Equipment Glossary Acknowledgements


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Appendix A - Using Hydrometers

A hydrometer measures the difference in gravity (density) between pure water and water with sugar dissolved in it by flotation. The hydrometer is used to gauge the fermentation progress by measuring one aspect of it, attenuation. Attenuation is the conversion of sugar to ethanol by the yeast. Water has a specific gravity of 1.000. Beers typically have a final gravity between 1.015 and 1.005. Champagnes and meads can have gravities less than 1.000, because of the large percentage of ethyl alcohol, which is less than 1. Hydrometer readings are standardized to 59F (15C). Liquid gravity (density) is dependent on temperature and temperature correction tables are usually sold with the hydrometer or are available from chemistry handbooks.

A hydrometer is a useful tool in the hands of a brewer who knows what wort gravity is and why he wants to measure it. Beer recipes often list the Original and/or Final Gravities (OG and FG) to better describe the beer to the reader. For an average beer yeast, a rule of thumb is that the FG should be about 1/4 to 1/5 of the OG. For example, a typical beer OG of 1.040 should finish about 1.010 (or lower). A couple of points either way is not unusual.

It needs to be emphasized that the stated FG of a recipe is not the goal. The goal is to make a good tasting beer. The hydrometer should be regarded as only one tool available to the brewer as a means to gauge the fermentation progress. The brewer should only be concerned about a high hydrometer reading when primary fermentation has apparently ended and the reading is about one half of the OG, instead of the nominal one forth. Proper yeast preparation should prevent this problem.

Beginning brewers often make the mistake of checking the gravity too frequently. Every time you open the fermenter, you are risking infection from airborne microbes. Check the gravity when you are ready to pitch the yeast, then leave it alone until the bubbling in the airlock stops. Checking the gravity in-between will not change anything except to possibly contaminate it. Also, always remove a sample of the wort to test it. Don't stick the hydrometer into the whole batch. Use a sanitized siphon or Wine Thief (turkey baster) to withdraw a sample of the wort to a Hydrometer Jar (tall, narrow jar) and float the hydrometer in that. There is less chance of infection and you can drink the sample to see how the fermentation is coming along. It should taste like beer even though it may taste a bit yeasty.

The hydrometer temperature correction table is shown below. Hydrometers are standardized at 15 C (59F). When discussing specific gravities of worts and beers with other brewers, always quote the standardized value. Measure the specific gravity of your wort, take the temperature and add the correction (Delta G) value given in the table. The correction number is added to the specific gravity number, 1.0XX.

For example:
If the wort temperature is 108 F, and the gravity of the sample is 1.042, the Delta G value that would be added is between .0077 and .0081. Rounding it off to the third decimal place gives us .008, which is added to 1.042 yielding 1.050.

Table 18 - Hydrometer Temperature Corrections

T C

Delta G

T F

T C

Delta G

T F

0

-0.0007

32.00

25

0.0021

77.00

1

-0.0008

33.80

26

0.0023

78.80

2

-0.0008

35.60

27

0.0026

80.60

3

-0.0009

37.40

28

0.0029

82.40

4

-0.0009

39.20

29

0.0032

84.20

5

-0.0009

41.00

30

0.0035

86.00

6

-0.0008

42.80

31

0.0038

87.80

7

-0.0008

44.60

32

0.0041

89.60

8

-0.0007

46.40

33

0.0044

91.40

9

-0.0007

48.20

34

0.0047

93.20

10

-0.0006

50.00

35

0.0051

95.00

11

-0.0005

51.80

36

0.0054

96.80

12

-0.0004

53.60

37

0.0058

98.60

13

-0.0003

55.40

38

0.0061

100.40

14

-0.0001

57.20

39

0.0065

102.20

15

0

59.00

40

0.0069

104.00

16

0.0002

60.80

41

0.0073

105.80

17

0.0003

62.60

42

0.0077

107.60

18

0.0005

64.40

43

0.0081

109.40

19

0.0007

66.20

44

0.0085

111.20

20

0.0009

68.00

45

0.0089

113.00

21

0.0011

69.80

46

0.0093

114.80

22

0.0013

71.60

47

0.0097

116.60

23

0.0016

73.40

48

0.0102

118.40

24

0.0018

75.20

49

0.0106

120.20

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Appendix A - Using Hydrometers
Real Beer Page

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Appendix A - Using Hydrometers
Appendix B - Brewing Metallurgy
Appendix C - Chillers
Appendix D - Building a Mash/Lauter Tun
Appendix E - Metric Conversions
Appendix F - Recommended Reading

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All material copyright 1999, John Palmer