circuit output voltage U01. Will be when the brick is 14% water content equal to zero. When the brick water content was 14% +0.1 x%, A1, A2 conversion amplification proportionally, so x LM3914 driving a LED display (or x-bit LED display), so when the measured water content value will be equal to the LM3914 brick drive The LED display of thousands of scores with 14%, the same can be launched, K1 point to other files, the measured value is equal to the number of stalls with LED display indicates the value of thousands of points. In this way, through the stall converter, the circuit in the moisture content
13% -16% within the measurement range of 0.1% is always able to distinguish the indication changes. Figure 1 switch K2 to point / line switch, as needed, to achieve points graph LED display or line graph display.

Second, testing and production
Debugging can be drying experiment results measured as the standard value, adjust the resistance divider (R1 ~ R4) to meet the requirements of the circuit measurement accuracy. Here to "13%" file, for example, debug as follows: First set at 13% of the knife switch K1 "file place the sensor probe into the exact moisture content of 13% of mud brick, the small adjustment of the value of R1, so that A1 output voltage is zero, then the probe into the exact moisture content of 14% of mud brick, adjust the value of R6 or R7, the LM3914 is just a 10-bit LED display date, then that measurement is 14%, R6 or R7 adjustment After, after the commissioning of the file can not adjust the value.
Finally, the above steps by adjusting the file in order to adjust the sub-piezoelectric resistance, accuracy of the measurement of water to meet the requirements.
To achieve high accuracy measurement, the stability of the resistance elements should use a good metal film resistor or wire wound resistors. The circuit in Figure 1, precision op amp selected OP07, if the request is not very high, an LM358 dual op amp optional.

To make the circuit work properly, should be used ± 5V dual supply. LED display brightness determined by the R5, the general election R5 is 1 ~ 2.7kΩ. Sensor probe with a bakelite and stainless steel, dimensions shown in Figure 3. Sensor probe insertion depth for the test results have a certain influence (maximum of more than 0.5%), so the probe into the mud every time the depth of the testing process should ensure consistent.
Because mud temperature, texture and impurities have a certain influence on the measurement results (such as testing by the author, every change in temperature 10 ℃, the absolute error of indication it is about 0.15% increase), so in actual use when the brick moisture tester It should also be small-scale value correction. To remove mud composition differences in different parts of the test results should be used several ways to measure the average value.