To extract soil water in the unsaturated zone the soil water tension (retention force) has to be surpassed by the sampler‘s potential - a vacuum needs to be applied.
 
The vacuum should be as close as possible to the in-situ soil water tension, as i.e. carbon will fall out in high vacuum.
Variations of the pressure difference between the sampler’s inside and outside will lead to different filtration results - thus, memory effects occur. This is prevented by a tension controlled vacuum unit.
 
Please note that water can only be extracted if water is available. The bigger the soil pores are, the less water is available at rising tensions!
 
Sampling is possible in stony soils up to pF 1, in sandy soils up to pF 2, and in clay soils up to pF 2.7.
 
Suction cups always act as chemical and physical filter. High vacuum applied at soils close to saturation cause transport of small particles into the sampler‘s pores. There is nearly no chance to reverse this process even by applying pressure, as around the cup an area of small particles will get accumulated.
 
The effective active suction force is the difference of soil water tension and applied vacuum.
 
 

Evacuate your sampling bottle down to approx. 50 kPa. If the soil water tension is lower than 50 kPa, soil water solution will be extracted until vacuum and soil water tension are equalized. When the samples are collected, the bottle is evacuated again.
 

Applications

• For qualitative analysis of soil water

Benefits

• Low cost
• Easy handling

Limits

• Discontinuous and undefined sampling

Instruments

• Pore water samplers
• Manual vacuum pump VPS1 or portable vacuum case VacuPorter

A constant vacuum is continuously maintained by a regulated vacuum pup The vacuum can be set between atmospheric pressure and 85 kPa.
Leachate samplers for example are supplied with approx. 6 kPa, while pore water samplers in silt and loam are supplied with 10 to 30 kPa. As clay soils retain water even at higher tensions a vacuum from 30 to 85 kPa could be applicable.
 

Applications

• Long term monitoring projects
• Studies on leachate
• Soil water extraction from a certain pore size with a vacuum which is exactly suitable to the pore size.

Benefits

• Defined sampling

Limits

• Constant vacuum ignores changing soil water tensions

Instruments

• Pore water or leachate samplers
• Vacuum station VS without controlling Tensiometer

A Tensiometer measures the soil water tension. The programmable vacuum station VS automatically supplies a vacuum in correspondence to the measured tension.
 
Due to the numerous functions of the unit an optimal adaptation to the sampling task is possible.
 

Benefits

• Constant sorption and constant filter effects
• Prevents memory effects

Limits

• Takes samples from various pore sizes depending on the current vacuum

Tools

• Pore water or leachate samplers
• Vacuum station VS with controlling Tensiometer