Limitations and accuracy

Limitations

There may be some limitations in the use of the pressure bomb.

• If the stems of plants are very weak, vessels are obliterated by the pressure of the sealing sleeve into the specimen holder. The reading is then in accurate because the sap can not flow easil past the constriction to the cut end.
  
• Even though this device is supplemented with different diameters of rubber sleeves, a good seal may not be possible if plants are very small.
  
• If the pressure bomb readings are to be used a to schedule irrigation, it is necessary to run a large number of samples in order to compensate for variability of the plant status.
  
• The use of the pressure bomb may be limited when the internal water status of plants is under equilibrium. Data is highly affected by pre-dawn conditions.Measurements should be taken when internal water status is in some way stabilized.
  
  
  

Accuracy

When the pressure technique is carefully executed, it will give the sap pressure that existed at the time the sample was cut. The pressure at which sap comes through the end of the branch is equivalent to the negative pressure that existed in plant when the sample was cut. Therefore, pressure bomb accuracy is high. However, some factors may influence accuracy such as:

• Evaporation loss from the twig must to be avoided. Keep all the water inside the plant tissue before the reading is taken to avoid turgor loss inside the leaves, which in turn may give a wrong reading.
  
• When sampling, sample size should match the pressure vessel size to avoid a new sample cutting or a trimming.
  
• In order to increase the level of accuracy it is necessary to carry out a large number of samples. In other words, the higher the number of samples, the higher the accuracy.
  
• It is important to consider the period of time from the sample cutting until the final reading of the pressure bomb. Consistent durations of time per sample results in higher levels of accuracy.
  
• If ice is used to keep plant tissues cool enough to avoid moisture loss, the ice must not touch samples because may cause membrane damages, which may produce inaccurate readings.
  
• Also, twigs exposure time during the time of the day may affect greatly the accuracy because there is more variation in leaf water potential during morning than at midday.
  
• It is essential to use healthy samples, free of diseases, insects, mechanical and other kind of damages.
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