Viscosity and Viscosity Index of Compressor Oils

Compressor oil plays a key role in the appropriate and long term functioning of compressors. As it is responsible for making a proper film layer between the moving parts (e.g. piston, cylinder), and for cooling the components, therefore it is very important that what kind of combination ability the specific compressor oil has with the used refrigerant, and what kind of viscosity index belongs to the given oil.

Viscosity of Compressor Oils

One of the important criteria in case of compressor oil is its viscosity. The term viscosity describes the given liquid’s resistance to flow, so an oil with bigger viscosity flows less, and an oil with lower viscosity is thin fluid. The kinematic viscosity describes the oil’s flowing time on a given temperature, its unit in the SI-system is mm2/s, or the commonly used centiStoke (cSt). Thus 1 cSt ≈ 1 mm2/s. In the base of this kinematic viscosity the ISO 3448:1992 standard defines a viscosity group system which determines the minimal and maximal kinetic viscosity point of a given oil on 40° Celsius. The average value derived from this two points indicates the name of the given group. Accordingly, e.g. an oil which has a minimal viscosity of 28 mm2/s, and a maximal viscosity of 35.2 mm2/s, takes place in the group ISO VG 32. (The VG means Viscosity Group). Though viscosity is affected by several factors (e.g. the kinetic speed of the components, and the quality of machining), essentially it needs to be striving after picking the lower but still satisfying viscosity classification.

Viscosity Groups based on 3348:1992 ISO standard
ISO 3448 Viscosity Group (40°C)	Minimal Kinetic Viscosity (mm²/s)	Maximal Kinetic Viscosity (mm²/s)	Kinetic Viscosity Midpoint (mm²/s)
ISO VG 2	1.98	2.42	2.2
ISO VG 3	2.88	3.52	3.2
ISO VG 5	4.14	5.06	4.6
ISO VG 7	6.12	7.48	6.8
ISO VG 10	9	11	10
ISO VG 15	13.5	16.5	15
ISO VG 22	19.8	24.2	22
ISO VG 32	28.8	35.2	32
ISO VG 46	41.4	50.6	46
ISO VG 68	51.2	74.8	68
ISO VG 100	90	110	100
ISO VG 150	135	165	150
ISO VG 220	198	242	220
ISO VG 320	288	352	320
ISO VG 460	414	506	460
ISO VG 680	612	748	680
ISO VG 1000	900	1100	1000
ISO VG1500	1350	1650	1500

Determining the Viscosity Index of Compressor Oils

The other critical characteristic of compressor oils which is important in case of cooling the components too is the alteration of viscosity due to temperature changing. The alteration of viscosity is described by the viscosity index (VI) which shows that how the viscosity of the given oil changes due to temperature changing, so the index number indicates the alteration of viscosity, not the viscosity itself. If the temperature of the oil rises, its viscosity will decrease; and if the temperature of the oil is down, its viscosity will get larger. The viscosity of the oils (known as ’paraffinic oils’) that has the best viscosity index (= 100) alters less, while in case of the oils (known as ’naphthenic oils’) which has the worst viscosity index (= 0) the viscosity changes in a large measure. So in case of the paraffinic and of naphthenic oils, too, there is a viscosity index based sequence, by which the kinematic viscosity of the specific oil is given. To determine the viscosity index of a specific oil, it needs to be picking one from those paraffinic and one from those naphthenic base oils, which have the same value of viscosity index at 100°C as the wanted oil has at 100°C. The following table summarizes the kinematic viscosity of the base oils at 100°C, in addition it contains a value (L) for the base oils which have the viscosity index of 0 at 40°C, and another value (H) for the base oils which have the viscosity index of 100 at 40°C.

The base oil values (ν, L and H) used for determining the viscosity index of wanted oils
Kinetic Viscosity (ν) (100°C. mm²/s)	L	H
2.00	7.994	6.394
2.10	8.640	6.894
2.20	9.309	7.410
2.30	10.00	7.944
2.40	10.71	8.496
2.50	11.45	9.063
2.60	12.21	9.647
2.70	13.00	10.25
2.80	13.80	10.87
2.90	14.63	11.50
3.00	15.49	12.15
3.10	16.36	12.82
3.20	17.26	13.51
3.30	18.18	14.21
3.40	19.12	14.93
3.50	20.09	15.66
3.60	21.08	16.42
3.70	22.09	17.19
3.80	23.13	17.97
3.90	24.19	18.77
4.00	25.32	19.56
4.10	26.50	20.37
4.20	27.75	21.21
4.30	29.07	22.05
4.40	30.48	22.92
4.50	31.96	23.81
4.60	33.52	24.71
4.70	35.13	25.63
4.80	36.79	26.57
4.90	38.50	27.53
5.00	40.23	28.49
5.10	41.99	29.46
5.20	43.76	30.43
5.30	45.53	31.40
5.40	47.31	32.37
5.50	49.09	33.34
5.60	50.87	34.32
5.70	52.64	35.29
5.80	54.42	36.26
5.90	56.20	37.23
6.00	57.97	38.19
6.10	59.74	39.17
6.20	61.52	40.15
6.30	63.32	41.13
6.40	65.18	42.14
6.50	67.12	43.18
6.60	69.16	44.24
6.70	71.29	45.33
6.80	73.48	46.44
6.90	75.72	47.51
7.00	78.00	48.57
7.10	80.25	49.61
7.20	82.39	50.69
7.30	84.53	51.78
7.40	86.66	52.88
7.50	88.85	53.98
7.60	91.04	55.09
7.70	93.20	56.20
7.80	95.43	57.31
7.90	97.72	58.45
8.00	100.0	59.60
8.10	102.3	60.74
8.20	104.6	61.89
8.30	106.9	63.05
8.40	109.2	64.18
8.50	111.5	65.32
8.60	113.9	66.48
8.70	116.2	67.64
8.80	118.5	68.79
8.90	120.9	69.94
9.00	123.3	71.10
9.10	125.7	72.27
9.20	128.0	73.42
9.30	130.4	74.57
9.40	132.8	75.73
9.50	135.3	76.91
9.60	137.7	78.08
9.70	140.1	79.27
9.80	142.7	80.46
9.90	145.2	81.67
10.0	147.7	82.87
10.1	150.3	84.08
10.2	152.9	85.30
10.3	155.4	86.51
10.4	158.0	87.72
10.5	160.6	88.95
10.6	163.2	90.19
10.7	165.8	91.40
10.8	168.5	92.65
10.9	171.2	93.92
11.0	173.9	95.19
11.1	176.6	96.45
11.2	179.4	97.71
11.3	182.1	98.97
11.4	184.9	100.2
11.5	187.6	101.5
11.6	190.4	102.8
11.7	193.3	104.1
11.8	196.2	105.4
11.9	199.0	106.7
12.0	201.9	108.0
12.1	204.8	109.4
12.2	207.8	110.7
12.3	210.7	112.0
12.4	213.6	113.3
12.5	216.6	114.7
12.6	219.6	116.0
12.7	222.6	117.4
12.8	225.7	118.7
12.9	228.8	120.1
13.0	231.9	121.5
13.1	265.0	122.9
13.2	238.1	124.2
13.3	241.2	125.6
13.4	244.3	127.0
13.5	247.4	128.4
13.6	250.6	129.8
13.7	253.8	131.8
13.8	257.0	132.6
13.9	260.1	134.0
14.0	263.3	135.4
14.1	266.6	136.8
14.2	269.8	138.2
14.3	273.0	139.6
14.4	276.3	141.0
14.5	279.6	142.4
14.6	283.0	143.9
14.7	286.4	145.3
14.8	289.7	146.8
14.9	293.0	148.2
15.0	296.5	149.7
15.1	300.0	151.2
15.2	303.4	152.6
15.3	306.9	154.1
15.4	310.3	156.6
15.5	313.9	157.0
15.6	317.5	158.6
15.7	321.1	160.1
15.8	324.6	161.6
15.9	328.3	163.1
16.0	331.9	164.6
16.1	335.5	166.1
16.2	339.2	167.7
16.3	342.9	169.2
16.4	346.6	170.7
16.5	350.3	172.3
16.6	354.1	173.8
16.7	358.0	175.4
16.8	361.7	177.0
16.9	365.6	178.6
17.0	369.4	180.2
17.1	373.3	181.7
17.2	377.1	183.3
17.3	381.0	184.9
17.4	384.9	186.5
17.5	388.9	188.1
17.6	392.7	189.7
17.7	396.7	191.3
17.8	400.7	192.9
17.9	404.6	194.6
18.0	408.6	196.2
18.1	412.6	197.8
18.2	416.7	199.4
18.3	420.7	201.0
18.4	424.9	202.6
18.5	429.0	204.3
18.6	433.2	205.9
18.7	437.3	207.6
18.8	441.5	209.3
18.9	445.7	211.0
19.0	449.9	212.7
19.1	454.2	214.4
19.2	458.4	216.1
19.3	462.7	217.7
19.4	467.0	219.4
19.5	471.3	221.1
19.6	475.7	222.8
19.7	479.7	224.5
19.8	483.9	226.2
19.9	488.6	227.7
20.0	493.2	229.5
20.2	501.5	233.0
20.4	510.8	236.4
20.6	519.9	240.1
20.8	528.8	243.5
21.0	538.4	247.1
21.2	547.5	250.7
21.4	556.7	254.2
21.6	566.4	257.8
21.8	575.6	261.5
22.0	585.2	264.9
22.2	595.0	268.6
22.4	604.3	272.3
22.6	614.2	275.8
22.8	624.1	279.6
23.0	633.6	283.3
23.2	634.4	286.3
23.4	653.8	290.5
23.6	663.3	294.4
23.8	673.7	297.9
24.0	683.9	301.8
24.2	694.5	305.6
24.4	704.2	309.4
24.6	714.9	313.0
24.8	725.7	317.0
25.0	736.5	320.9
25.2	747.2	324.9
25.4	758.2	328.8
25.6	769.3	332.7
25.8	779.7	336.7
26.0	790.4	340.5
26.2	801.6	344.4
26.4	812.8	348.4
26.6	824.1	352.3
26.8	835.5	356.4
27.0	847.0	360.5
27.2	857.5	364.6
27.4	869.0	368.3
27.6	880.6	372.3
27.8	892.3	376.4
28.0	904.1	380.6
28.2	915.8	384.6
28.4	927.6	388.8
28.6	938.6	393.0
28.8	951.2	396.6
29.0	963.4	401.1
29.2	975.4	405.3
29.4	987.1	409.5
29.6	998.9	413.5
29.8	1011	417.6
30.0	1023	421.7
30.5	1055	432.4
31.0	1086	443.2
31.5	1119	454.0
32.0	1151	464.9
32.5	1184	475.9
33.0	1217	478.0
33.5	1251	498.1
34.0	1286	509.6
34.5	1321	521.1
35.0	1356	532.5
35.5	1391	544.0
36.0	1427	555.6
36.5	1464	567.1
37.0	1501	579.3
37.5	1538	591.3
38.0	1575	603.1
38.5	1613	615.0
39.0	1651	627.1
39.5	1691	639.2
40.0	1730	651.8
40.5	1770	664.2
41.0	1810	676.6
41.5	1851	689.1
42.0	1892	701.9
42.5	1935	714.9
43.0	1978	728.2
43.5	2021	741.3
44.0	2064	754.4
44.5	2108	767.6
45.0	2152	780.9
45.5	2197	794.5
46.0	2243	808.2
46.5	2288	821.9
47.0	2333	835.5
47.5	2380	849.2
48.0	2426	863.0
48.5	2473	876.9
49.0	2521	890.9
49.5	2570	905.3
50.0	2618	919.6
50.5	2667	933.6
51.0	2717	948.2
51.5	2767	962.9
52.0	2817	977.5
52.5	2867	992.1
53.0	2918	1007
53.5	2969	1021
54.0	3020	1036
54.5	3073	1051
55.0	3126	1066
55.5	3180	1082
56.0	3233	1097
56.5	3286	1112
57.0	3340	1127
57.5	3396	1143
58.0	3452	1159
58.5	3507	1175
59.0	3563	1190
59.5	3619	1206
60.0	3676	1222
60.5	3734	1238
61.0	3792	1254
61.5	3850	1270
62.0	3908	1286
62.5	3966	1303
63.0	4026	1319
63.5	4087	1336
64.0	4147	1352
64.5	4207	1396
65.0	4268	1386
65.5	4329	1402
66.0	4392	1419
66.5	4455	1436
67.0	4517	1454
67.5	4580	1471
68.0	4645	1488
68.5	4709	1506
69.0	4773	1523
69.5	4839	1541
70.0	4905	1558

To determine the viscosity index, we can use the following formula:

The symbols that are used in the above formula are the following:

VI: viscosity index
U: the kinematic viscosity of the wanted oil sample at 40°C (mm2/s)
L: the viscosity index of that base oil at 40°C (mm2/s), which has a viscosity index of 0, and its viscosity at 100°C is the same as the viscosity of the wanted oil
H: the viscosity index of that base oil at 40°C (mm2/s), which has a viscosity index of 100, and its viscosity at 100°C is the same as the viscosity of the wanted oil

The following graph shows the determining of viscosity index:

Viszkozitási index kiszámítása grafikon

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