changed in time while the measurement range decreased. Secondly, the osmotic tensiometer had a slow In an osmotic tensiometer, the hydrostatic pressure of a confined response on changing temperature and soil water potenpolymer solution is measured to infer the soil water potential. Essential in the operation is the osmotic potential of the polymer solution. tial. Thirdly, these researchers argued that the influence Because the osmotic potential depends on the temperature, a signifi-of the temperature on the osmotic potential of the polycant and undesired pressure lag may develop when the osmotic tensimer solution limits application of the osmotic tensiomeometer is used under conditions of varying temperature. To underter to temperature-controlled laboratory experiments. stand and eliminate the pressure lag, a dynamic transport model was We have developed an osmotic tensiometer that may derived that describes the time response of the osmotic tensiometer overcome these difficulties (Fig. 1). In our version, a while assuming complete temperature equilibration and a resistance porous inorganic filter is used that consists of a thick to water transfer that is fully located within the filter. Essential elesupport layer and a thin microporous membrane. These ments of the model are the filter conductance and the sensitivity of inorganic membranes are durable in any environment the pressure transducer, for which submodels are set up. The transport (De Vos and Verweij, 1998), and for large polymers model is validated with measurements on an osmotic tensiometer that uses an inorganic membrane as filter. The experiments were done for (e.g., mol. wt. Ͼ 20 kg mol Ϫ1), polymer retention was instrument-limited conditions by placing the osmotic tensiometer in close to 100% for a long period. The response time can free water. For these conditions, the model describes measurements be minimized for an osmotic tensiometer by combining with reasonable accuracy. The new transport model clarifies the bea permeable filter with a stiff housing, a small cell volhavior of the osmotic tensiometer for changing temperatures and can ume, and a sensitive pressure transducer. Finally, the be used to design an osmotic tensiometer with a lower pressure lag. influence of temperature on the osmotic potential can be substituted readily in the pressure transducer software used to read the osmotic tensiometer pressure, which

In this paper, a new method of calibrating an acoustic particle velocity sensor using laser Doppler anemometry (LDA) is discussed. The results were compared and were in good agreement with the results obtained by conventional methods, where the sensitivity of the microflown is obtained with the use of a reference microphone and a standing wave tube. The LDA signal generated by the acoustic particle motion was analysed using the photon-correlation method, where the signal is considered to consist of a series of discrete photon events. The photon-correlation system is used to measure particle velocity amplitude next to the microflown particle velocity sensor in a standing wave. Measurements are performed for frequencies between 250 Hz and 4 kHz and velocities between 5 mm s−1 and 25 mm s−1 (root-mean-square (rms) values) which are equivalent to sound fields of 100 and 114 dB SPL in free field. From the output voltage of the probe microflown and the LDA-derived particle velocity in a standing wave, the sensitivity of the microflown is obtained. The two different calibration methods are in good agreement showing a discrepancy of 1 dB for the frequency range of 250 Hz–4 kHz.

Summary An accurate field method that directly measures the entire range of relevant soil water potentials for periods of months to years is not yet available. The osmotic tensiometer (OT) has the potential to fulfil these demands but is still in an early stage of development. We have designed a novel OT that should overcome several drawbacks of former designs by the use of cross‐linked polymeric gel grains instead of an aqueous solution of linear chain polymers inside the porous ceramic cup. For this reason, the cup can have large pores without polymer leakage. This ensures that water exchanges rapidly with the soil, resulting in a negligible pressure lag with changing temperatures, which is the most important source of a systematic measurement error of an OT. A second reason for better performance might be the inherent elasticity of the grains which should inhibit formation of a polymer cake at the filter. To study several of the key variables of the new sensor, transient and stationary experiments were performed in an aqueous solution the temperature, salt type and salt concentration of which were varied. Under similar conditions the swelling of the polymer grains was studied in more detail. The OT that we developed responds rapidly to changes in water potential, but is very susceptible to the influence of salts. To study and minimize salt influence, swelling tests of unconfined grains are proposed in combination with tests of the OT itself.