Re: meter with continuity test and diode test on same setting

ehsjr wrote:

John, the test is flawed. When you add a meter in series to
measure current, the added meter changes the current supplied
by the diode testing meter, due to the internal resistance of
the added meter (relatively large on the low mA scales). That automatically upsets the "calibration" such as it is, of the
reading on the diode testing meter.

I have no argument with your analysis. My only defense is that adding the milliamp meter across the diode check meter leads or in series with the diode junction changed only the least significant digit of the diode check reading. I was not trying to check the accuracy so much as determine whether these two meters were displaying voltage or resistance. I think the voltage check proves that they are not displaying voltage, and the combination of voltage and current measurement confirms, pretty closely that these meters are displaying something close to resistance. Am I wrong about these conclusions?

My tests say nothing about what other meters display, and I am sure that there are many variations. I have to repeat this test for every meter I use and put a label on the back that reminds me what that meter displays on the diode check scale.

You may happen to get
a reading on the diode testing meter that agrees, but you
would need to make multiple readings across a range of
resistances that always agreed to make a proper conclusion.

That would certainly increase my confidence.

Also, it sounds like you used only 2 meters. You would need
3 to be sure your setup is measuring what you think it is.
From your description: "Then I measured a diode junction with
each meter while measuring the test current and voltage drop
with the other meter."
You can't measure both current and voltage simultaneously
with the other meter. When you remove it from a series
connection for measuring current, to a parallel connection
for measuring voltage, you change the current in the circuit.

Agreed, if accuracy were important. Also measuring the resistance of each meter on its 20 mA scale would be a good idea, so I knew what resistance they added when measuring the current through a test resistor.

There are two ways you can test that come to mind that do
not upset that calibration:
1) Diode test normally and measure the voltage
across the diode with a second meter.
2) Diode test, except use a low value resistor,
like 150 ohms.

I've performed test #1 a number of times in the
past, and the reading on the second meter (which
is voltage drop) has always been close to the
reading on the first.

I just performed test #2. My 150 ohm resistor
read 220 on the Webtronics meter in diode test.
(I believe that is the same meter the op has.)
On the ohmmeter setting, it reads 149.2

I then tested the same way with an Alfa 2360 meter.
It reads 149.8 ohms on the ohmmeter setting, and
.154 volts on the diode setting.

Surprisingly, when I performed the test on the
cheap $3.00 meter from Harbor Freight, it read
149 on both diode test and ohms.

The two I posted results for are like your cheap one. It is clear that there is no standard for what is displayed on the diode test scale.

Ed


--
Regards,

John Popelish
.

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