> for those using this meter from harbor freight, this thing has a
> fuse inside that protects the milliamp position. after blowing two
> of these 1/4 amp fuses, (and this is easy to do in this setup) i
> picked up a 4 pak of 1/2 amp fuses at radio shack, and these solve
> the problem. the only reason i went to 1/2 amp instead of 1/4 amp,
> was that radio shack was out of the 1/4's.
Very good reply, Jim, but you really should go back to 1/4A fuses. The
internal circuit is not designed to handle sustained currents above this
level, and you could damage the instrument.
You can prevent blowing fuses very easy. Since you have a 27 V supply,
just put a small resistor in series with either lead. The value is
calculated by
R = E / I
= 27 / 0.25
= 108
Use a 100 ohm, 1/4 watt resistor. This will have little or no effect on
the voltage and current measurements, but will prevent blowing the fuse
when you inadvertently short the silver rods together.
I found an easy way to mount the rods is to take the plastic lid from a
jar of peanut butter and use an awl to put two holes an inch or so apart.
Find a glass that is small enough so the plastic lid fits with a bit of
play.
Place a 6-32 screw with two washers through the hole from the underside,
and add a washer on top. Bend the silver into a U-shape, and put it
between the two washers. When you tighten the nut, there will be enough
thread showing on top to connect the alligator clips.
Add a mark on the glass for the fill level. Add another mark 10% higher
to add seed solution from a previous batch. This will get the current
started much faster, and you won't have to wait so long. Monitor the
current and stop when you reach about 1 mA per square inch on the anode.
To calculate the area, measure the rod diameter or refer to the wire
gauge chart below.
-----------------------------------------------------------------------
Gauge Diameter Resistance
(AWG) (in) (Ohms/M-ft.)
-----------------------------------
0 0.3249 0.09827
1 0.2893 0.1239
2 0.2576 0.1563
3 0.2294 0.1970
4 0.2043 0.2485
5 0.1919 0.3133
6 0.1620 0.3951
7 0.1443 0.4982
8 0.1285 0.6282
9 0.1144 0.7921
10 0.1019 0.9989
11 0.09074 1.260
12 0.08081 1.588
13 0.07196 2.003
14 0.06408 2.525
15 0.05707 3.184
16 0.05082 4.016
17 0.04526 5.064
18 0.04030 6.385
19 0.03589 8.051
20 0.03196 10.15
21 0.02846 12.80
22 0.02535 16.14
23 0.02257 20.36
24 0.02010 25.67
25 0.01790 32.37
26 0.01594 40.81
27 0.01420 51.47
28 0.01264 64.90
29 0.01126 81.83
30 0.01003 103.2
--------------------------------------------------------------------
Measure the wetted length of the anode. The area is
A = pi * D * L
You can use pi = 3 for very small diameters:)
Best Regards,
Mike Monett
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