Testo 6740 Data sheet
Humidity Monitoring in Compressed Air
Increase safety - Cut costs
°C tpd
°F tpd
°C td
atm
°F td
atm
% RH
ppm
V
mg/m
3
°C / °F
testo 6740
2
The humidity sensor
2
Optimal for trace humidity measurement
The testo humidity sensor is generally calibrated at several points to ensure minimum deviations. For trace
humidity measurement, a high-precision reference measurement (dewpoint mirror) is used to help carry out a
calibration at -40 °CtP (pressure dewpoint).
To demonstrate the accuracy of testo’s humidity sensors, five sensors were extensively tested in a large number
of international calibration institutes over a 5-year period. All the measurement results confirmed the high
accuracy of 1% RH.
Monitor trace humidity, avoid damage
Dry air, compressed air and dry gases are used in all areas of industry. Humidity is normally
undesirable because it can cause damage or impair the quality of the end product, as the
graphic underneath shows.
04/96
10/96
12/96
07/97
09/98
10/98
03/99
10/00
03/01
08/96
10/96
09/98
10/98
04/00
08/01
PTB
P
T
B
12/00
NRC
CRM
09/00
KRISS
J
Q
A
INTA
N
P
L
IMGC
N
IST
C
E
T
IA
T
·
That’s why you need the testo 6740 for effective trace humidity measurement
>>
Compression Distribution Consumer
Corrosion in pipes and
function elements
Pneumatic drives:
- interrupted lubrication
- mechanical damage
Formation of ice in
cold and external zones
Compressor Drier
e.g. medical
compressed air
Moist powder
conglutinated
Bacterial growth
(European drug
legislation)
e.g. transport air for
pharmaceutical powder
·
Gas engineering: humidity causes damage and reduces the
value of the gas in the system
Granulate drying: dry air is a
requirement for product quality
Medical compressed air:
minimum humidity as a hygiene
requirement
Compressed air systems: drier
monitoring to avoid damage
caused by humidity
05/00
Dielectric layer
Polymer, changes dielectricity as
a function of relative humidity
Top electrode
Allows humidity to penetrate to
the dielectric layer and protects
against dirt
Carrier
Ceramic substrate for
mechanical protection
Connection pins
Special anti-
corrosion design
Bottom electrode
33
Safeguard quality - Cut costs
What is compressed air quality?
The international standard ISO 8573 defines seven classes of
compressed air quality and lays down the humidity, the oil
content, the particle content etc. which the compressed air is
allowed to have. Class 1 represents the highest requirements.
Class 4 is satisfied if, for instance, the pressure dewpoint does
not exceed 3 °CtP or 37 °FtP or an absolute humidity of 6 g
water vapour per m3 or 8,150 ppmV(parts per million, relative to
the volume).
The main way of ensuring compliance with a quality class
involves installing a suitable drier. Its monitoring and, where
appropriate, its control (see below), is handled by the
testo 6740.
How can costs be reduced?
Of course, the main reason for using the testo 6740 is to
monitor and avoid excessive humidity in the system so as to
avoid damage (cf. p. 2 below). In addition, dryer operating costs
can be reduced sharply.
Adsorption driers:
If chamber switchover is humidity-controlled
using the testo 6740 rather than being time-
controlled (see diagram on the right), the dry
phases (blue) are normally much longer than the
regeneration phases (red). During this time no
regeneration air must be generated, so that the
compressors can be reduced from 100% to
about 85% volumetric flow rate. This results in
significant savings in operating costs.
Dry
compressed air
Humid
compressed air
Condensate
Humid
compressed air
Cooling driers:
In non-critical systems, low-temperature driers
can be switched off completely when air humid-
ity is low (e.g in the winter). The testo 6740
supplies the humidity measurement here as
well. If the humidity surges that occur when the
low-temperature driers are switched on are to
be avoided, a downstream low-temperature
drier can be kept in continuous operation to
trap this humidity. This results in tangible
savings in operating costs.
On -22°
Vessel A
Pressure dewpoint
(°Ctpd)
Drying
Regeneration
Drying
Regeneration
Vessel B
On -22°
Off -28°
Vessel A
Drying
Regeneration
Drying
Regeneration
Vessel B
Time
10°
2°
Low-
temperature
driers
Pressure
dewpoint
temperature
TimeOff
On
Off
Equipment Compressed air drier
Monitoring/
Control testo 6740
Typical application
Semi-cond. prod.
Granulate drier
Transport air
Pneu. tube conveyor
Vacuum eng.
Working/energy air
Blow air
ISO 8573
Class
1
2
3
4
5
6
7
Trace humidity
°Ctpd °Ftpd g/m
3
ppm
v
-70 -94 0.003 4
-40 -40 0.12 163
-20 -4 0.88 1200
3 37 6 8150
7 44 7.8 10600
10 50 9.4 12800
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