3 Determining the MH Value
Determining the M and MH Values for Silica Gel
M Value: To compare effectiveness among different types of silica gels, it is necessary to know the gel’s moisture buffering capacity within a specified RH range, defined as the gel’s specific moisture reservoir or M value.
THE M VALUE IS THE AMOUNT OF MOISTURE (IN GRAMS) THAT IS ADSORBED OR DESORBED BY A KILOGRAM OF SILICA GEL FOR A 1% CHANGE IN RH
MH Value: Since the M value can change based on whether the gel is adsorbing or desorbing moisture (referred to as hysteresis), the measurement that takes hysteresis into account is the MH value. This gives a more accurate measurement than the M value for determining the silica gel’s buffering capacity for a specific application. The (H) designates that the M value’s hysteresis is accounted for within a specified RH range.
THE MH VALUE IS DETERMINED BY CYCLING GEL MULTIPLE TIMES WITHIN A SPECIFIC RH RANGE UNTIL THE AMOUNT OF WATER THAT IS ADSORBED AND DESORBED ARE THE SAME
The MH values used in the Moisture Buffering Capacity table and throughout this website are derived from two sources.
Canadian Conservation Institute’s Technical Bulletin 33 on Silica Gel
Unpublished data based on hysteresis studies done by Art Preservation
MH Value of Rhapid Gel
There is a relatively small change in the MH value of Rhapid Gel within the range of 0-60% RH.
MH = 4.4 within the range of 20-30% RH
MH = 5.5 within the range of 40-60% RH
Table 1: MH Value for Rhapid Gel
Note on the MH value above 60% RH: Rhapid Gel has a high M value up through 70+% and then falls off to a lower MH value. Since there is no data on MH performance within the range of 60-75% RH, an MH value of 5.5 is recommended.
MH Value for Arten Gel
There is a relatively large change in the MH value of Arten Gel within the range of 0-60% RH.
MH = 5.5 within the range of 20-30% RH
MH = 1.9 within the range of 40-60% RH
The amount of silica gel recommended for RH stabilization for a “typical” microclimate application must take into consideration the change in the MH value, depending on the RH range in which Arten Gel will be used.
To determine the amount of Arten Gel required for a specific RH range, the following table of MH values can be used.
Table 2: MH value for Arten Gel
Between 5.0 and 6.0