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11-18-HVAC-Skills-Workshop.Pdf HVAC&R MODULE Skills 117 CALIBRATION Workshop AND USE OF PROUDLY SPONSORED BY INSTRUMENTS The HVAC&R technician is often required to commission new installations, analyse the performance of existing plant and locate faults on new and existing plant. To carry out this work, a variety of measuring and test Skills summary instruments are available. n What? Some are simple, low-cost instruments, and others are highly expensive, A guide to the calibration and sophisticated units beyond the scope (and budget) of the average use of air-measuring instruments. technician. No matter how much they cost, they require calibration. n Who? Relevant for anyone involved in the installation, This Skills Workshop looks at the calibration and use of instruments, operation, maintenance and assessment specifically those used for air measuring. of HVAC&R systems. All instruments are sensitive to damage and must or diaphragm. Recording-type barometers are be regularly calibrated against the manufacturers’ available in either a 24-hour or seven-day drum master instruments or an accepted standard. rotation. These are similar in appearance to the recording thermometers previously described. However, experience shows that most errors result from: SERVICE GAUGES (a) the misuse of the instrument PULLOUT AND MANIFOLD (b) air turbulence where readings are made The most important tool of the refrigeration (c) calculation errors – particularly in taking technician is the service gauge and manifold. air velocity readings in m/s as indicating It is used for checking system pressures, charging volume in m3/s. refrigerant, evacuating the system, adding oil and for many other purposes. The student or operator, when first receiving an air-measuring instrument, should experiment The gauge manifold consists of a compound with it by taking readings in different locations gauge, calibrated from -100kPa to 700kPa, on the one system, at different angles to the or from -100 to 2,100kPa for R410A, and a high-pressure gauge, calibrated from 0kPa airflow and at different distances from filters, to approximately 2,800kPa (0 to 5,500kPa for coils, dampers, outlet grilles or fans – and noting R410A), mounted on the manifold assembly, any differences. Once a technique is developed which has isolating valves and service line to give acceptable readings in a given situation, connections. The left-hand gauge is the high- stick to that technique when using that instrument. or discharge-pressure gauge. Flexible service Figure 1: Refrigeration service gauge and manifold with digital read-out. (Courtesy of Testo Pty Ltd.) Air turbulence occurs for a distance of six or seven hoses with rubber sealing fittings are used to make connections from the manifold to the system. times the outlet diameter from any restriction. accuracy at very low pressures, and is fragile Accurate readings (i.e., those unaffected by location Figure 1 shows an electronic gauge manifold and not without problems if carelessly handled. situations) are only possible under ideal conditions. with a digital read-out, which – with the possibility It is possible, for example, to fill the gauge with of connecting various probes for temperature, compressor oil, which is serious enough, but it is Calculation errors can be minimised only by current and oil pressure – provides for a wide also possible to have the mercury drawn into the the operator recording all test figures, as read, range of measurements and data storage and compressor – with absolutely disastrous results. and then showing all calculations to convert management, including superheating and to “standard” units, so that someone else subcooling calculation, vacuum measurement, Thermocouple gauge can recheck the correctness of the working. temperature-compensated tightness testing The thermocouple gauge, or electronic vacustat, Do not do conversions in your head. and refrigerant management. gives a direct dial scale of the vacuum based on the number of electrons which strike, and BAROMETERS therefore cool, the heater thermocouple. The VACUUM GAUGES better the vacuum, the fewer are the molecules Barometers are used to measure atmospheric Normal hydrostatic gauges such as the Bourdon – and the hotter the thermocouple. Figure 3 pressure. Barometers used in air conditioning tube or diaphragm dial gauges are accurate at best shows the circuit of the chamber connected to are usually of a different type. The barometric to 133Pa (1mm) only. The mercury manometer is the vacuum. The electronic service gauge shown pressure is measured by the deflection of a bellows capable of low readings but is of questionable in Figure 1 also has a vacuum measurement function. Spring/Summer 2018 | www.airah.org.au/nation | HVAC&R Nation | 15 HVAC&R Skills Workshop low high low high low high Purging Charging or adding oil Bypassing low high low high A B E Both manifold valves turned all the way i ystem pumping C vapour and both low- and high-side Gauge reading pressures being read. D Figure 2: Schematic of gauge manifold installation on external drive compressor with service valves. (A – manifold suction valve; B – manifold discharge valve; C – compressor discharge service valve; D – compressor suction service valve; E – service opening.) It is important to use the tube in an upright position to keep out foreign matter. NOTE Never allow the system pressure to enter a high-vacuum gauge. base connections X Y Y X heater Figure 4: High-vacuum thermocouple gauge. filament vacuum Vacuum scales chamber Gauges may be calibrated in millimetres of mercury (torr) or microns (μm) – more thermocouple correctly called micrometres – of mercury. Gauges may also be calibrated in pascals (Pa) and the relationship between the various scales is shown in Figure 5 (parts a, b and c). However, the international unit accepted in Australia at the moment appears to be the torr, X connect to galvanometer connection with one torr equal to one millimetre of mercury (Hg); to manifold therefore 0.1 mm (100 microns) equals 10−1 Torr Y connect to electrical etc, as shown. power source The continued change to the SI metric system will eventually make the pascal the standard unit Figure 3: Principle of thermocouple vacuum gauge. for vacuum measurement. 16 | HVAC&R Nation | www.airah.org.au/nation | Spring/Summer 2018 HVAC&R Skills Workshop (a) (b) Millimetres of mercury Equivalents 1 2 Microns Millimetres Torr Pascals 0.5 5 10 1 .001 mm 10−3 Torr 0.1333 Pa 20 ATM 1000 10 .010 mm 10−2 Torr 1.3332 Pa 500 microns 0 microns 100 .100 mm 10−1 Torr 13.3322 Pa 300 microns 150 .150 mm 15 × 10−2 Torr 20.0 Pa 100 microns 500 .5 mm 5 × 10−1 Torr 66.66 Pa 1000 1.0 mm 1 Torr 133.32 Pa (c) Absolute units Torr (mm Hg) Microns Inches mercury Pa kPa Actual ("Hg) Expressed as value 760 000 760 760 30 101 592 101.592 500 000 500 500 19.685 66 661.1 66.661 1 250 000 250 250 9.842 5 33 330.5 33.330 5 100 000 100 100 3.937 01 13 332.2 13.332 2 75 000 75 75 2.952 75 9990 9.99 50 000 50 50 1.968 50 6660 6.66 25 000 25 25 0.984 25 3330 3.33 20 000 20 20 0.787 40 2660 2.66 15 000 15 15 0.590 55 1990 1.99 10 000 10 10 0.393 70 1330 1.33 5000 5 5 0.196 85 667 0.667 4000 4 4 0.157 48 533 0.533 PULLOUT 3000 3 3 0.118 11 400 0.400 2000 2 2 0.078 74 267 0.267 1000 1 1 0.039 37 133 0.133 750 75 × 10−2 0.75 0.029 53 99 0.099 500 5 × 10−1 0.50 0.019 69 67 0.067 250 25 × 10−2 0.25 0.009 84 33 0.033 100 1 × 10−1 0.10 0.003 94 13 0.013 50 5 × 10−2 0.05 0.001 97 6.6 0.006 6 25 25 × 10−3 0.025 0.000 98 3.3 0.003 3 10 1 × 10−2 0.01 0.000 39 1.3 0.001 3 5 5 × 10−3 0.005 0.000 19 0.7 0.000 7 1 1 × 10−3 0.001 0.000 04 0.13 0.000 13 0.5 5 × 10−4 0.000 5 0.000 02 0.07 0.000 065 Figure 5: Comparison of vacuum measurements units EDITION 05 This month’s Skills Workshop has been taken from AUSTRALIAN REFRIGERATION AND AIR-CONDITIONINGNATIONAL RESOURCE Australian Refrigeration and Air-Conditioning Volume 1, T TO YOU H BY UG O R B 0 2 9 1 G E I V C I N N I G S B Y A R C T K S T DU O T H E I N by Graham Boyle, F.AIRAH. VOLUME 01 PROUDLY SPONSORED BY Next month: Controlling condensation in mechanical services. Spring/Summer 2018 | www.airah.org.au/nation | HVAC&R Nation | 17.
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