How to use a multimeter to detect the quality of the heating tube?

Calculate the resistance of the heater element based on the parameters, then measure it using a multimeter with the appropriate resistance range. In reality, resistance generally differs between cold and hot states, with the difference being as much as a factor of 10 depending on the material. Therefore, testing isn't a fixed standard. As long as it's not zero (short-circuit the test leads first, look at the displayed data, and subtract the short-circuited resistance from the measured value), or if it's greater than 100 ohms, the heater element is functioning properly. Otherwise, there's a problem.

Digital Multimeter

Of course, you can use a multimeter to measure AC and DC (multimeters come in two types: digital and analog):

To measure AC voltage with a digital multimeter: Connect the black test lead to - or ground (marked inside the test lead jack), and the red test lead to the AC (or V plus one) jack. Set the range to 750 (sometimes 1000) in the AC~ column (if measuring 220V mains). Then touch the two test leads to the two jacks of the power outlet. The reading is the AC voltage.

Measuring DC voltage with a digital multimeter: Same as measuring AC, but connect the red probe to the + hole and also touch the + terminal of the power supply. Set the range to DC (or the "V" with a dash under it). The range depends on the specific situation, from highest to lowest. For example, if measuring a AA battery, select the 2V range. If the polarity is reversed, a negative polarity will be displayed before the reading.

Measuring AC voltage with a digital multimeter: Connect the black test lead to negative or ground, the red probe to +, and set the range to AC~500V. Use the same digital multimeter as with both probes. Read the number corresponding to the maximum range on the right. For example, if the 500V range is selected, read the number on the right side of the dial marked with the highest number, 50. If the pointer points to 20, the reading is 200V.