How to know when to tune up a digital multimeter?

I have bought the multimeter with temperature measurement from the following website:

http://futurlec.com/Multimeters.shtml

Currently I use the voltage, resistance, and short circuit detection functions on it.

The short circuit detection function works fine. but now when I measure high resistance values over 100K, the results are inaccurate. For example, the 560K resistor was measured at 408 instead of 560 on the meter (I turned the knob to the 2000K setting). When I measured a 1K resistor, the meter reported 0.8 instead of 1.0 on the 200K setting.

All resistors measured were carbon film 1/4 watt with 5% tolerance.

One time when I opened up a multimeter to replace the battery, I noticed it had a fuse and a potentiometer as part of its circuit.

Would adjusting the potentiometer fix this issue? I mean the low battery symbol never appeared on the meter so I don't know if battery is an issue. Could I hack the meter somehow and measure something that could give me an indication if the battery is dying?

• The best 'hack' is to replace the battery with a fresh one. If the low battery indicator does not work reliably, then consider replacing the meter with a fresh one. What good is a reading you can't depend upon? – Spehro Pefhany Aug 31 '18 at 0:51
• Not sure I would expect a lot from a multimeter that sells for \$5.90. I wouldn't mess with the pot, you would probably make things worse on the other scales. If a fresh battery doesn't help maybe invest in a better meter – John D Aug 31 '18 at 0:53
• There are no adjustments for Resistance or V/A Readings. The pot may be for Diode/Continuity buzzer threshold. – Sunnyskyguy EE75 Aug 31 '18 at 2:06
• What do you expect from a meter that costs a dollar to make, they are disposable, get a new one, or a real one – laptop2d Aug 31 '18 at 5:08

When I measured a 1K resistor, the meter reported 0.8 instead of 1.0 on the 200K setting.

That's an error of $\frac {0.2}{200} = 0.1\%$ of full scale on that range. That's much better than might be expected on such an instrument.

For accuracy you should always make your measurements on the lowest range that will accommodate the value being measured. If unsure what to expect then start on the highest range and work down so that you minimise the chance of applying an over-voltage to the electronics. (This isn't important on the resistance measurement ranges as the meter itself is the voltage source.)

You don't know what the potentiometer is for so don't mess with it. You don't have any reference to measure from anyway.

For the 560k resistor reading, if you had your fingers touching both probes, you body resistance would be in parallel with the resistor, so would lower the meter reading. This effect would be insignificant with a 1k resistor, however.

If all of the other ranges are reading correctly, you have a bad resistor which sets the gain for that particular range. Without a schematic, there is no way to tell which it is, and I'm assuming it's a SMD part anyways. So, no, the pot won't fix the problem - or if it does it will cause all the OTHER ranges to be wrong. The problem is probably fixable, but not without both knowing what you're doing and having access to documentation.

Generally, unless you're just in it for the experience, you're better off either just not using that scale or trashing the meter - DMMs are dirt cheap these days, so why put up with the hassle?