English Heritage The Effect of Air Tightness on RH Control · English Heritage Thomsen (1977) N MB...

English HeritageEnglish Heritage

The Effect of Air Tightness on RH Control

David Thickett David Thickett Collections Conservation TeamCollections Conservation Team

English HeritageEnglish Heritage AER Measurements

0 20 40 60 80

desktop

medium <1m

high >1m

historic

print frames

MC frames

stewart boxes

0

0.2

0.4

0.6

0.8

1

1.2ai

r ex

chan

ge r

ate

/day

English HeritageEnglish Heritage RH measurement

0

5

10

15

20

25

30

1991 THGs + 5%1994 squirrel+ 3%1996 Shinea + 3%2000 Vaisala humicap + 2%

2006 Rotronic hygroclip + 1%

1 pt3 ptdata failure

English HeritageEnglish Heritage Thomsen (1977)

NMBt 4

2/1 =

t ½ is the hygrometric half life (days)M is the loading of silica gel in the box (0.1kg/m3)B is the specific moisture reservoir of silica gel (8 g/kg/%)N is air exchange rate (0.35 and 0.65 /day)

Thermal Equilibrium between enclosure and roomPerfect and rapid mixing in enclosure

English HeritageEnglish Heritage Weintraub and Tetrault (2003)

DNCMFBt

eq

HRH =

tRH is time to reach a specifed RH (days)F is targeted range of RH fluctuation (irrelevant for this application but taken to be16 %)BH is specific moisture reservoir corrected for hysteresis (this effect is negligible for regular silica gel below 30% RHCeq is equilibrium concentration of water vapour (calculated g/m3)D is decimal difference between external RH and chamber (0.59)

Perfect and rapid mixing in enclosure

English HeritageEnglish Heritage Dryer than Room


Stewart Boxes for Archaeological Metals

01020304050607080

18/01/200500:00

09/03/200500:00

28/04/200500:00

17/06/200500:00

date

RH (%

)

room model box

Thickett &Thickett & OdlyhaOdlyha, , Conservation Arch Materials, 2007Conservation Arch Materials, 2007

English HeritageEnglish Heritage Porchester


0

20

40

60

80

100

17-Feb-0509-M

ar-0529-M

ar-0518-Apr-0508-M

ay-0528-M

ay-0517-Jun-0507-Jul-0527-Jul-0516-Aug-0505-Sep-0525-Sep-05

RHmodel

tt½½ = 4MB/n = 4MB/n M = 10kg/m3, B = 8, tM = 10kg/m3, B = 8, t½½ = 187days= 187days


0102030405060708090

100

0 50 100 150 200 250 300 350

time (days)

RH

(%)

Thickett & Thickett & LuxfordLuxford, , Metal 07, 2007Metal 07, 2007

English HeritageEnglish Heritage Wetter than Room

English HeritageEnglish Heritage Down House, London

0

10

20

30

40

50

60

70

80

90

100

10/10/200600:00

20/10/200600:00

30/10/200600:00

09/11/200600:00

19/11/200600:00

29/11/200600:00

09/12/200600:00

19/12/200600:00

29/12/200600:00

08/01/200700:00

Time (days)

0

5

10

15

20

25

English HeritageEnglish Heritage Above and Below Room


Werner Collection, Rangers House London

0

10

20

30

40

50

60

70

80

90

100

0.4 0.6 0.6 0.8 0.8 1 1.6 2.4 2.6 2.6 4.5

Thickett et al, Thickett et al, Conservator, 2005Conservator, 2005


0

10

20

30

40

50

60

70

0 5 10 15 20 25 30 35

Time (days)

RH

(%)

English HeritageEnglish Heritage Mechanical Conditioning

DehumidifiersDehumidifiersHumidifiersHumidifiersMixedMixedFlow Through, Hahn RK2, Flow Through, Hahn RK2, RecirculationRecirculation, , MiniclimaMiniclima


Dehumidifier PerformanceBronzes, 42%

0

10

20

30

40

50

60

70

80

<20 21-25 26-30 31-35 36-40 40-42 >42

20.4610.46.524.093.71

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

40-42 >42

20.46

10.4

6.52

4.09

3.71

English HeritageEnglish Heritage Dehumidifier Specification

0

5

10

15

20

25

30

0 7 15 22 30 37 45 52 60 67 75 82 90 97 105

112

120

127

time (days)

wat

er v

apou

r (g/

hr)


y = -0.31x + 8.6582

7.27.47.67.8

88.28.48.68.8

0 1 2 3

days

ln [

CO

2 pp

m]

00.20.40.60.811.21.41.61.82

Inst

anta

neou

s A

ER(d

ay-1

)


Dehumidifier PerformanceBones, 70%

0

10

20

30

40

50

60

70

80

<40 40-50 50-60 60-70 70-80 80-90

room yearroom0.520.640.83

English HeritageEnglish Heritage Kenilworth Environment

0

10

20

3040

50

60

7080

90

13/05/200400:00

12/06/200400:00

12/07/200400:00

11/08/200400:00

10/09/200400:00

10/10/200400:00

date

-20

-10

0

10

20

30

40

50

60

English HeritageEnglish Heritage Conditioning Performance

40

50

60

70

80

90

100

0.11 0.59 0.89 0.69

English HeritageEnglish Heritage Tapestry Case RH

5

15

25

35

45

55

65

21/08/200600:00

23/08/200600:00

25/08/200600:00

27/08/200600:00

29/08/200600:00

31/08/200600:00

02/09/200600:00

04/09/200600:00

date

0

5

10

15

20

25

30

35

40

English HeritageEnglish Heritage Tapestry Case RH

5

15

25

35

45

55

65

21/08/2006 00:00 26/08/2006 00:00 31/08/2006 00:00 05/09/2006 00:00 10/09/2006 00:00 15/09/2006 00:00 20/09/2006 00:00

date

0

5

10

15

20

25

30

35

40

English HeritageEnglish Heritage Conditioning Failure

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60 70 80 90 100

Time (days)

RH

(%)

room

casemodel


0%10%20%30%40%50%60%70%80%90%

100%

0 1 2 3 4 5 6 7

air exchange rate (/day)

prop

ortio

n [m

etha

noic

aci

d]

English HeritageEnglish Heritage Ethanoic acid

0

1000

2000

3000

4000

5000

6000

7000

0 0.5 1 1.5 2 2.5 3

conc

entr

atio

n (p

pb)

julydecember

English HeritageEnglish Heritage Conclusions

Modelling internal Modelling internal RHs RHs fairly well, well enoughfairly well, well enoughTemperature effectsTemperature effectsWoodWood

Predict performance of casesPredict performance of casesSpecify AER required, cost implicationsSpecify AER required, cost implicationsMethanoic acid, equation works wellMethanoic acid, equation works wellEthanoic (sealed MDF), apparent ‘threshold’ Ethanoic (sealed MDF), apparent ‘threshold’ ~0.5day~0.5day--11

English HeritageEnglish Heritage Acknowledgements

Philip Fletcher, British MuseumPhilip Fletcher, British MuseumAndrew Andrew CalverCalver, Museum of London, Museum of LondonSarah Sarah LambarthLambarth, ICON/HLF, ICON/HLFUniversity of StrathclydeUniversity of Strathclyde

English Heritage The Effect of Air Tightness on RH Control · English Heritage Thomsen (1977) N MB...

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