This chart is an example of superheat charging
charts that are supplied by A/C manufacturers with
each fixed orifice air conditioner. Note that you must
measure indoor wet bulb and outdoor dry bulb to be
able to determine the target superheat. For wet bulb
measurements, you can use Fieldpiece ARH4 rela-
tive humidity head or the Fieldpiece ATWB1 wet
bulb thermocouple.
Although this chart may be similar to many charts
supplied by manufacturers, you should use the
chart supplied by the manufacturer of the air condi-
tioning equipment. Fieldpiece bears no responsibil-
ity of results in using this chart.
Generic Target Superheat Chart
(for fixed orifice systems)
*
Measuring actual superheat
Superheat is the difference between the boiling
point temperature of the refrigerant in the evapora-
tor coil and the actual temperature of the refrigerant
(gas) as it leaves the evaporator. After boiling, the
refrigerant continues to warm up. The number of
degrees it “warmed up” after boiling is called the
superheat. Under worst case conditions (low load),
the refrigerant in the evaporator boils off near the
end of the evaporator coil. To make sure liquid
doesn’t enter the compressor under the worst case
condition (low load), the AC manufacturers publish
charts indicating what the superheat should be at a
given indoor wet bulb measurement and outdoor air
temperature.
Measuring superheat is your best indication on a
fixed orifice system of the proper refrigerant charge
and opperating conditions. If everything else is
working properly and the actual superheat is too
high, add refrigerant. If it’s too low, remove refriger-
ant.
Using superheat to determine proper charge
enables an air conditioner to deliver the maximum
cooling power, the best reliability, and the best effi-
ciency.
The ASH3 should only be connected to the low
pressure side of an A/C refrigerant circuit.
The hose must have a schraeder depresser on
one end to release the refrigerant from the suction
line. This is the same type of hose available with
most pressure gauge sets. We suggest EPA sanc-
tioned “no leak” hoses.
Exercise caution whenever working with any
electricity and high pressure liquid or gas. Follow all
instructions provided with equipment being serviced
or installed.
Target superheat
To determine the target superheat, you will need
three things. Outdoor dry bulb (outdoor air temper-
ature), indoor wet bulb, and the manufacturers tar-
get superheat chart.
You can use the ARH4 Fieldpiece acessory head
for both indoor wet bulb and outdoor dry bulb. Or
you can use any Fieldpiece meter that has a tem-
perature function along with a ATWB1 wet bulb ther-
mocouple.
Set up for the ASH3
(meter not shown)
A: Compressor
B: Service port
C: Pipe clamp thermocouple
connected to suction
line under insulation
D: Suction line
E: Evaporator
AUTO-
OFF
T/C
Cal
STABLE
Pressure
Temp
Superheat
ON
LO BATT
Superheat
Accessory
Head
ENGLISH
METRIC
R22
R410A
Set
ATM
ASH3
A
C
B
D
E
*
