DMM Mastech MS8229

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This is a fairly cheap DMM with all common function and some environment functions.

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It arrived in a box with a picture of another meter, because the box is used for multiple models. Mastech has 3 models in the MS8229 series as can be seen on the back of the box and this is the "high end" model.

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It included the DMM, a pair of probes, a thermocoupler and a manual.

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The probes has removable tip covers. The CAT rating is marked on the tip and will change when the cover is removed.

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The plug is fully shrouded. The pin inside is shorter than standard probes.

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The meter includes a cheap thermocoupler.

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The tilting bale is stable enough to use the switch and buttons, but the meter may slide around on a smooth surface.

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The environment sensors.

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Display

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The above picture shows most of the segments on the display, not all environment segments are not included.

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Typical display during usage, it will show the number and what measurement is selected.
The display will always show humidity and ambient temperature, but they need a few second to initialize when powering on.



Functions

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Buttons:
Rotary switch:

Input

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The terminals are not deep enough to cover standard banana plugs, but they do connect.
Each terminal has a light that will light up if a probe has o be inserted and flash if the probe is in the wrong terminal. This feature is very nice.



Measurements
1uF

A look at the capacity measurement waveform.

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Tear down

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Four screws and the back could be removed.

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Two more screws and the circuit board could be lifted out, but I also had to remove two screws in the environment sensor unit to remove that.

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Holes and white dome for sensor unit.

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There is a lot of wires between the sensor unit and the circuit board, all with the same 3 pin connectors.


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Sensor unit has two diodes for temperature measurement, a humidity sensitive resistor , a photo diode and a microphone.

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And there is also electronic for signal conditioning and adjustment.

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It is a fairly complicated circuit, I will not try to descibe much of the circuit (It is difficult to find data on the different chips).
Some obvious details:
The two resistors that is visible below the 10A input terminal are the mA and uA shunt and the diodes (D3..D6) below the input circuit board are protection for them. The 10A fuse is marked 250V on the circuit board, this do not really match with the 1000V input marking.
The two resistors (R21A & R21B 2x5Mohm) are probably an input resistor, but distance are very low for any high voltage. Close to them is the only PTC (R25) in the circuit.
The meter is adjusted with trimmers, this is not the most stable solution, but can be nice for DIY calibration.

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The front shield is mounted with two screws at the front, the lcd display is connected with the orange zebra stripe.

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There is nothing on this side of the circuit board, except switches and connection to the display.

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The light on the four input terminals is controlled with a PIC16F54 processor, the two led around each terminal and the switches on the other side. The big brown part is a PTC that is used for overcurrent protection on the mA and uA range.




Conclusion

Generally the DMM calibration is very good on this DMM, i.e. within a few count, but the environment measurements are not that impressive, but except for sound works acceptable.
The automatic mA/uA fuse is nice, but it means a very large voltage drop when measuring current in the high mA range (Use A range instead).
Generally it is a nice meter for occasionally use and and can work well on a workbench, but it is not for high voltage and high current. As usual the CAT ratings is way too high.


Notes

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