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13.7 Selecting and Using Meters for Electrical Testing
Most on-site trouble shooting requires a test
instrument called a multimeter which can be
obtained from an electric supply, electronic
supply, automotive or hardware store. It will
measure DC and AC volts, current (amps) and
resistance (ohms, symbolized by &!). Here are
some criteria for selecting a meter for testing.
Digital or analog meter?
A digital meter is
best. An
autoranging
digital
meter is easiest
to use, especially for a beginner. An analog
(mechanical) meter is good if it is of high
quality and at least 3" (75 mm) wide.
Resistance ranges
The meter must read in
the 0-10 Ohm range to one decimal place.
This includes all but the smallest and
cheapest digital meters, and analog meters
that have at least three resistance ranges.
Ammeter ranges
These are the options, with
notes about cost and benefits. $ costs are
US$, typical in USA.
1. Multimeter with milliamp range but no Amp
range (under $35) – This will be useful for
voltage and resistance measurement, but
not for current. It is useful, but not ad-
equate for all troubleshooting.
2. Multimeter with amp range to 10A or more,
without clamp-on capability ($25-$150) –
This will measure PV array current and
pump running current. A wire must be
disconnected in order to measure current
(current must flow through the meter).
Inexpensive meters are delicate and not
suited to professional use.
3. DC/AC clamp-on ammeter ($50-$300) – a
clamp-on meter allows measurement of
current without disconnecting wires. We
strongly recommend this type of meter for
solar pumps and other electrical equip-
ment. It makes the job much safer and
easier. Fluke Model 33 or 36 are the
professional favorites. Cheaper ones are
less reliable and should be checked
periodically for accuracy.
Use two meters and clips for easier testing.
It is often helpful to measure voltage and
current simultaneously. An inexpensive meter
is adequate for voltage, because precision is
not necessary. Clip-wires or clip-on probes
are very helpful if you don’t have three hands.
Resistance readings
are always taken with
NO POWER applied to the circuit. Always use
the LOWEST range that produces a reading
13.8 Measuring Solar Energy
Intensity
To accurately evaluate a solar-direct solar
pump, it is necessary to measure sun inten-
sity. For example, if the solar pump is produc-
ing around 60% of the specified maximum
flow, and you measure the sun intensity (in
the same plane as the array) as 60% of full
sun, you know the system is working properly.
The
Daystar Meter
is a hand-held instrument
that measures sun intensity using a solar (PV)
cell similar to those used to power your solar
pump. It displays the solar potential in watts
per square meter (W/m
2
). The industry-
standard for full sun intensity is 1000 W/m
2
,
so a reading of 600 indicates 60% intensity.
The meter costs less than $150 (in USA).
Order directly from the manufacturer:
Daystar, Inc., 3240 Majestic Ridge, Las
Cruces, NM 88011 USA, tel. (505) 522-4943
[email protected] www.raydec.com/
daystar
(RX1 is the lowest range). An “auto-ranging”
meter will adjust its range automatically.
Zero adjustment
Some meters have a zero-
adjustment to insure accuracy. This applies to
analog meters when measuring resistance,
and to clamp-on DC ammeters. Be sure to set
the zero if necessary!
WARNING Read
the instructions
that come with
your meter, and
follow the safety
warnings.
WARNING
Attempting to
read current
(amps) between
the two poles of
a power circuit
causes a poten-
tially dangerous
short circuit.
Connect the
probes IN
SERIES with the
circuit – see
your meter’s
instruction
manual. To read
voltage, the red
probe must NOT
be in the Amps
socket. This will
cause a short
circuit.
