NMR Lab. Air Supply ----- Dew Point, Oil free filters

The site requirement for NMR instruments

Lab. Air Supply Requirement
System Pressure(psig/kpa) Flow Rate(LPM/SCFH) Dew Point
200 - 400 MHz
500 and 600 MHz
Solid State NMR

Reference: Varian Installation Planning Guide

The house compressed air supply must provide a source of air that is clean, dry and free of contaminats, with a dew point -40 C minimum.

RH% caculator: http://einstein.atmos.colostate.edu/~mcnoldy/Humidity.html

Input the temperature of air supply, and relative humidity reading from the meter, the dew point could be calculated.

For example: the temperature is 22C and RH reading is 50%, then dew point is - 43.20C



Definition of dew point:
The dew-point temperature is the temperature to which the air must be cooled to reach saturation (assuming air pressure
remains the same). When the temperature cools to the dew point, fog or dew can occur, and the relative humidity becomes

The dew point is associated with relative humidity (RH%) . A highrelative humidity indicates that the dew point is closer to the current air temperature. Relative humidity of 100% indicates that the dew point is equal to the current temperature (and the air is maximally saturated with water). When the dew point stays constant and temperature increases, relative humidity will decrease.

Calculation of the dew point:
The Magnus formula relates the saturation vapour pressure and dew point. At a temperature T (in °C), the
saturation vapour pressure EW (in hPa) over liquid water, is


For the range from –45°C to 60°C, Magnus parameters are given by a =6.112 hPa, b =17.62 and c =243.12 °C. By restating
equation, the dew-point temperature Dp (in °C) can be expressed from vapour pressure E

Inserting the definition of relative humidity RH (in %), i.e. E=RH*EW/100, into equation (2) and using equation (1) leads
to the calculation of the dew point Dp from temperature T and relative humidity RH

The following simple program calculates the dew point Dp from the relative humidity RH and temperature T according to
equation (3). All temperatures are in Celsius.
H = (log10(RH)-2)/0.4343 + (17.62*T)/(243.12+T);
Dp = 243.12*H/(17.62-H); // this is the dew point in Celsius

Dewpoint Calculator:




Example: RH=10%, T=25°C -> Dew point = -8.77°C
RH=90%, T=50°C -> Dew point = 47.90°C
This formula is a commonly used approximation.


RRelative Humidity sum

RH% is used to describe the amount of water vapor that exists in a gaseous mixture of air and water.

The relative humidity defined as the ratio of the partial pressure of water vapor in the mixture to the saturated vapor pressure of water at the defined temperature. Relative humidity is normally expressed as a percentage and is defined in the following manner:

RH= (Pwater)/(Psaturated) *100%

where: RH% is the relative humidity of the mixture being considered; Pwater is the partial pressure of water vapor in the mixture; and Psaturated is the saturated vapor pressure of water at the temperature of the mixture.

Good Resources for Temperature and Humidity Control:




air filter: