Hi,

I am measuring water potentials with the WP4 meter.

When I read samples - I usually read the standard, then read the sample, then read the standard, then read the sample again, then the standard again, etc.

Previously I have measured the standard, then measured the sample, then measured the sample again, then measured the sample again (repeated until I get two readings within 0.1 MPa)

The latter leads to a reduction in output readings - for example:

-2.20 - standard
-1.45 - sample reading 1
-1.33 - sample reading 2
-1.16 - sample reading 3
-1.02 - sample reading 4
-0.96 - sample reading 5

When reading a standard, then sample, then standard, then sample etc - little or no reduction is seen.

Which is the correct reading for the sample?

Please advise

I just wanted to add some additional information to this thread that might be useful to a number of forum users. First, let me say that John’s analysis of the drift phenomenon mentioned above is right on the money. From my experience with any of the WP4 models, the presence of any contaminants on the sensor block will adsorb or desorb water vapor from the sample and severely slow the process of getting the atmosphere in the measurement chamber into vapor equilibrium, which is crucial for an accurate water potential measurement. In the case of DF’s data, the -2.2 MPa KCl standard dried the air in the sensor block to -2.2 MPa. When the wetter (less negative) sample is introduced, it is clearly taking a long time (10s of minutes) to wet up the sample chamber due to the sensor block contamination.

It is important to note that in very moist samples (i.e. water potential wetter than -0.5 MPa), this slow wet-up will be present even with a perfectly clean sensor block, simply from vapor adsorption on the sensor block surfaces. The new WP4C has an extremely hydrophobic coating on the sensor block to minimize the vapor equilibration time, but even with this coating, the WP4C will not give best precision in precise mode, and certainly not in fast mode. If you want the best possible precision on measurements of moist samples, I am convinced that you need to run the WP4C in continuous mode, log the data using a terminal emulator, and continue the measurement until you see a stable, fully equilibrated reading. Let me show some real data to illustrate the point.

The first column is time (minutes) and the second column is measured water potential (MPa)
3.5    -0.29
7.3    -0.25
11.0    -0.24
14.8    -0.23
18.6    -0.22
22.5    -0.22
26.3    -0.21
30.2    -0.21
34.1    -0.20
37.9    -0.20
41.8    -0.20
45.7    -0.20
49.5    -0.20

The data above were collected on a -0.20 MPa KCl salt solution starting with an air dry (~-120 MPa) sensor block. If the WP4C had been configured in Fast mode, it would have reported the water potential of the first reading (-0.29 MPa), yielding an error of -0.09 MPa. In Precise mode, the WP4C waits for two consecutive readings to be within 0.03 MPa of each other, so it would have reported -0.24 MPa, yielding a -0.04 MPa error. However, if you are patient and your lab has decent temperature stability (more on temperature stability below), you can achieve precision to the 0.01 MPa level.

If you use this technique carefully and are able to really dial in your precision, then you are half way there. We all know that it is possible to have great precision but poor accuracy, so our next step is to make our precise measurements accurate. The WP4C has a built-in offset adjustment sequence that you can use in conjunction with the 0.5 molal KCl standard that we provide to calibrate the instrument. This calibration sequence uses Precise mode to measure the water potential of the KCl standard and then adjusts all subsequent readings by the difference between the known and measured water potentials of the KCl solution. If you calibrate the WP4C using the built-in offset adjustment function, then I typically expect the wet end accuracy to be about ±0.04 MPa due to the limitations of the Precise mode measurement of the KCl standard. My strategy for really dialing in the accuracy is to run the KCl standard in continuous mode until an equilibrium value is reached. Then, apply the offset between the known and measured water potential to all subsequent data during post processing in Excel or whatever program you are using for data analysis. When I did some careful evaluation of the wet end accuracy of the WP4C with some precision KCl solutions, I was able to achieve accuracy to about ±0.02 MPa between -2.2 MPa and 0. Here is an example data set. Note that all data were adjusted with the offset on the -2.238 MPa standard as described above.

The first column is known water potential(MPa), the second column is offset-corrected measured water potential (MPa), and the third column is absolute error (MPa)
-2.238    -2.238    0.000
-1.346    -1.363    -0.017
-0.223    -0.238    -0.015
-0.132    -0.138    -0.006
-0.040    -0.053    -0.013
0.000    0.012    0.012

An additional important consideration when measuring moist samples is the type of sample cup that you should use. The disposable plastic cups are fine for dry samples, but not for wet samples. In the wet end, slight thermal gradients across the plastic sample cups can yield errors up to 0.05 MPa. The stainless steel cups have high enough thermal conductivity to become isothermal and prevent these errors. It is also important that you calibrate the instrument using the stainless steel cups if you are going to use them for wet end measurements.

Now I’d like to include some final thoughts on temperature stability. If you are measuring dry samples, small fluctuations in the sample temperature won’t be noticeable, but if you are measuring in the wet end, these fluctuations can affect your repeatability, precision, and accuracy. The WP4C has internal temperature control to ensure thermal stability of the sample. This works quite well under most conditions. However, if there are rapid swings in the ambient temperature, the temperature stability can suffer. I’ll give you an example of the type of rapid temperature fluctuation that can affect a measurement. In my office, the most convenient place to set up the WP4C for long term measurements happens to be directly in line with an HVAC vent. When the air conditioner turns on, there is enough cold air flow to cool the case of the WP4C about 1.5˚C in less than 5 minutes. This type of temperature disturbance is enough to induce about 0.05 MPa error into the measurement. To help ensure temperature stability, I have placed the WP4C in a simple cardboard box with a large vent hole in the box at the rear of the WP4C to allow the heat from the WP4C to be exhausted out of the box. Note that the vent hole in the box is essential for good measurements. Without it, the box will heat up and the WP4C won’t be able to control its temperature, which is worse than having it in the air conditioner stream.

Ok, I hope the discussion above is interesting. I think that it will help some of you who are “expert” users get more accurate wet end measurements out of your WP4’s. Let us know if you have comments or questions.

Doug Cobos, Ph.D.
Research Scientist
Decagon Devices
(509) 332-2756