Why are there differences in the values ​​displayed by different megohmmeters?

Due to the non-ideal voltage source of the high-voltage megohmmeter test power supply, different internal resistance Ri, different series resistance Rm of the measuring circuit, different dynamic measurement accuracy, and unreasonable or wrong on-site measurement operations, different models of megohmmeters will have different measurement results for the same tested product. In actual measurement, various possible measurement errors should be minimized in combination with the particularity of the megohmmeter insulation test conditions: 

(1)  When different types of insulation meters measure the same test product,  the same voltage level and wiring method should be used. For example, when measuring the insulation of the high-voltage winding of a power transformer, when the winding lead-out terminal is always connected to the L terminal of the megohmmeter, there are: the  direct method in which the E terminal is connected to the low-voltage winding and the shell, and the G terminal is left hanging;  the shell shielding method (low potential shielding) in which the E terminal is connected to the low-voltage winding and the G terminal is connected to the shell; the two types of bushing shielding methods (high potential shielding) in which the G terminal is connected to the surface thickness meter of the high-voltage winding bushing , and the E terminal is first connected to the low-voltage winding, and then connected to the shell or not connected to the shell respectively.  The E terminal is connected to the shell, and the G terminal is connected to the low-voltage winding.  The G terminal potential is different for megohmmeters of different structures and standards, and the placement position of the G terminal on the bushing surface should also change accordingly. (KD2677 is low potential shielding, that is, the G terminal is low potential). 
(2)  Different types of megohmmeters have different ranges and indications, different scale resolutions, and different levels of measurement accuracy, which will cause differences between indications. In order to ensure accurate measurement of power equipment, it is necessary to avoid using megohmmeters with low accuracy and inconvenient use. 
(3)  Most test products contain capacitive components and dielectric polarization. Even if the test conditions are the same, it is difficult to obtain ideal data repeatability. 
(4)  During measurement, the temperature of the insulating medium and the oil temperature should be consistent with the ambient temperature, and the difference is generally allowed to be ±5%. 
(5)  The measured value should be read as quickly as possible within the allowable time difference range of a specific time period. In order to make the measurement error not higher than ±5%, the time for reading R60S is allowed to have an error of ±3S, and the time for reading R15S should not differ by ±1S. 
(6)  The high-voltage test power supply is not an ideal voltage source. When under heavy load (small insulation resistance value of the test product), the output voltage is lower than its rated value, which will cause the measurement accuracy of the single-branch direct reading measurement method megohmmeter to decrease due to the change of the conversion coefficient.