I added on to my accuracy verification spreadsheet covering earth ground resistance and resistivity testers. I added the ETCR2000A+ and the Simpson 444 micro-ohmmeter. The ETCR2000A+ is pretty much identical to the UT276A with different features enabled. The Simpson meter isn't made for earth ground measurements but measures four wire resistance via a low AC (or DC) voltage and current, and I thought that it would be useful for comparison. The Simpson's low test voltage is helpful in that it won't arc over a corroded connection which may be destined to later have more serious arcing.
The UT276A and ETCR2000A+ were both made by the same manufacturer as shown by their outward appearance and their circuit boards shown on the attached pictures. It seems more likely that ETCR made both. They both have the same IC's with their numbers scratched off. They are still fairly legible in the Uni-T meter. Those IC's include the Exar SP3232EEN RS232 transceiver IC and the NXP HEF4066BT switch in just the UT276A since only it has the RS232 interface. Both have TI IC's labeled 72500 14T CZ90 and 22521 08M AZN2. I haven't been able to translate those numbers yet.
Both meters have good accuracy. The ETCR meter can only measure up to 200 ohms while the UT276A can measure up to 1,200 ohms. The lower limit on the ETCR meter isn't important for my use. Plus, for measurements above 200 ohms the UT276A accuracy falls off and the precise contact of the clamping jaws becomes much more critical with high resistance readings due to the low measurement current for that range.
Only the UT276A has a computer interface. That is just via RS232 and the current software isn't the best. Attached are pictures of the sampling and memory screens. It doesn't tell you much more than the meter's front display and the graphing part of it doesn't work with my computer. The memory shown on the computer isn't too useful other than seeing all entries at once. There isn't any time information stored or anything else to identify the specifics of the reading.
The UT521 takes 2 and 3 point readings. While I've never had a Yokogawa EY200 meter, they both look like they are very similar and probably made by the same manufacturer. The Uni-T meter is typically much less expensive. The UT521 has a STC 12C5410AD microcontroller, ATMEL 24C04N 512x8 EEPROM, 3201-BI ADC, HEF4053BT and CD4052BM multiplexers, OP07C, OP97FS and LM324DG op amps, MAX660CSA regulator and more IC's.
I found that the test figures on my prior spreadsheet were a bit inaccurate for the UT521, particularly for low resistance tests, due to my not noticing extra resistance on a test cable used. Those figures were updated on the attached spreadsheet.. The UT521 had pretty good accuracy, generally meeting its specs. As with all of the resistance/resistivity meters, due to their limited resolution, a one or two digit error can cause a significant percentage change. And while all spec sheets shows accuracy per cent +/- so many least significant digits, I haven't included those digit figures in my accuracy figures. And with the readings dependent on soil moisture and temperature and well as composition, the earth ground resistance readings are not constant.
The DUOYI DY4300A is a couple of steps above the UT521. I can also take 4 point readings, adding resistivity measurements to the earth ground resistance measurement which the UT521 can make. It also has greater microcontroller power and includes the individual resistances between the adjacent points to its front display. It also measures keeps track of any interference and displays the frequency and level of the interference. Its memory includes time and location information. The DY4300A appears to be the same thing as the Kyoritsu KEW 4106, with different features activated.
The main IC's in the DY4300A include the STM32F103ZET6 microcontroller, AMIC LP62S16256FV-70LLTF 256k x 16 CMOS SRAM, 24LC028 EEPROM, TI TL1453 pulse width modulation controller, DG409CY+ multiplexer, TI LM2575S and PS767D318 voltage regulators and probable op amps labeled 2904, LT521, LT121 and LT423.
The DY4300A has good accuracy and has the choice of four test frequencies. Or, the frequency can automatically selected to that having the least noise level.
The accuracy of my personally built unit is also included. It was designed to be most accurate on the lower frequencies. The resistance/resistivity values need to be calculated based on the five digit voltage and current displays. While in one sense that's inconvenient, knowing those values is useful when evaluating the best probe separation distance.
The accuracy of the Simpson 444 was good. I included some additional accuracy measurements of 10, 25 and 100 milliohms. Those extra resistors all had 1% accuracy, so the meter should have been more accurate than the resistor. I could have used my 34401A measured resistance as the standard for all equipment rather than the resistors value. However, due to most meters having just 2.5 to 3 digit displays that appeared to be overkill.
While still accurate, the Simpson 444 is an old design. It uses a MAX7129CPL ADC, some 4000 series logic IC's and LT1001ACN8, LF412ACN, KA1458 and LTC1052CN8 op amps.
As said before, while the CN0359 has good accuracy, it's not made for earth ground resistance or resistivity testing. You will have problems using it for that. It's made for fluid conductivity testing. Or, it can be used with completely isolated components (like my test resistors).
So, here is addition information, with two extra meters being covered. This is already too long with too many attachments, so I'll leave it at that.