It is impossible to install or service air-conditioning and refrigeration units and systems without using gages and instruments.
A number of values must be measured accurately if air-conditioning and refrigeration equipment is to be operated properly. Refrigeration and air-conditioning units must be properly serviced and monitored if they are to give the maximum efficiency for the energy expended. Here, the use of gages and instruments becomes important. It is not possible to analyze a system’s operation without the proper equipment and procedures. In some cases, it takes thousands of dollars worth of equipment to troubleshoot or maintain modern refrigeration and air-conditioning system.
Instruments are used to measure and record such values as temperature, humidity, pressure, airflow, electrical quantities, and weight. Instruments and monitoring tools can be used to detect incorrectly operating equipment. They can also be used to check efficiency. Instruments can be used on a job, in the shop, or in the laboratory. If properly cared for and correctly used, modern instruments are highly accurate.
Pressure Gages
Pressure gages are relatively simple in function. See Fig. 1-21. They read positive pressure or negative pressure, or both. See Fig. 1-22. Gage components are
Fig. 1-21 Pressure gage. (Weksler)
Fig. 1-22 This gage measures up to 150 psi pressure and also reads from 0 to 30 for vacuum. The temperature scaled runs from
–40¥ to 115¥F (–40¥ to 46.1¥C).
relatively few. However, different combinations of gage components can produce literally millions of de- sign variations. See Fig. 1-23. One gage buyer may use a gage with 0 to 250 psi range, while another person with the same basic measurement requirements will or- der a gage with a range of 0 to 300 psi. High-pressure gages can be purchased with scales of 0 to 1000, 2000, 3000, 4000, or 5000 psi.
There are, of course, many applications that will continue to require custom instruments, specially designed and manufactured. Most gage manufacturers have both stock items and specially manufactured gages.
Fig. 1-23 Bourdon tube arrangement and parts of a gage. (Marsh)
Gage Selection
Since 1939, gages used for pressure measurements have been standardized by the American National Standards Institute (ANSI). Most gage manufacturers are consistent in face patterns, scale ranges, and grades of accuracy. Industry specifications are revised and up- dated periodically.
Gage accuracy is stated as the limit that error must not exceed when the gage is used within any combination of rated operating conditions. It is expressed as a percentage of the total pressure (dial) span.
Classification of gages by ANSI standards has a significant bearing on other phases of gage design and specification. As an example, a test gage with ±0.25 percent accuracy would not be offered in a 2 in. dial size. Readability of smaller dials is not sufficient to permit the precision indication necessary for this degree of accuracy. Most gages with accuracy of ±0.5 percent and better have dials that are at least 4.5 in. Readability can be improved still further by increasing the dial size.
Accuracy How much accuracy is enough? This is a question only the application engineer can answer. However, from the gage manufacturer’s point of view, increased accuracy represents a proportionate increase in the cost of building a gage. Tolerances of every component must be more exacting as gage accuracy increases.
Time is needed for technicians to calibrate the gage correctly. A broad selection of precision instruments is available and grades A (±1 percent), 2A (±0.5 percent), and 3A (±0.25 percent) are examples of tolerances available.
With the advent of modern electronic gages and more sophisticated equipment it is possible to obtain heretofore undreamed of accuracy automatically with equipment used in the field.
Medium In every gage selection, the medium to be measured must be evaluated for potential corrosive- ness to the Bourdon tube of the gage.
There is no ideal material for Bourdon tubes. No single material adapts to all applications. Bourdon tube materials are chosen for their elasticity, repeatability, ability to resist “set” and corrosion resistance to the fluid mediums.
Ammonia refrigerants are commonly used in refrigeration. All-steel internal construction is required. Ammonia gages have corresponding temperature scales. A restriction screw protects the gage against sudden impact, shock, or pulsating pressure. A heavy- duty movement of stainless steel and Monel steel pre- vents corrosion and gives extra-long life. The inner arc on the dial shows pressure. The other arc shows the corresponding temperature. See Fig. 1-24.
Fig. 1-24 Ammonia gage. (Marsh)
Line Pressure
The important consideration regarding line pressures is to determine whether the pressure reading will be constant or whether it will fluctuate. The maximum pressure at which a gage is continuously operated should not
exceed 75 percent of the full-scale range. For the best performance, gages should be graduated to twice the normal system-operating pressure.
This extra margin provides a safety factor in pre- venting overpressure damage. It also helps avoid a permanent set of the Bourdon tube. For applications with substantial pressure fluctuations, this extra margin is especially important. In general, the lower the Bourdon tube pressure, the greater the overpressure percentage it will absorb without damage. The higher the Bourdon tube pressure, the less overpressure it will safely absorb.
Pulsation causes pointer flutter, which makes gage reading difficult. Pulsation also can drastically shorten gage life by causing excessive wear of the movement gear teeth. A pulsating pressure is defined as a pressure variation of more than 0.1 percent full-scale per second. Following are conditions often encountered and suggested means of handling them.
The restrictor is a low-cost means of combating pulsation problems. This device reduces the pressure opening. The reduction of the opening allows less of the pressure change to reach the Bourdon tube in a given time interval. This dampening device protects the Bourdon tube by the retarding overpressure surges. It also improves gage readability by reducing pointer flutter. When specifying gages with restrictors, indicate whether the pressure medium is liquid or gas. The medium determines the size of the orifice. In addition, restrictors are not recommended for dirty line fluids. Dirty materials in the line can easily clog the orifice. For such conditions, diaphragm seals should be specified.
The needle valve is another means of handling pulsation if used between the line and the gage. See Fig. 1-25. The valve is throttled down to a point where pulsation ceases to register on the gage.
In addition, to the advantage of precise throttling, needle valves also offer complete shutoff, an important safety factor in many applications. Use of a needle valve can greatly extend the life of the gage by allowing it to be used only when a reading is needed.
Liquid-filled gages are another very effective way to handle line pulsation problems. Because the movement is constantly submerged in lubricating fluid, re- action to pulsating pressure is dampened and the pointer flutter is practically eliminated.
Silicone-oil-treated movements dampen oscillations caused by line pressure pulsations and/or mechanical oscillation. The silicone oil, applied to the movement, bearings, and gears, acts as a shock absorber.
Fig. 1-25 Different types of needle valves. (Marsh)
This extends the gage life while helping to maintain accuracy and readability.
Effects of Temperature on Gage Performance
Because of the effects of temperature on the elasticity of the tube material, the accuracy may change. Gages calibrated at 75°F (23.9°C) may change by more than 2 percent at:
- Full scale (FS) below -30°F (-34°C)
- Above 150° F (65.6°C)
Care of Gages
The pressure gage is one of the service person’s most valuable tools. Thus, the quality of the work depends on the accuracy of the gages used. Most are precision- made instruments that will give many years of depend- able service if properly treated.
The test gage set should be used primarily to check pressures at the low and high side of the com- pressor. The ammonia gage should be used with a steel Bourdon tube tip and socket to prevent damage.
Once you become familiar with the construction of your gages, you will be able to handle them more efficiently. The internal mechanism of a typical gage is shown in Fig. 1-23. The internal parts of a vapor tension thermometer are very similar.
Drawn brass is usually used for case material. It does not corrode. However, some gages now use high- impact plastics. A copper alloy Bourdon tube with a brass tip and socket is used for most refrigerants. Stainless steel is used for ammonia. Engineers have found that moving parts involved in rolling contact will last longer if made of unlike metals. That is why many top-grade refrigeration gages have bronze- bushed movements with a stainless steel pinion and arbor.
The socket is the only support for the entire gage. It extends beyond the case. The extension is long enough to provide a wrench flat enough for use in attaching the gage to the pressure source. Never twist the case when threading the gage into the outlet. This could cause misalignment or permanent damage to the mechanism.
NOTE: Keep gages and thermometers separate from other tools in your service kit. They can be knocked out of alignment by a jolt from a heavy tool.
Most pressure gages for refrigeration testing have a small orifice restriction screw. The screw is placed in the pressure inlet hole of the socket. It reduces the effects of pulsations without throwing off pressure readings. If the orifice becomes clogged, the screw can be easily removed for cleaning.
Gage Recalibration
Most gages retain a good degree of accuracy in spite of daily usage and constant handling. Since they are precision instruments, however, you should set up a regular program for checking them. If you have a regular program, you can be sure that you are working with ac- curate instruments.
Gages will develop reading errors if they are dropped or subjected to excessive pulsation, vibration, or a violent surge of overpressure. You can restore a gage to accuracy by adjusting the recalibration screw. See Fig. 1-26. If the gage does not have a recalibration screw, remove the ring and glass. Connect the gage you are testing and a gage of known accuracy to the same pressure source. Compare readings at midscale. If the gage under test is not reading the same as the test gage, remove the pointer and reset.
Fig. 1-26 Recalibrating a gage. (Marsh)
This type of adjustments on the pointer acts merely as a pointer-setting device. It does not reestablish the original even increment (linearity) of pointer travel. This becomes more apparent as the correction requirement becomes greater.
If your gage has a re-calibrator screw on the face of the dial, as in Fig. 1-26, remove the ring and glass. Relieve all pressure to the gage. Turn the recalibration screw until the pointer rests at zero.
The gage will be as accurate as when it left the factory if it has a screw recalibration adjustment. Reset- ting the dial to zero restores accuracy throughout the entire range of dial readings.
If you cannot calibrate the gage by either of these methods, take it to a qualified specialist for repair.
