Because of the EPA regulations concerning venting and the use of refrigerants for leak testing, the recommended method for leak testing uses a trace amount of HCFC-22 refrigerant mixed with inexpensive dry nitrogen. The system is first pressurized with the trace quantity of HCFC-22 refrigerant to a pressure of about 10 psig. Dry nitrogen is then used to further increase the system pressure to about 125 psig. The trace refrigerant in this mixture is enough to be detected by an electronic leak detector, while the nitrogen provides the system pressure needed to accomplish the test. After the leak testing is completed, EPA regulations allow the mixture of trace refrigerant and nitrogen to be vented to the atmosphere.
Use of the HCFC-22/nitrogen mixture for leak testing is not limited to systems that use HCFC-22. This mixture is also used for testing systems that normally use other refrigerants such as CFC-12, CFC-500, or CFC-502.
Be aware chat EPA regulations prohibit the addition of nitrogen to a charged system for the purpose of leak detection. The system refrigerant must be recovered before the system is pressurized with the recommended trace refrigerant and nitrogen leak-testing mixture. Pure CFCs and HCFCs, when released during leak detection, are considered a violation of the Clean Air Act.
Pressurizing a system with a trace refrigerant (HCFC-22) and dry nitrogen in preparation for leak testing is performed:
• When a system is without a refrigerant charge (empty).
• When a system with a partial charge of refrigerant has insufficient pressure to support leak detection. In this case, the partial charge of refrigerant must first be recovered and then the system pressurized with the refrigerant and nitrogen mixture, as would be done with an empty system.
Pressurizing systems for leak testing using a mix of refrigerant and nitrogen requires that certain precautions be followed:
• Never use oxygen, compressed air, or flammable gas to pressurize a system! A n explosion will result when oil and
oxygen are mixed.
• Nitrogen is a high-pressure gas (about 2,000 psig). A t full cylinder pressure, nitrogen can rupture a refrigerant cylinder and/or the refrigeration system.
• When charging the system with both a refrigerant and nitrogen, always put the refrigerant in first. Valve off and remove the refrigerant cylinder before connecting the nitrogen cylinder.
• To prevent personal injury and control the system test pressure to the safe test pressure limits established by the system manufacturer, connect the nitrogen to the system using a gauge-equipped accurate pressure regulator on the nitrogen tank and a pressure relief valve in the pressure feed line to the system. The relief valve should be adjusted to open at about 2 psi above the system test pressure, but never more than 150 psig. Figure SP-1-3 shows a typical nitrogen pressure regulator system.
• When pressurizing the system in preparation for leak testing, be sure not to exceed the maximum safe test pressures stamp ed on the unit’s name plate or listed in the manufacturer’s service literature for the unit. A safe maximum is 125 psig. Once the test pressure is established, locate any leaks using an electronic leak detector, bubble solution, or both.
A common method of leak detection is the use o f a leak detecting solution such as a water/soap solution (Figure SP-1-2). Its main advantages are low cost and ease of use. The solution is brushed over the area suspected of leaking. G as coming through the solution will cause bubbles to form. If the leak is very small, several minutes may pass before a bubble will form. Popular commercial leak detection solutions give better, longer-lasting bubbles and more accurate results than plain soapy water. The bubble solution should be removed from the tubing or fittings after checking for leaks as some solutions may corrode the metal.
Electronic leak detectors (Figure SP-1-1) can typically detect leak rates of about 1/2 ounce per year. Normally, the leak detector sensing device is placed next to each component in the system and slowly moved (about one inch per second) above and below areas suspected of leaking. When a refrigerant leak is detected, the leak detector typically gives off an audible alarm, turns on a bright flashing light, or both. For best results, operate the leak detector according to the manufacturer’s instructions. If possible, minimize drafts by shutting off fans and other devices that cause air movement.
If the system contains an adequate refrigerant charge, a visual check of the system might reveal the source of the leak. Since oil is mixed with refrigerant inside the system, the presence of oil around tubing, fittings, and on coil surfaces indicates a leak. Check tightness of all mechanical fittings, since vibration can loosen fittings over time. Use your eyes and ears. Large leaks can sometimes be heard.
If the sight and sound method fails to locate the leak, use an electronic leak detector, leak detection fluid such as soap bubbles, or both, to locate the leak. The electronic leak detector method is easier and gives more accurate results than the bubble method. Normally, an electronic leak detector will be needed to find very small leaks. Make sure your leak detector can sense the refrigerant.
Making current measurements with a multimeter is different than making other measurements with a multimeter. Current measurements are made in series, unlike voltage or resistance measurements, which are made in parallel. The entire current being measured flows through the meter. As previously discussed, the clamp-on ammeter is used to make most of the higher A C current measurements needed for HVAC servicing. The multimeter is used mainly to make low-level DC current measurements in electronic control circuit modules.
Make current measurements as follows (Figure 3-28):
1. Turn off power to the equipment or circuit. Disconnect a component or circuit to make a place where the multimeter probes can be inserted in series with the circuit to be measured.
2. Use the function/range switch to select DC amps. Select A C amps if A C is being measured.
If using an analog meter, always select the highest range. If necessary, switch to a lower range when making the measurement. For accuracy, select the range where the indicator reads in the upper half of the scale face.
3. Plug the test probes into the meter jacks. Usually the black probe is connected to the common (COM) or minus (—) jack. Connect the red probe to the input jack marked for the DC range of the expected reading, the plus (+ ) jack, or other jack as applicable.
4- Connect the test probe tips in series with the circuit so that all current flows through the meter. Measurement is easier if alligator clips are used to connect the meter leads to the circuit. Be sure to turn the circuit power off before connecting the alligator clips.
If using an analog meter to measure DC current, you must observe correct polarity (+/—)- Connect the red test probe to the positive side of the circuit and the black test probe to the negative side or circuit ground. If you reverse the connections, the meter movement goes off the scale in the opposite direction and damage to the meter can result. If using a digital meter with auto polarity, the reading will display a minus sign to indicate negative polarity.
5. Turn the circuit power on. View the reading on the digital meter readout. Be sure to note the unit o f measurement indicated. If using an analog meter, read the voltage value indicated by the pointer on the DC current scale. Make sure to use the scale that matches the selector switch current setting.
Analog (VOM) and digital (DMM) multimeters are used to measure voltage (volts), resistance (ohms), and current (amperes). Some can be used to make other measurements such as temperature or high current; these usually involve the use of one or more special probes or accessories that are readily available.
Figure 3-23 shows an analog multimeter.
The multimeter is used mainly to measure:
• High-level A C voltages in power and load circuits.
• Low-level A C and D C voltages and DC currents in control and electronic module circuits.
• Resistance and continuity on system and component wiring, including motor windings, relay coils, and motor starter/contactor coils.
• Run and start capacitors for a shorted or open condition.
Voltage measurements are usually made to determine source voltage, voltage drop, and/or voltage imbalance. Be sure to always connect the multimeter across (in parallel with) the circuit being measured. Know the capabilities and limitations of your multimeter before attempting any measurements. Read and follow the manufacturer’s instructions.
See Figure 3-24. Make voltage measurements as follows:
1. Use the function/range switch to select A C or DC volts. If using an analog meter, always select a range higher than the highest anticipated reading. For example, if you expect to measure 24 volts AC, select the 300-volt A C range on the meter. Once a reading is obtained, switch to a lower range when making the measurement. For accuracy, select a range where the meter pointer reads in the mid to upper half of the selected range scale.
2. Plug the test probes into the meter jacks. Usually the black probe is connected to the common (COM) or minus (—) jack and the red probe to the plus ( + ) or V-Ohm jack.
3. Connect the test probe tips to the circuit in parallel with the load or power source. Measurement is easier and safer if an alligator clip is used on one of the leads. Be sure to shut off power to the equipment before attaching the alligator leads.
If using an analog meter to measure DC voltage, you must observe correct polarity (+/—). Connect the red test probe to the positive side of the circuit and the black test probe to the negative side or circuit ground. If you reverse the connections, the meter movement will go off the scale in the opposite direction and damage to the meter may result. If using a digital meter with auto polarity, the reading will display a minus sign to indicate negative polarity.
4. View the reading on the digital meter readout. Be sure to note the unit of measurement indicated. If using an analog meter, read the voltage value indicated by the pointer on the A C or D C voltage scale. Make sure to use the scale that matches the selector switch voltage setting.
Multimeters can measure the resistance in ohms (£2) of all or any part of the circuit. Resistance values o f HVAC components can vary greatly from a few ohms to several million ohms. Most multimeters can measure below one ohm; some measure as high as 300 million (meg) ohms. Multimeters contain their own battery power for use when making resistance measurements. Resistance measurements must be made with the equipment power off and all capacitors in the circuit discharged, otherwise damage to the meter can result. Some multimeters have high voltage protection (500 volts or more) in the resistance mode in case o f accidental contact with voltages. This level of protection varies greatly between models.
Resistance measurements are usually made to determine the resistance of a Load; e.g., relay, contactor or starter coils; motor windings; and electronic components such as diodes and resistors.
Make resistance measurements as follows (Figure 3-25).
1. Turn off power to the equipment or circuit.
2. Select resistance (ohms or Q) using the function/range switch.
If using an analog meter, select the lowest resistance range that will accurately read the value. If unknown, start at the highest, then work your way down to a lower range when making the measurement. For accuracy, select the range where the pointer reads in the mid to upper half of the scale.
3. Plug the test probes into the meter jacks. Usually the black probe is connected to the common (COM) or minus (-) jack and the red probe to the plus ( + ) or V-Ohm jack. If using an analog meter, always zero the meter before the first measurement and whenever you change range scales. To zero the meter, touch the tips of the test probes together, then use the zero adjustment knob (Figure 3-23) to set the pointer to zero.
4. Before measuring resistance, make sure to electrically isolate
the component being measured by disconnecting at least one lead of the component from the circuit- This is important in order to get an accurate resistance reading. Otherwise, the meter will read the combined resistance of all components that are connected in parallel with the component to be measured.
5. Connect the test probe tips across the component or portion of the circuit you want to measure.
6. View the reading on the digital readout or analog scale.
If using a digital meter, be sure to note the unit of measurement: ohms (Q), kilohms (kQ), or megohms (MQ) shown for the reading.
If using an analog meter, determine the resistance value by multiplying the scale reading by the number (R x 1, R x 10, etc.) next to the function/range switch.
As shown on Figure 3-26, the reading “5 ” is multiplied by the selector setting R x 1, yielding a resistance of 5 ohms. At the R x 100 setting, the same reading would be 500 ohms, and at the R x 10,000 setting, it would be 50,000 ohms.
A continuity check is a go/no-go resistance test used to test for open and closed circuits. Examples are shown in Figure 3-27.
A good fuse offers no resistance. If using an analog meter for the measurement as shown, the pointer moves all the way to the right, displaying continuity (zero ohms). If using a digital meter, it may beep to indicate that continuity was detected and the digital readout will also show zero ohms. The level of resistance required to trigger the digital beeper varies from model to model. This beep feature is helpful because it allows continuity checks to be made without having to look at the meter reading.
If the fuse is bad, it has no path for current flow. The analog meter has no deflection and the pointer remains at the far left as shown, displaying infinite resistance or infinity (°°). This indicates an open circuit. If using a digital meter, it would indicate infinite resistance by reading “OL” , flashing digits, or a similar message on the display indicating that the resistance is greater than the digital meter can measure.
Unless specified by the manufacturer, the VOM/DMM should not be used to make resistance measurements on an electronic device. This is because when set up to measure resistance, the test voltage supplied by the internal battery of the VOM/DMM may exceed the safe voltage level for the electronic device under test, resulting in damage to the device.
Ammeters measure current flow through a circuit. Depending on the level of the current being measured, the ammeter reads in amperes, milliamperes (one thousandth of an ampere) or microamperes (one millionth of an ampere). Current must flow directly through the ammeter meter movement for it to record. For this reason, standard (in-line) ammeters such as those incorporated in multimeters, must always be connected in series with the load. This requires disconnecting or opening the normal circuit to insert the ammeter. Because of this inconvenience, as well as for safety reasons, the A C clamp-on ammeter is used almost exclusively for HVAC field service.
The clamp-on ammeter is most often used to measure the total AC current being drawn by a system or by individual loads like the compressor, fan motors, or heaters (Figure 3-20). For servicing three-phase systems, it is used to measure the current drawn by each phase to determine the percent of current imbalance* if any.
Clamp-on ammeters have a movable set o f jaws that can be opened and placed (clamped) around each of the wires to be measured, one wire at a time. The clamp-on ammeter works like a transformer. The wire being measured acts like the primary of a transformer and the “jaws” of the ammeter as the secondary. Current flowing through the wire creates lines of force that induce a current in the jaws. The induced current passes through the meter movement, providing an indication of how much current is passing through the wire. A clamp-on ammeter is easy to use, but a review of a few basic techniques will help you to obtain maximum performance.
• The jaws must be clean and properly aligned, or an error in the reading can result.
• Always start to measure within the highest possible measurement range and work toward the lower range to prevent
damage to the meter.
• Do not turn a motor off and then on with the meter clamped around the motor lead. This precaution prevents damage to the meter from current surges.
• Do not clamp the jaws of the meter around two different wires at the same time. This will cause the meter to read an incorrect value of current.
Sometimes, the current being measured will be so low that it is difficult to get an accurate measurement, even on the lowest scale of the meter. One way to overcome this problem is to coil the wire through the jaws of the meter (Figure 3-21).
Winding one loop of the wire (two passes) around the jaws doubles the strength of the magnetic field, resulting in a meter reading that is twice the amount of current than is actually flowing in the circuit. If one loop is passed through the jaws, you must divide the meter reading by two (because of two passes) to determine the actual current. If two loops are passed through the jaws, you divide by three (three passes), and so on.
The adjustment of a room thermostat heat anticipator is a practical example of a procedure where this method is commonly used. The thermostat heat anticipator must be set to match the current draw of the components (heat control, gas valve, etc.) connected in the R-W circuit. As shown in Figure 3-22, an insulated wire is looped nine times around the jaws (ten passes through the jaws) of a clamp-on ammeter, then connected to the R and W terminals of the thermostat. For the purpose of this example, assume that the ammeter reads 5 amperes. Based on this measurement, you would adjust the heat anticipator to 0.5 amps (5 amps h- 10 passes = 0.5 amps).
The procedure used for disconnecting the gauge manifold set varies, depending on the type of service valves installed in the unit.
If the unit being serviced contains back-seating service valves, the charge remaining in the hoses can be drawn back into the operating system using the following procedure:
1. On the gauge manifold set, close (front-seat) both valves. Make sure you close the valve on the refrigerant cylinder (if used).
2. Start and run the system to check temperatures and pressures.
If they are correct, back-seat the system high-side service valve. This traps refrigerant in the high-side hose and the utility hose.
3. Open both valves on the gauge manifold set to allow any refrigerant trapped in the high side and utility hoses to be drawn into the system through the low-side service valve.
4. Once both manifold gauge pressures have equalized at the low-side pressure, back-seat the low-side service valve. This allows only a minimal amount of refrigerant to escape to the atmosphere when all hoses are disconnected.
If the system is equipped with Schrader valves instead of back-seating service valves, service hoses equipped with fast self-sealing fittings must be used with the gauge manifold set for this procedure to work. To remove the gauge manifold hoses from a system with Schrader valves, use the following procedure:
1. Turn off the equipment or system.
2. Close (front-seat) both gauge manifold set valves. If applicable, make sure you close the valve on the refrigerant cylinder- Remove the high-side hose from the service valve or service port. Leave the low-side hose attached to the service valve or service port. Th is traps refrigerant in the high-side hose at the high-side pressure. It also traps the refrigerant in the utility hose (if used) at the cylinder pressure.
3. Start the equipment or system, then open (crack) both gauge manifold valves. This allows the refrigerant trapped in the high side and utility hoses to be brought down to a lower and safer low-side (suction) pressure by drawing it into the operating system.
4. Remove the low-side hose. This traps the remaining refrigerant in the low-side hose at suction pressure.