CPD Programme
atmospheric pressure, p
Figure 2 - Assmann
at
, of 101.325kPa. The lines and identifying air points, as it builds in
other properties may then be read directly off
Psychrometer (source:
a clearly visible tolerance, or uncertainty, that
www.mesdan.com)
the chart with the specific enthalpy obtained reflects the real world challenge in obtaining
by aligning a straight edge through the plotted an ‘accurate’ measurement.
air point so that the same value is read on both wet-bulb or electronic A future article will apply the Psychrometric
the ‘laddered’ specific enthalpy scales (shown sensor) is susceptible to Chart to processes and systems.
with the black dashed line in Figure 1). contamination by both air
These values may also be calculated. So borne contamination as © Tim Dwyer
using the values obtained from the earlier well as finger grease. The
example for air at θ = 20.0°C and θ’=13.9°C. wetted wick on a wet-bulb Further Reading
v was calculated as 0.839 m
3
·kg
-1
needs to stabilise at the wet Air Conditioning Engineering, Jones WP,
(compares with 0.84 m
3
·kg
-1
from the bulb temperature before Butterworth 2001, Chapter 2
chart) the temperature may be ASHRAE Fundamentals 2009, Chapter 1
p
v
is 1.179kPa (previously calculated) recorded. Tap water or other impure water CIBSE Guide C1, CIBSE 2007
and so can cause measurement errors when used
g = 0.622 p
v
/(p
at
– p
v
) = to wet the wick, and if the wick (covering the References
0.622 x 1.179/(1013.25-1.179) bulb of a wet-bulb thermometer) is too long [1] A Sprung, Uber die Bestimmung
= 0.0073 kg·kg
-1
(0.0074 kg/kg from it will be affected by heat conducted from the der Luftfeuchtigkeit mit Hilfe des
chart) thermometer. Assmannschen Aspirations psychrometers.
To determine the percentage saturation the Small variations in recorded temperature 1888. A.Sprung
value of the saturated moisture content at the can translate into significant variations in [2] CIBSE Guide C - Reference Data. CIBSE.
dry bulb temperature of 20 deg C is required. percentage saturation. For example the 2007.
This can be obtained using the formula for g gradations on the stem of a thermometer are [3] R C Legg, Air Conditioning Systems -
but using saturated vapour pressure (at the likely to be 0.5 deg C – this alone will lead Design, Commissioning and Maintenance.
dry bulb temperature), p
s
, in place of p
v
to to an uncertainty of +/-2.5 per cent in the Batsford, 1991.RC, Legg
determine g
s
. And p
s
(at 20 deg C) can be derived percentage saturation. To reduce [4] T C Dwyer, The properties of air. CIBSE,
obtained from possibilities of operator error devices such as 2009.T C Dwyer
log p
s
= 30.5905 – 8.2 log (20 + 273.16) + the Assmann Psychrometer may be used to [5] R C Legg, Private communication on
2.4804 x 10
-3
(20 + 273.16) – [3142.31 / (20 measure room air conditions (see Figure 2) – Psychrometric Analysis. 2009.R C,Legg
+ 273.16)] and so p
s
= 2.342 kPa. they have finer gradations on the temperature [6] Thomas H Kuehn, J W R James
g
s
= 0.622 p
s
/(p
at
– p
s
) = 0.622 x 2.342/ scales and provide a more consistent method L.Threlkeld. Thermal Environmental
(1013.25-2.342) = 0.0147 kg·kg
-1
of measurement with air being drawn over the Engineering (3rd Edition).
μ = g/g
s
x 100 = (0.0073/0.0147) x 100 = thermometers by a small motor driven fan.
50 per cent (compares with 50 per cent This ensures that the air velocity is drawn
on chart) across the wet bulb at the minimum required
[6]
Finally the Specific Enthalpy, h may be 1 m·s
-1
.
determined from In terms of practical application of the
h = ( 1.005 θ) + g (1.89 θ +2501) = (1.005 psychrometric chart it has been observed that
x 20) + 0.0073 (1.89 x 20 + 2501) = 39 many seasoned users prefer to use a thicker
kJ·kg
-1
(the same value as on chart). pen rather than a sharp pencil when plotting
Psychrometric accuracy
There is evidently reasonable correlation
Temperature - Celsius °C - normal symbol θ
between reading information off the chart
Absolute Temperature - Kelvin K (normal symbol T) = θ°C + 273.15
and the calculated values and this should
Energy - joule J
provide some confidence (and will be
Power - watt W (normal symbol P)
similarly accurate across the whole chart).
1W = 1J·s
-1
= 1kg·m
2
·s
-3
However it is important to keep in mind
Pressure - pascals Pa (or bar) normal symbol p
that the accuracy of the output data can be
1 Bar = 1000mbar = 100000 Pa = 100kPa
no better than the measurements that have
Other symbols (not units)
been taken on-site or, perhaps, read from a
V = volume, m
3
screen of a building management system
m = mass, kg
(BMS). The perception may be that since, for
R = gas constant, J·kg
-1
·K
-1
example, a BMS output screen shows several
g = moisture content kg·kg
-1
da
Subscripts
significant figures that the measurement is
ρ = density kg m
-3
v = water vapour
precise. This is unlikely to be the case. Legg
[5]
v = specific volume m
3
·kg
-1
s = saturated
reports that the practical uncertainties in the
h = enthalpy kJ·kg
-1
a = dry air
measurement of wet-bulb temperatures can
μ = percentage saturation % da = dry air
lead to significant uncertainties.
θ’ = wet-bulb temperature °C (normally sling) at = atmospheric
The wet-bulb sensor (whether a true
www.cibsejournal.com August 2009 CIBSE Journal 51
CIBSEaug09 pp49-52 cpd.indd 51 7/23/09 4:06:43 PM
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