DMM Mastech MS8218

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This is the top Mastech meter with very high precision and PC connection. The meter is a fairly old design.

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I got the meter in an envelope with the meter in its bag.

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Everything was inside the bag.

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It included the DMM, a pair of probes, a shoulder strap for the bag, the bag, a CD with software, a opto to RS232 cable, some instructions sheets and a calibration certificate in Chinese.

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Probes are branded with Mastech and has removeable tip covers. The CAT rating is the usual with with II without covers and III & IV with covers.

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The plug is fully shrouded and standard probe plug size.

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The meter is heavy and the tilting bale can hold it while the range switch is used or the buttons is pressed.

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The optical port is one led, i.e. only transmitting of data, it is not possible to send command to the meter.


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Display

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The above picture shows all the segments on the display.
The 51000 is used to show selected range when using manual range, i.e. it can show 5, 50, 500, 5000 and 1000

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Typical display during usage, it will show the main number and what measurement is selected.



Functions

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Buttons (Many buttons, but not that many functions): Rotary switch:
Input

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Measurements 1uF

A look at the capacity measurement waveform.

DMMInputVoltageSweepHz

Frequency iput resistance.

DMMschema

Shows -40VDC when measuring DC voltage on 230VAC



Software

The supplied software looks fairly standard for Chinese multimeter software. This means fairly simple and with some limitations.
One limitation is COM ports, it would not work with COM3, but it worked with COM1 (I did not find any settings for COM port).

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The data logging cable uses an optical port and a RS232 connector. The RS232 is rather old fashion today, but a RS232 to USB cable will help.

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The data logging cable connected to the optical port.

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The software has 5 areas: Analog meter, Digital meter, Chart, Table and min/max. Neither the chart or table keeps the data for long, due to the 0.5s sampling range.

2

It is possible to reduce the sampling rate, this also makes it possible to record for longer time (The software has a rather small data buffer).

4s

Here I varied the input voltage fast enough and got a curve in the chart area.

FileLog

The data can first be saved to a file after they are recorded. The format looks like a standard CSV file and is easy to handle.



Tear down

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4 screws and the back could be removed.

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The circuit board is fairly rectangular in shape and uses most of the space in the meter. There is shielding over part of the circuit board.

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Two screws (one between the fuses and one holding the shield) and I could remove the circuit board.
I am a bit surprised about the fairly bad moounting of the circuit board (2 screws are not much). The lid may apply some pressure at a few points.

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The two fuses are good quality, but 500V fuses in a 1000V meter? The mA and uA current shunts are on the other side of the circuit board, but here is the diode protection of the shunts (D1..D5). There is also resistors for the input terminal sensing (R23A..R23D and R24A..R24D: 8x2Mohm).
There is two input transistor clamps (Q1 & Q2 and Q3 & Q4) they work together with two PTC's on the other side of the circuit board.
There is a big black box (R9-R13) that contains the resistor network, the big capacitor is used before the input resistor on AC voltage. At the input a precision OpAmp is used (U4: MAX6166), there is a true rms converter (U6: AD737J). Two chips marked GZ650 (U5 & U7, probably OpAmps) and a memory chip (U8: 93LC56B) for calibration.
The actual DMM frontend chip (U1: FS9704) and a microprocessor to handle it (HD64738024). Notice the contact pattern next to it, it is probably for calibration, there is a "unused" switch pattern on the circuit board.

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On this isde are the current shunts for uA and mA) (R18 & R19) and also A (The black wire). The voltage input terminal has two paths, one with about 940ohm (R1 & R2) and one with about 660ohm (R3, R4), both clamped with a GDT (Sparkgap) to the minus terminal. This works fine for low energy pulses, but will fry the resistors on a high energy pulse. The two PTC's (PTC1 & PTC2) are used for ohm output and low voltage input.

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Below the display is two crystals, one for the DMM chip and one for the microprocessor.



Conclusion

As usual I doubt the CAT rating, but it is fairly well protection and I do not see any problems using it at mains voltages (120VAC & 230VAC).
This is a very precise multimeter, but for this high end multimeter I am missing a few functions like temperature and average, I would also have prefered a faster bargraph. The RS232 connection shows that it is a old design. The Hz input is a bit special in requiring square wave input.
For the price it is a great meter, if you need precision.



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