Two Examples of Negative Pressure Failure Repairs for Inspection Equipment

Many testing equipment, such as biochemical analyzers, hematology analyzers, blood gas analyzers, etc., are equipped with negative pressure devices, which play the role of sucking samples, reagents, and waste liquids. At work, we often encounter faults caused by negative pressure systems.

Example 1: Negative pressure failure of KX-21 blood cell analyzer of Japan East Asia Company

After turning on the power, the instrument is initialized and enters the working screen. Occasionally, an error alarm appears, and the screen prompts "Replace diluent". However, after replacing the diluent, the fault still exists.

Fault Analysis The diluent detection sensor of the KX-21 blood cell analyzer is a float switch. Under normal circumstances, the diluent outside the machine must be drawn into the buffer bottle inside the machine. First, the solenoid valves SV-1 and SV2-2 are instructed to work to form a negative pressure in the buffer bottle, so that the diluent in the buffer bottle rises to a certain height within the specified time, so that the float triggers the liquid level switch to turn on and transmit the signal to the CPU: However, if the liquid level does not reach the required level within the specified time, the CPU cannot detect the diluent level connection signal, and the instrument prompts "Replace diluent" error. Therefore, the following situations will cause this fault: ① Insufficient diluent; ② Insufficient negative pressure added to the buffer bottle; ③ Failure of the float switch; ④ CPU board failure.

Fault repair According to the above analysis, we adopt the elimination method one by one. First, we check the diluent, which is sufficient and no problem; second, we use a multimeter to measure the float switch. The resistance value changes from infinity to zero between the lower and upper positions of the float block, indicating that the float switch is working normally. Then we carefully observe the fault phenomenon and find that the diluent in the buffer bottle rises very slowly when it is sucked. Therefore, it can be determined that the possibility of CPU damage is very small, and the focus is on checking the insufficient negative pressure in the buffer bottle. The negative pressure in the bottle is provided by the original negative pressure of the compressor. The original negative pressure value is -50mmHg directly measured by the negative pressure gauge. It is completely normal. According to the pipeline diagram, the original pressure is introduced into the buffer bottle one way and into the SVR rotary drive cylinder another way. Therefore, it is suspected that there is occasional leakage during the working process. We repeated the specimen to reproduce the fault, and then used soapy water to check the leakage of each component of the gas circuit joint one by one. Obviously, there is leakage in the middle of the cylinder driving the SVR shaft cylinder. Carefully disassemble the cylinder and find that it is caused by the aging and deformation of a sealing O-ring. This O-ring can be found on the market for a few cents and the instrument worked perfectly after the replacement.

Example 2: Negative pressure failure of Hitachi 7020 fully automatic biochemical analyzer

After testing several samples, it was found that a large amount of liquid in the reaction cup overflowed onto the reaction disk. There was no error alarm on the instrument screen, but the measurement could not continue.

Fault analysis: After careful observation, it was found that there was liquid left at the A suction nozzle among the 13 suction nozzles of the cleaning arm, and the A suction nozzle is used to attract the reaction liquid after the measurement, so it was judged that the negative pressure of the A suction nozzle was insufficient. According to the pipeline diagram, the causes of this fault are: ① The suction nozzle is blocked; ② There is air leakage in the pipeline; ③ The electromagnetic valves SV3, SV11, and SV14 are not working properly; ④ The vacuum pump is faulty.

Fault repair: Based on the above analysis, we also adopted the method of eliminating one by one. Because the other steps requiring negative pressure are normal, the vacuum pump fault can be ruled out; after cleaning the suction nozzle with the 0.5mm stainless steel wire provided, the fault still exists, so the suction nozzle blockage can be ruled out; checking the inlet pipe of the negative pressure buffer bottle, it is found that there is negative pressure, so the solenoid valve SV11, SV14 and the upper level pipeline leakage can be ruled out, and the buffer bottle is checked for cracks and leaks. It is initially judged that the solenoid valve SV3 is not working properly. The working voltage DC 24V is measured with a multimeter, which is normal. It is suspected that the solenoid valve is rusted. The solenoid valve is removed, and it is found that the small section of silicone tube connecting the buffer bottle and the solenoid valve has been broken. It turns out that the problem lies here. After replacing the silicone tube, the instrument returned to normal.

Maintenance Summary

Although the two negative pressure failures occurred in different instruments, their causes are similar. If you encounter a negative pressure failure in the inspection equipment, if you do not analyze it carefully, it is easy to misjudge it as a failure of the negative pressure pump or electrical components such as the solenoid valve. After replacing it at a high cost, the failure will recur. When repairing, you should carefully check the pipeline diagram and first focus on eliminating pipeline leakage, so as to avoid detours and eliminate the fault as soon as possible.