The reason for this phenomenon:
After the sphygmomanometer was used, it was not tilted more than 450 to return the mercury back to the mercury storage bottle, causing the mercury in the glass tube to come into contact with air for an extended period of time to cause an oxidation reaction, and black oxidized mercury was generated on the inner wall of the glass tube. During use, since the convexity of the mercury column in the glass tube cannot be clearly seen on the zero scale, the reading of the sphygmomanometer will cause an error and affect the accuracy of the measurement.
Remove the glass tube, screw one end of a long thin wire onto a small cotton ball, and the other end through the glass tube. Pull the cotton ball to wipe the inner wall of the glass tube. If you encounter stubborn stains, you can pick up the right amount of alcohol. Care should be taken to wear a mask when cleaning to avoid inhaling black mercury oxide powder.