Troubleshooting Guide and FAQs
Superheat/Subcooling
SSX34, ASH3, ASX14, ASX24
The ‘cold beer’ test is good enough no longer. You must check
superheat or subcooling to charge a modern air conditioner properly for efficient
operation. Below you’ll find the most common technical questions and
answers associated with superheat/subcooling measurements and the use of Fieldpiece
Instruments superheat/subcooling tools. For a description of superheat and
subcooling, see Superheat
Overview and Proper
Use of Superheat Measurements Saves Energy and Prevents System Damage
on the Fieldpiece website.
1.
What equipment do I need?
2.
How do I know if I should be measuring superheat or subcooling?
3. What is superheat and subcooling?
4. When should I take my actual and target superheat/subcooling
measurements?
5. How do superheat subcooling tools work?
6. How do I use either subcooling or superheat to charge a system?
7. How do I calculate ACTUAL superheat?
8. How do I calculate ACTUAL subcooling?
9. How do I measure refrigerant temperature (the ACTUAL temperature
of the refrigerant)?
10. What do I do with my ACTUAL superheat or ACTUAL subcooling
measurements?
11. How do I calculate TARGET superheat?
12. How do I calculate TARGET subcooling?
13. What if my ACTUALS are far different from my TARGETS, but
the system seems to be running properly?
14. Why is my meter displaying pressure when it is not hooked
up to any pressure source?
15. Why does my pressure reading keep changing when I am testing
superheat / subcooling?
16. Why is my PRESSURE READING displayed as “OL”
(or) “-OL”?
17. Why is my TEMPERATURE reading displayed as “OL”
or “-OL”?
18. Why is my SUPERHEAT / SUBCOOLING reading displayed as “OL”
or “-OL”?
19. My ambient temperature reading is off. How do I calibrate
temperature on my Fieldpiece tool?
You are probably here
because you have a question regarding a Fieldpiece superheat/subcooling instrument.
Here are the products currently available and their capabilities. All measure
pipe temperature and refrigerant pressure. Make sure you have the one that
will do the job you need.
SSX34 –
Stand-alone superheat/subcooling unit for A/C and Refrigeration.
Measures suction/liquid line pressure and temperature to calculate and display
actual superheat/subcooling. For use with R-134A, R-404A, R-410A, or R22 refrigerants.
ASH3 – Accessory head for A/C superheat. Measures suction
line pressure and temperate to calculate actual superheat. For use with R-22
and R-410A refrigerant.
ASX14 – Accessory head for A/C superheat/subcooling.
Measures suction or liquid line pressure and temperature for actual superheat/subcooling
for A/C. For use with R-22 or R-410A refrigerants.
ASX24 – Accessory head for Refrigeration superheat/subcooling.
Measures suction or liquid line pressure and temperature for actual superheat
subcooling for Refrigeration. For use with R-134A or R-404A refrigerants.
In addition, you
will need an instrument to measure air temperature and indoor wet bulb. The
ARH4 will measure both. The ATWB2 wet bulb thermocouple can be used to measure
indoor wet bulb. It can be used with any instrument that accepts the standard
K-type thermocouple plug.
2. How do I know if I should be measuring superheat or subcooling?
A: Using superheat or subcooling is the best way to properly charge an AC/refrigeration system.
On systems with a TEV/TXV (Thermostatic Expansion Valve), measure the liquid line pressure and temperature to get actual subcooling.
On systems using a fixed capillary restrictor, calculate superheat measuring the evaporator pressure (low-side) and pipe temperature reading to get actual superheat
3. What is superheat and subcooling?
A: In short, superheat is the temperature rise above the boiling point of the refrigerant after the evaporator. Too high, and the refrigerant boils off early in the evaporator, and ‘wastes’ most of the effect of evaporator. Too low and you risk liquid going into the compressor.
Subcooling is the temperature decrease below the boiling point (same as the condensing point) in the condenser. Too high and refrigerant condenses too early in the condenser and ‘wastes’ most of the effect of the condenser. Too low and a mixture of gas and liquid can be delivered to the expansion valve, reducing efficiency. See Superheat Overview and Proper Use of Superheat Measurements Saves Energy and Prevents System Damage on the Fieldpiece website for a more detailed explanation.
4. When should I take my actual and target superheat/subcooling measurements?
A: Superheat and subcooling readings must be taken when the system is in a steady state condition. Typically a residential system will be at steady state after ~10-15 minutes, but can be longer. Turn the system on and set the thermostat low to ensure that the system doesn’t turn off during your testing. Once the system’s temperatures and pressures are no longer noticeably changing the system is at steady state.
All measurements must be taken at the same time. In other words, wet bulb temperature and outdoor dry bulb temperature must be taken within a short amount of time from the actual superheat or subcooling reading to obtain valid measurements. Conditions can change that affect your target superheat subcooling number by the minute.
For example, you might take a dry bulb reading outside of 75°F at 1:00 pm. You come inside, take a wet bulb reading, then look at the manufacturers target charts, and finally set up your pressure and pipe temperature measurements. By 1:30 pm the reading outside dry bulb reading could be 10°F higher, changing your target superheat by say 6°F, and affecting your charge requirements.
The ideal procedure for accomplishing accurate results quickly is to connect the meter to the pressure taps and connect the temperature probe on the appropriate refrigerant line. Then take the indoor wet bulb and outdoor dry bulb temperature readings while the meters are connected. Look up the values for target superheat or subcooling on the chart and make the comparison to the actual superheat subcooling being read by the meter.
5. How do superheat subcooling tools work?
A: For superheat, they measure suction line pressure and suction line temperature, and calculate the actual superheat. For subcooling, they measure the liquid line pressure and liquid line temperature and calculate actual subcooling. The refrigerant charts for calculations are incorporated into the microprocessor of the unit. Just measure temperature and pressure and read superheat or subcooling directly.
6. How do I use either subcooling or superheat to charge a system?
A: Measure the refrigerant pressure and temperature to determine the actual superheat or subcooling. Then use the manufacturer’s charts (usually requiring indoor wet bulb and outdoor dry bulb measurements) to determine the target superheat or subcooling. Add or subtract refrigerant as needed to cause the actual reading to match the target.
7. How do I calculate ACTUAL superheat?
A: Manually (the hard way): on systems with a fixed restrictor you need two measurements: suction line pressure and temperature. To get superheat, measure the suction line pressure and convert that pressure to the boiling point temperature (or saturation temperature) using a chart or gauge for that specific refrigerant. Make sure the chart or gauge is for the right refrigerant. Each refrigerant requires a different chart or gauge. Subtract the boiling point temperature of the refrigerant from the actual pipe temperature after the end of the evaporator. This gives you your actual superheat reading. For more information on superheat and subcooling please refer to the “What is superheat and subcooling?” question on this page.
A: Using superheat instruments (the easy way): select the proper refrigerant. Connect the pressure port on the instrument to the suction line. Connect the thermocouple clamp to the suction line. The instrument will display actual superheat.
8. How do I calculate ACTUAL subcooling?
A: Manually (the hard way): actual subcooling is measured in much the same way as superheat except liquid line (high-side) temperature and pressure are used in the calculation.
Take the temperature of the liquid line as close to the expansion valve as possible, before the metering device. Take the liquid line pressure and convert it to a boiling point temperature using a chart or gauge. Make sure you have the correct chart or gauge for the refrigerant used.
Subtract the liquid line temperature from the boiling point (saturation temperature at pressure) on the liquid side of the system to get subcooling.
A: Using a subcooling instrument (the easy way): select the proper refrigerant. Connect the pressure port on the instrument to the liquid line. Connect the thermocouple to the liquid line. The instrument will display actual subcooling.
9. How do I measure refrigerant temperature (the ACTUAL temperature of the refrigerant)?
A: Measure by taking the surface temperature of the pipe. An accurate reading can be achieved using the ATB1 beaded thermocouple by wrapping it around the line with a Velcro strap and then insulating the back of it.
An easier way is to use the ATC1 (included with the purchase of your Fieldpiece Superheat/Subcooling tool) or ATC3 pipe clamp thermocouples. They offer faster and more accurate method for measuring temperature. Place the metal sensor on the ATC1 or ATC3 at approximately 90° (+/- 45°) from the vertical plane of the pipe (see FIG. 2) on horizontal suction line pipes for a more accurate reading. In other words, you want the metal sensor to be on the sides of a horizontal pipe, not on the top and not on the bottom. For vertical pipes, the angle is unimportant. Be sure there is no insulation between the pipe and the temperature instrument contact.
The reasoning is that the gas part of the refrigerant mixture in a horizontal pipe tends to house the top of the vertical portion of the pipe interior while the liquid portion tends to travel along the bottom of the pipe interior. To get the most accurate reading, you want to take the temperature of both states in the mixture.
10. What do I do with my ACTUAL superheat or ACTUAL subcooling measurements?
A: Compare actual readings against the target superheat/subcooling as determined by the system manufacturer’s specifications. These charts are supplied with the equipment.
As a general rule, if superheat is too high, add refrigerant. If too low, remove refrigerant. If subcooling is too high, remove refrigerant. If subcooling is too low, add refrigerant. Consult the manufacturer’s specifications before adding or recovering refrigerant, as further diagnostic tests may need to be performed.
11. How do I calculate TARGET superheat?
A: On a fixed restrictor system, most manufacturers provide a “look-up table” that requires two measurements:
1. Outdoor dry bulb air temperature taken from the air that is going into the condenser coil.
2. Indoor wet bulb temperature taken by a wet bulb thermometer that has been soaked in water (or equivalent digital wet bulb thermometer such as the Fieldpiece ARH4) and held in front of the indoor return grill or, better yet, just in front of the evaporator coil.
Note:
Some manufacturers assume a 50% relative humidity and do not require a wet
bulb measurements. Superheat measurements can vary significantly if the actual
humidity is significantly different from 50%. A majority of superheat charts
are the same across the industry. Generic charts are available from several
sources if manufacturer-specific charts are unavailable. Heed all manufacturer
specifications before any other source. It is the only way to be certain the
system you are testing has proper superheat.
12. How do I calculate TARGET subcooling?
A: Different manufacturers have various methods for determining target subcooling. For superheat, most manufacturers use the same charts, but this is not true of subcooling. There is no substitute for having the manufacturer’s subcooling chart.
13. What if my ACTUALS are far different from my TARGETS, but the system seems to be running properly?
A: Many poor performing systems can have actual superheat/subcooling measurements well outside the target. The system may be drastically overcharged or undercharged. Comparing the actual superheat/subcooling readings with the manufacturer’s equipment specific target superheat/subcooling can often confirm the system’s condition. Sometimes there is something else wrong.
First, make sure the system has been in operation for at least 15 minutes and is stable. Make sure it continues to operate throughout the test. Take your readings in as short a time period as possible. Things can change, even for a system that appears stable.
Make certain the unit selected corresponds to the reading you are taking. For example, for superheat on the SSX34 make sure superheat (SH) is selected on the LCD. For superheat on accessory heads (ASX14, ASX24), make certain the large switch on the face of the head is switched to superheat and small switch is in the “SH” position. (The ASH3 calculates superheat exclusively and therefore has no small “SH” switch.)
Calibrate the temperature (for
‘ice bucket’ field calibration procedure, see “Q: My ambient
temperature reading is off. How do I calibrate temperature on my Fieldpiece
tool?” on this page).
Calibrate the pressure (‘zero’ the reading) on your unit. (See
“Q: Why is my meter displaying pressure when it is not hooked up to
any pressure source?” on this page for further information..
Check that the proper refrigerant has been selected on your meter.
Check pressure hoses and fittings for leaks.
Check the batteries in your DMM, meter and/or accessory head.
Make certain that you are testing for superheat/subcooling at the proper location in the system. Test the evaporator/suction line side of the system for superheat. Test the condenser/high side of the system for subcooling.
If you still get readings that don’t seem right, you can perform a manual test using your gauges, a wet sock for wet bulb (ATWB1 wet bulb thermocouple), thermometer, refrigerant chart, and manufacturer’s target superheat chart.
If it still appears your unit is reading superheat/subcooling incorrectly, call Fieldpiece technical support at 714.257.9060.
14. Why is my meter displaying pressure when it is not hooked up to any pressure source?
A: You need to ‘zero’ the meter. Disconnect your Fieldpiece superheat/subcooling unit from all pressure sources. Remove the black cap on the “T”. Press calibration button to “zero” the instrument at ambient (surrounding) pressure.
On the SSX34, the calibration button is called “ATM”.
On the accessory heads
(ASH3, ASX14, ASX24), remove the rectangular gray rubber shield from the face
of the head. Find internal push-button over the “Set ATM”. Depress
the button.
If the unit does not respond to ATM calibration please contact Fieldpiece
technical support at 714.257.9060.
15.
Why does my pressure reading keep changing when I am testing superheat / subcooling?
A: If your readings are changing when measuring line pressure, in most cases it means the system has not reached a steady state. Suction line pressure continues to drop as a system is coming to equilibrium, while the liquid line pressure will continue to rise. A good rule of thumb is to let the system run for ~10-15 minutes before taking readings to determine actual superheat or subcooling (See “Q: When should I take my actual and target superheat/subcooling measurements?” earlier in the document). If pressure readings continue to fluctuate, check the system for leaks.
If the system is stable and there are no leaks, your testing equipment itself might cause fluctuations in your pressure readings. Leaks in hoses or at a fitting are possible sources for fluctuations as well. A simple way to perform a hose test is to hook the unit up to a positive pressure source such as a refrigerant recovery unit and isolating the system (the pressure reading unit/gauge and the hose) from the recovery unit with an adjustable valve hose. A good hose will hold a constant pressure reading. Most recovery units allow you to pressurize the hoses with air, avoid leaking any refrigerant into the atmosphere. You may also test hoses with a stable vacuum source and micron gauge to read vacuum.
If the hose check reveals no problems, the problem may be a loose or “leaking” “T”. Tighten all of the fittings on the “T” (including the terminator fitting on the non-hose side). Hook the entire unit, with a properly working hose to a positive pressure source and isolating the system as outlined in the hose test paragraph above. Check the pressure. If it holds, the problem has been corrected. If you are still losing pressure, there may be a problem with the isolation hose or the Fieldpiece unit. Test on another isolation/adjustable flow hose. If the unit is still loosing pressure, please contact Fieldpiece technical support at 714.257.9060.
16. Why is my PRESSURE READING displayed as “OL” (or) “-OL”?
A: Pressure reading will display “OL” or “-OL” if the maximum pressure range is exceeded for the given Fieldpiece unit. Pressure ranges are established in accordance with the type of refrigerant that each particular unit is rated on. The units rated for R410-A have a much higher maximum pressure rating than those of strictly R-22. If testing modified or custom-built systems, pressure readings, although unlikely, may be out of the reading range.
If the unit is still reading “OL”
or “-OL” and you have calibrated (“zeroed-out” your
meter to ambient pressure – ATM); verified your pressure is within Fieldpiece’s
specs range (located in the user’s manual) with a gauge; that you have
selected the proper refrigerant; that you are measuring in the correct location
for your reading; you’ve checked all testing equipment batteries; and
that you have no leaks in your testing equipment – please call Fieldpiece
for technical support at 714.257.9060.
17. Why is my TEMPERATURE reading displayed as “OL” or “-OL”?
A: An “OL” or “-OL” reading means out of range. There are several possible causes:
The thermocouple may not be connected properly Make sure the ‘fat’ plug is in the ‘fat’ hole.
The thermocouple wire may be broken. Try another thermocouple or fix your thermocouple. K-type thermocouples tend to break at or near the plug after extensive use. If there are screws on the plug, open it up, cut a couple inches off the thermocouple wire, strip, and reconnect.
Check the clamp end of your thermocouple for physical signs of damage. If the clamp is damaged please call Fieldpiece Support at 714.257.9060 to discuss warranty / repair options.
If the unit is still reading “OL” or “-OL” and you have verified your thermocouple is working; and that all of the batteries in your testing equipment have an adequate charge – please call Fieldpiece for technical support at 714.257.9060.
18.
Why is my SUPERHEAT / SUBCOOLING reading displayed as “OL” or
“-OL”?
A: An “OL” reading for superheat or a “-OL” for subcooling could mean that the system is severely undercharged, however this level of undercharge is rarely achieved and therefore, check the following:
Make certain thermocouple is connected properly and the temperature has been calibrated.
Make certain the proper refrigerant is selected on your Fieldpiece tool.
Make certain hoses and fittings are fastened securely, in the proper location for your measurement, and free from leaks. Also make certain that the unit has been calibrated to ambient pressure.
A negative “-OL” in superheat or a positive “OL” in subcooling indicates that the system is severely overcharged and therefore the system may be flooding liquid refrigerant into the compressor. Check all suspicious measurements with your own calculations before condemning any unit.
If it appears your unit is functioning improperly, contact Fieldpiece technical support at 714.257.9060.
19. My ambient temperature reading is off. How do I calibrate temperature on my Fieldpiece tool?
A: This problem can often be solved with a quick temperature calibration.
Calibrate the instrument to a known temperature using the calibration adjustment. Ice water works well because you can count on it being very close to 32°F (0°C). Turn the “Temp-Cal” on the instrument to calibrate the temperature to match 32°F (0°C).
On the accessory heads, you need to remove the gray rubber strip protecting the T/C pot. Turn the pot using an eyeglass-style (~1.7mm) flat-head screwdriver to.
With the calibration performed, let the thermocouple/pipe clamp return to ambient temperature. If the reading remains “off” make certain your backup temperature device is working properly.
If the unit cannot be calibrated to a known temperature, please contact Fieldpiece technical support at 714.257.9060.
Fieldpiece
Instruments, Inc.
580 W. Central, Suite A
Brea, CA 92821
(714) 257-9060 FAX (714) 257-9069
fporders@fieldpiece.com
