Delta OHM HD32.3, Operating Manual

Page: 16 / 74

HD32.3
- 16 -
V1.4
•
In case of forced convection:
( ) ( )
273
10 1 . 1
273
4 / 1
4 . 0
6 . 0 8
4
−
⎥
⎥
⎦
⎤
⎢
⎢
⎣
⎡
−
×
× ×
+ + =
a g
g
a
g r
t t
D
v
t t
ε
where:
D = globe thermometer diameter
ε
g
= 0.95 globe thermometer predicted emissivity
t
g
= globe thermometer temperature
t
a
= air temperature
v
a
= air speed
The medium radiant temperature does not coincide with the temperature: if within a room, there are
areas which have a temperature much higher than the one belonging to the air (think, for example,
about the flame of a fireplace); the medium radiant temperature is an average in which the presence
of this very hot area is significant.
The medium radiant temperature is detected by the globe thermometer, it’s a temperature probe
consists of a copper sphere with 50 mm diameter, painted with matt black, with emissivity equal to
ε g = 0.95 (as required by ISO 7726 ), with inside a Pt100sensor. The temperature of globe ther-
mometer may be significantly higher than the air temperature, as in the case of a little cabin in the
mountains, where the air is 0 ° C, but where the presence of a fireplace produces medium radiant
temperature of 40 ° C, ensuring a situation of comfort. Under normal conditions, maintaining a cer-
tain difference between the medium radiant temperature and the air temperature (where
T
MR
is sig-
nificantly higher than T
A
) is a merit in terms of ambient quality. In homes where there are no more
fireplaces or stoves, usually the medium radiant temperature coincides with the air temperature, or
even it’s inferior. These situations (the main case is represented by buildings with large glazed sur-
faces) are not particularly healthy as the warm and wet air facilitates the development of pathogens.
From this point of view, the heating with lamps or radiant panels is much healthier. It’s more hygi-
enic to ensure the comfort conditions with a medium radiant temperature greater than the air tem-
perature. The laws erroneously stipulate as an evaluation parameter for heating plants the air tem-
perature and not the medium radiant temperature.
To calculate the medium radiant temperature it’s necessary that are connected the following
probes:
•
TP3276.2 or TP3275 globe thermometer probe
•
HP3217.2 or HP3217R combined probe for the measurement of relative humidity and
the air temperature.
•
AP3203.2 or AP3203 probe with warm wire for the measurement of the air speed.
To calculate the medium radiant temperature you can see the regulation:
•
ISO 7726