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Relative Humidity or Humidity Deficit


									                                                                                                      Update No. 7
                                                                                                                 19 March 2009

                           Relative Humidity or Humidity Deficit

In recent years the accepted norm for growers of edible crops such
as tomatoes, cucumbers and peppers has been to control humidity
based on humidity deficit (HD). This is because conventional
wisdom suggests that maintaining a minimum HD promotes
transpiration which in turn ensures optimised plant development
and growth.

In contrast, ornamental crop growers typically use relative humidity
(RH). This is because RH theoretically gives a better indication of
the impending risk of condensation (and the subsequent possibility
of disease formation), and historically ornmentals growers have
been more concerned about disease control than maximum growth.

However, energy prices have now increased, and growers of all
crops have become more concerned about providing reliable
disease control at an acceptable cost.

As a result the question that is now being asked by edible crop
growers is, “Should I control humidity based on RH”?

Our Focus Nurseries are using RH rather than HD
Both of our spring 2009 GrowSave Focus Nurseries have             greenhouse environment and energy use are.
chosen to challenge the conventional thinking for edible
                                                                  In the meantime, the information below gives some
crops and use RH for humidity control in preference to HD.
                                                                  background on the use of RH and why it might be
At first this might be seen as a bold move, but it is an          the preferred approach when energy saving is a key
approach that many edibles growers in Holland are already         consideration.
using, This then begs a number of questions including:
                                                                  Both Red Roofs Nursery and Eric Wall are controlling
•   If Dutch growers are using it then why shouldn’t we?          to a maximum RH of around 85% to 90%. This range of
                                                                  settings has been chosen to provide good disease control at
•   Does plant performance suffer at the expense of disease
                                                                  acceptable heating pipe temperatures / energy inputs.
                                                                  So how does this compare in HD terms, especially where
•   Will I save energy?
                                                                  growers using HD based control typically try to achieve a
Ultimately the answers relating to questions about plant          minimum of 3.0g/m3 during the day and 2.0g/m3 overnight?
performance and the effectiveness of disease control can
                                                                  The graphs below show the results from Red Roofs Nursery
only be seen at the end of the season, but over the next few
                                                                  on two days, a first which had excellent light levels and a
weeks our updates will hopefully give you an insight into how
                                                                  second when the light levels were significantly lower.
the approach can be implemented and what the effects on

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                                                                                                                                                                                    Update No. 7
                                                                                                                                                                                                19 March 2009

                                                                   Relative Humidity or Humidity Deficit
 High light level day
                                    28                                                                                            110
                                                                                                                                                                     This graph shows that the humidity
                                                                                              Temperature = 25oC
                                    26                                                                                                                               was below the RH set point although
                                    24                                                                                                                               it was not excessively low. For the

                                                                                                                                                                     majority of the time, the resulting HD

         Greenhouse temperature -

                                                                                                                                                                     was also above the minimum setting

                                                                                                                                                 RH% & HD g/m X 10
                                    20                                                                                            60
                                                                                                                                                                     that might be used by a grower using

                                                                                   RH = 84%
                                    18                                                                                            50
                                                                                                                                                                     HD control.
                                                                                                                                                                     What is particularly striking is that
                                                                                                                                                                     the HD’s were particularly good
                                    12                                             HD = 4.7g/m3                                   10
                                                                                                                                                                     throughout the daytime period
                                    10                                                                                            0
                                                                                                                                                                     because of solar gain.
                                         00           03         06    09            12           15         18     21

                                              Greenhouse temperature                 Greenhouse HD                  Greenhouse RH

 Low light level day
                                    28                                                                                        110
                                                                                                                                                                     This second graph shows an
                                                                                            Temperature = 25oC
                                    26                                                                                                                               interesting contrast because,
                                                                                                                                                                     although the control of RH was

                                                                                                                                                                     again satisfactory, HD levels were

         Greenhouse temperature -

                                                                                                                                                                     significantly lower.
                                                                                                                                       RH% & HD g/m X 10

                                    20                                                                                        60

                                                                               RH = 84%                                                                              This highlights that, had
                                    18                                                                                        50
                                                                                                                                                                     ‘conventional’ HD control been used,
                                    16                                                                                                                               the energy consumption would have
                                    14                                                                                                                               been appreciably higher since higher
                                                                                                                                                                     pipe temperatures would have been
                                    12                                             HD = 4.7g/m3                               10
                                                                                                                                                                     needed for most of the day.
                                    10                                                                                        0
                                         00           03         06    09           12            15        18     21

                                              Greenhouse temperature                Greenhouse HD                  Greenhouse RH

 What are the conclusions so far?
 The evidence provided by our focus nurseries so far highlights that using RH in preference to HD provides significant
 potential for energy savings. This is particularly the case on dull days, when acceptable RH levels are maintained at
 significantly lower heating pipe temperatures than those needed for conventional HD control.
 Doubts still remain that plant environment is not optimised on dull days and yields will ultimately fall. However,
 commercial evidence from growers in Holland and one of our Focus Nurseries (Eric Wall Ltd) who have been using
 RH control for a number of years suggests that these worries are unfounded. This is because growers using RH are
 achieving good yields and excellent disease control with reduced energy inputs.

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