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KSAT FAQs

If the porous plate is contaminated with soil particles, its conductivity changes. How do I fix this?

Usually, you can flush it with clear water from down side up to get rid of the soil particles. If the porous plate is dirty, try to clean it under water with a brush, or in the desiccator under vacuum. If this does not help, we recommend replacing the plate to eliminate unwanted conductivity changes.

Is it best to measure saturated conductivity in the field, as this covers the entire pore system of a soil? How can a small sample represent field conditions?

 

It is true that field data are always better, but many researchers still measure Saturated Hydraulic Conductivity (K_s) using core samples in the laboratory.  To help ensure that measurements using small samples are representative of field conditions, it is necessary to have more replicates that eliminate open paths. We recommend using five replicates to compare the results. If one or two have much higher K_s results than the others, leave these samples out of the final average. Instead, take the average of those readings with lower levels. The high conductivity data may result from open paths (pores), which are the results of cutting a core sample, but which are more or less passive in the field.

 

Why does the fitted falling-head curve not match my data?

There can be a variety of reasons for this: 

1. If your sample is not mounted properly, the base might not be tightly sealed. If this is the case, the water pressure will not approximate the value of zero hPa at the end, but tends to go to a negative value. To solve this problem, ensure that your sample is re-mounted properly. NOTE: In the early releases of KSAT, a bottom plate was used that sometimes failed to provide a tight-sealed connection to the sample, particularly if steel cylinders were scratched or dirty. The plates were replaced in the summer of 2015 by new plates with a soft, rubber sealing. Only these new ones should be used to ensure a tight connection between the sample and the dome. 

2. In some soils, particularly soils with a loamy texture, almost all water passes through a very small part of the soil sample (for instance, through macropores). Water flow in these macropores becomes turbulent if the pressure gradient becomes too large. In that case, the water flow is no longer proportional to the pressure gradient, and consequently the change of the hydraulic head with time is not exponential, invalidating Darcy’s law. KSAT is a precision measurement device which shows you this by a misfit of the exponential function: the fitted function will be less curved than the data. Also, you will notice in such a case that the calculated conductivity becomes larger as the size of the pressure head decreases. Under very small gradients, flow might still be laminar. To remedy this, repeat your measurement with a small gradient (for instance, an initial pressure head < 5 cm).

3. Soils are fragile, porous systems, and their permeability might change during the measurement process. There are different directions and reasons for this:

• If flow takes place primarily through macropores, these might erode during the measurement process, increasing conductivity. This will lead to a result similar to the previous case, with the difference being that the effect (increasing conductivity) is lasting.  
• Due to preferential flow through macropores, these can become sealed by sediment particles. In this case, conductivity will decrease during the measurement process. You will see this again by an apparent misfit of the exponential function, but in this case, the fitted exponential curve will be more curved than the data.

4. The offset of your pressure transducer might not be equal to zero. The reason for that can be that you have a temperature drift (if not all components of the measurement, i.e., KSAT, used liquid, and soil samples were equilibrated at the same temperature). To solve this problem, equilibrate all components to the same temperature, and perform the offset recalibration before the measurement.

 

Why does the water level in the burette not drop to zero, but remains at a positive value?

 

You might have air in the pipe connection between the burette and the tube. To remove it, fill the burette up to 20 cm height with water, and open the valve to the open dome quickly. Water will shoot through the pipe and drag any of the existing air with it.

 

The automatic detection of the start of the measurement does not work. What is the reason, and what can I do?

KSAT automatically detects the start of a measurement by a positive pressure jump in the signal. There are some possible reasons and coordinating solutions as to why the automatic detection does not work:

1. The opening of the valve is too slow. If this occurs, the increase in pressure will be too gradual, and the increase will not be recognized. To solve this problem, open the valve with a swift turn of the lever. 

2. The pressure transducer might be not reacting instantaneously due to layerings or sedimentation. If this occurs, clean the KSAT. 

3. The pressure transducer is defected. In this case, send the KSAT to METER. 

 

*IN ANY CASE: You can ALWAYS manually start your measurement by pressing the button “Restart manually.” This solution is also appropriate if you want to start a KSAT measurement “on the run”—for instance, if the valve connection to the burette is already open (intentionally or accidentally) when you want to start your measurement.

 

Which fluid should I used for my experiments?

Do not use distilled water!  In sandy soils, the ionic composition of water is not of big consequence, but in fine-textured soils, the width of the electric double layer will be greatly affected by the ionic strength and the ionic composition of the water. Furthermore, use of water with monovalent anions of distilled water can disperse the sample, thus reducing its saturated conductivity. In general, it is recommended to use water with a similar ionic composition as the soil under investigation; however, knowing a water’s ionic composition is not always easy. In practice, standard tap water is used in most cases, and it is good if you can specify the ionic strength. For some investigations, particularly with soil that can undergo dispersion, it is recommendable to use an electrolyte solution with bivalent cations, e.g. a 0.01 molar solution with calcium as cation. ALWAYS use water with the same temperature as the lab environment where you perform the measurements.

Water leaving the exhaust tube is not clear. Is that a problem?

 

Stop! The pressure head you applied is too high for your sample, which results in erosion and destroys your sample. The pressure transducer of your instrument is precise enough to work with minimum pressure heads. Adjust your pressure head to be between 2 – 5 cm. Additionally, you will usually get the best results with small pressure heads.

 

Nothing happens when I open the connection valve. Is my sample impermeable?

KSAT can record even extremely small percolation rates. If you have selected “Auto” for the sampling rate, a data point will only be shown if a minimum pressure head difference is recorded (default is 0.1 cm). You can do the following to see more points:

1. Select a smaller min. pressure head difference (down to 0.01 cm). 

2. Select a constant time interval instead of the automatic mode. 

3. Increase the initial pressure head. We always recommend starting measurements with a pressure head difference of no more than about 5 cm to minimize the risk of eroding or destroying the sample during the measurement. However, if your sample is obviously stable, then you may increase that up to 20 cm.

4. If conductivities are so low that even measurements with 20 cm initial pressure head difference appear extremely slow, use the burette extension mode for your measurement to accelerate the measurement again by a factor of 50. To do this, fill the burette completely to the top of the pipe with the constant head pipe on top. The KSAT will automatically detect that water is being delivered from the narrow pipe instead of the wide burette, and will calculate the proper conductivity value.

Do I need to wait until the defined measurement time is reached

 

You can stop the measurement before the defined measurement time is reached if the following parameters are met:

1. the fitting curve fits to the measurement values
2. r² is high enough (near 1)
3. enough measurement values are already taken (> 10)
4. the K_s value is constant

 

I cannot measure conductivities because all water passes through the sample before the automatic measurement even starts.

 

The upper limit of the range of measurable conductivities with KSAT is about 5000 cm/d. In this case, the initial water level passes through the sample in about 5 seconds, which is close to the temporal resolution of the KSAT data acquisition. To solve this measuring problem, you can use the “restart measurement” button to manually initiate the data recording immediately after opening the valve. This can slightly accelerate the recording of the first data.

 

Why does the fitted falling-head curve not match my KSAT data?

There can be a variety of reasons for this:

1. If your sample is not mounted properly, it might be not tightly sealed at its base. If this is the case, the water pressure will not approximate the value of zero hPa at the end but will tend to go to a negative value.

• Solution: Remount the sample properly.
• NOTE: In KSAT early releases, a bottom plate was used that sometimes failed to provide a tightly sealed connection to the sample, particularly if steel cylinders were scratched or dirty. The plate was replaced in summer 2015 by a new plate with a soft rubber seal. Only this updated plate should be used to ensure a tight connection between sample and dome.

2. In some soils, particularly of loamy texture, almost all water passes through a very small part of the soil sample (i.e., through macropores). Water flow in these macropores becomes turbulent if the pressure gradient becomes too large. If this is the case, the water flow is no longer proportional to the pressure gradient. Consequently, the change of the hydraulic head with time is not exponential, and Darcy’s law is not valid. If this is the case, the exponential function will not fit the data: the fitted function will be less curved than the experimental results. Also, you will notice in such cases that the smaller pressure heads give a larger calculated conductivity.

• Solution: Under very small gradients, flow still might be laminar. So, repeat the measurement with a small gradient (i.e., an initial pressure head < 5 cm).

3. Soils are fragile porous systems, and their permeability might change during the measurement process. There are different reasons for this:

• a. If flow takes place primarily through macropores, these might erode during the measurement process (i.e., conductivity increases). This will lead to a result similar to #2, however, the effect (increasing conductivity) will be lasting.
• b. Due to preferential flow, macropores can become sealed by sediment particles. In this case, conductivity will decrease during the measurement process. This will be indicated by an apparent misfit of the exponential function, but in this case, the fitted exponential curve will be more curved than the data.

4. The offset of your pressure transducer might not be equal to zero. You may have a temperature drift if all components of the measurement (i.e., KSAT, used liquid, and soil samples) were not equilibrated at the same temperature.

•  Solution: Equilibrate all components to the same temperature, and perform the offset recalibration before the measurement.

 

It's best to measure saturated hydraulic conductivity in the field, as this covers the entire pore system of a soil. How can you measure K_s (K_t) with only a soil core?

 

Many research institutions still measure K_s (K_f) with samples, but field data is always better.  If using a soil core, it is necessary to have five replicates to be sure open paths do not falsify the result. Compare the results. If one or two have much higher K_s results, don’t average those in, but average only those readings with lower values. The high conductivity data may result from open paths (pores), which were cut on the top and bottom of the soil core but which are more or less passive in the field.

 

Which fluid should I used for my experiments?

 

Do not use distilled water. In sandy soils, the ionic composition of water is not a big concern, but in fine-textured soils, the width of the diffuse double layer will be greatly affected by the ionic strength and ionic composition of the water. Furthermore, the use of distilled water and its monovalent anions can disperse the sample, greatly reducing its saturated conductivity. In general, we recommend using water with a similar ionic composition to the soil under investigation. This is not always easy to determine. In practice, standard tap water is used in most cases, and it is even better if you can specify the ionic strength. For some investigations, particularly with soil that can undergo dispersion, we recommend an electrolyte solution with bivalent cations (e.g., a 0.01 molar solution with calcium as cation). In addition, always use water that is the same temperature as the lab environment where you perform the measurements.

How does KSAT calculate the temperature correction to obtain the saturated conductivity at the specified reference temperature?

 

KSAT uses the temperature dependency of the viscosity of water to recalculate the reference conductivity (at your specified reference temperature) from the measured value (at the measured operation temperature). Details are specified on page 11 in the KSAT operation manual (available as a pdf from the Help menu in the KSAT software).

Does saturated mean that all soil pores are filled with water?

 

No! But this is also not the case in the field.

I cannot measure conductivities because all water passes through the sample before the automatic measurement even starts. 

 

The upper limit of the range of measurable conductivities with KSAT is about 10000cm/d. In this case, the initial water level passes through the sample in about 5 seconds, which is close to the temporal resolution of the KSAT data acquisition.  You might try using the Restart Measurement button to manually initiate the data recording immediately after opening the valve. This can slightly accelerate the recording of the first data point and help to push the upper measurement limit slightly higher.

When is my measurement finished?

Your measurement is finished automatically if either a minimum total pressure head (parameter H_end_abs) or a minimum relative pressure head (parameter H_end_rel) is reached, which is related to the initial pressure head. The default setting is that water percolates until the level goes down to 25 % of the initial value. You can change this setting in the parameter menu. The default values are very conservative. Often, measurements can be stopped much earlier. You can do this anytime by pressing Stop Measurement. As a rule of thumb, measurement can be stopped:

a) if the calculated conductivity becomes a stable value. This means that a sufficient number of measured data have been recorded (> 10) and that the signal shows a clear trend, and

b) if r² is high enough (r² > 0.999).

For samples with low permeability, a decrease by 1 cm pressure head is normally sufficient to stop the measurement. For example, a sample with a conductivity of 2 cm/d will take about 8 hours to reach 0.25 of its initial pressure head. In practice, you can start with 20 cm initial head and stop when reaching 19.5 cm (either manually, or by setting H_end_rel = 0.975), which occurs after approximately 15 minutes.

Can I visualize my data externally?

 

Yes. All your data and all parameters are written into an ASCII file in the csv format. You can use these data in order to re-visualize the measurement and the fitted curve with your own visualization software.