Calibration Checks of Dry- and
Wet-Bulbs
Observations From Kiln Audits
Marc Savard, Wood Drying Specialist
New England Kiln Drying Association
2015 Spring Meeting
May 14
Presentation Plan
1) Types of temperature sensors in kilns
2) Results of calibration checks from kiln audits
3) Calibration checks methods
4) Conclusions
2
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Main types of temperature probes in kilns
1) Dry-Bulb Temperature
▫
Resistance Temperature Device (RTD) Sensors
▫
Transistor Sensors
2) Air Humidity
3
▫
Dry- and Wet-Bulbs Psychrometer
▫
Electronic Dry- and Wet-Bulbs or Electronic RH Sensors
▫
Paper Wafer Electric Resistance Sensors
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Dry-Bulb Temperature Sensors
1) RTD Sensors
RTD = « Resistance Temperature Device »
Based on the fact that the electrical resistance of
platinum is dependent of its temperature
Copyright © Sechoir Mec
4
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Dry-Bulb Temperature Sensors
2) Transistor Sensors
Used mainly by European kiln controllers
▫
▫
▫
5
At constant current, the voltage measurement is a linear
function of the temperature
Advantage of being simple to manufacture and implement,
inexpensive and very linear
Correct measurement range for kilns (-50 ° C, 150 ° C)
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Air Humidity Sensors
1) Dry- and Wet-Bulbs Psychrometer
Wet-Bulb Temperature
▫
It is defined as the air temperature obtained by any
temperature measuring device whose sensitive portion is
kept wet by means of a cotton wick soaking in water the
whole being placed in a stream of air
Copyright © Secovac
6
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Air Humidity Sensors
1) Dry- and Wet-Bulbs Psychrometer
The temperature difference between the dry bulb and
wet bulb thermometers is a function of air relative
humidity
7
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Air Humidity Sensors
2) Electronic Dry- and Wet-Bulbs Temperature Sensors
VAISALA brand are the most used for dry kilns
Doesn’t require water in the kiln
Expensive
8
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Air Humidity Sensors
3) Paper Wafer Electrical Resistance
Used mainly by European
kiln controllers
▫
▫
▫
9
The paper wafer absorbs or
releases moisture depending on
the humidity of the air
The electrical conductivity of the
paper varies according to its
moisture
Does not require water in the
kiln
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Presentation Plan
1) Types of temperature sensors in kilns
2) Results of calibration checks from kiln audits
3) Calibration checks methods
4) Conclusions
10
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FPInnovations Kiln Audit Program
Standardized Audits (Keep It Simple and Short)
1.
2.
3.
4.
5.
11
Kiln Tightness
Mechanical Components
Controller (calibration check)
Airflow
Temperature Distribution
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Results from Kiln Audits
1) Example of Dry-Bulb Sensors Calibration Check
Vérification de la calibration des bulbes secs
190
185
Température
de l'air (ºF) (°F) .
Dry-Bulb
Temperature
180
175
170
165
160
Séchoir
Kiln #1# 1
Station 1
RTD 1
RTD 4
RTD 5
RTD 8
155
150
23.0
23.5
24.0
24.5
25.0
Temps
de séchage
(heures)
Recording
Time (hours)
12
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25.5
26.0
Results from Kiln Audits
1) Example of Dry-Bulb Sensors Calibration Check
Vérification de la calibration des bulbes secs
190
185
Température
de l'air (ºF)(°F) .
Dry-Bulb
Temperature
180
175
170
165
Séchoir # 1
Station 1
Ts6
RTD 1
Ts12
RTD 4
Ts4
RTD 5
Ts10
RTD 8
Kiln
160
155
150
23.0
23.5
24.0
24.5
25.0
Temps
de séchage
(heures)
Recording
Time (hours)
13
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#1
25.5
26.0
Results from Kiln Audits
1) Example of Dry-Bulb Sensors Calibration Check
220
Kiln
KILN#1
#2
215
B12
Dry-Bulb
Temperature
Température
de l'air (ºF)(°F) .
RTD #12
210
205
200
195
190
19.0
19.1
19.2
19.3
19.4
19.5
19.6
19.7
Temps
(heures)
Recording
Time (hours)
14
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19.8
19.9
20.0
Results from Kiln Audits
1) Example of Wet-Bulb Sensors Calibration Check
Vérification de la calibration des bulbes humides
190
Kiln
Séchoir#1
#1
Station B
WB FPI
TH11
WB FPI
Th12
WB 17
BH17
WB 18
BH18
185
Wet-Bulb Temperature
Température
humide (ºF)(°F) .
180
175
WB
SP
TH
Cons.
170
165
160
155
150
23.4
23.6
23.8
24.0
24.2
24.4
24.6
24.8
Temps de séchage
Recording
Time(heures)
(hours)
15
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25.0
25.2
25.4
Results from Kiln Audits
1) RTD Sensors
PT100
▫
▫
16
PT = Platinum (most linear, stable, repetitive et precise)
100 = Electrical resistance value in ohms at 0°C
Temperature
Tolerance
Class B
Tolerance
Class A
0°C (32°F)
±0.3°C (±0.5°F)
±0.15°C (±0.3°F)
100°C (200°F)
±0.8°C (±1.4°F)
±0.35°C (±0.6°F)
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Results from Kiln Audits
RTD Sensors
99
Probes
99RTD
sondes
RTD
Frequency
(# of sondes)
probes)
Fréquence
Fréquence
(nbre
30
25
Faster Drying
(more heating)
Slower Drying
(less heating)
20
15
32%
64%
4%
10
5
<=-10
-9
-8
-7
-6
-5
-4
-3
-2
-1
0
1
2
3
4
5
6
7
8
9
>=10
0
Reading
offset (°F)
Écart
de lecture
(°F)
17
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Calibration Checks
1) RTD Sensors
a) Ice Water and Boiling Water Method
▫
Only the sensing part of the probe is heated
Laboratory method, hard to do in a
dry kiln!
Necessary equipment:
- Hot plate
- Agitator
- Reference thermometer
18
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Calibration Checks
1) RTD Sensors
b) Hot water in a insulated bottle
▫
Only the sensing part of the probe is heated
Necessary equipment:
- Insulated bottle
- Reference thermometer
(calibrated) to tell water
temperature in the bottle
19
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Calibration Checks
1) RTD Sensors
c) RTD dry-wells
▫
20
Only the sensing part of the probe is heated
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Calibration Checks
1) RTD Sensors
d) In operation with a thermocouple
▫
Allows to check the cable and junctions
Make sure your
thermocouple meter has
been calibrated lately!
21
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Results from Kiln Audits
Transistor Sensors
26 sondes
Transistor
Probes
26
transistors
16
Frequency
(# of sondes)
probes)
Fréquence
Fréquence
(nbre
14
12
Faster Drying
(more heating)
Slower Drying
(less heating)
10
8
27%
65%
8%
6
4
2
<=-10
-9
-8
-7
-6
-5
-4
-3
-2
-1
0
1
2
3
4
5
6
7
8
9
>=10
0
Reading
offset (°F)
Écart
de lecture
(°F)
22
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Calibration Checks
2) Transistor Sensors
a) In operation with a thermocouple
▫
▫
Allows to check the cable and junctions
Hard to reach adjustment potentiometer
Make sure your
thermocouple meter has
been calibrated lately!
23
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Results from Kiln Audits
Wet-bulb using a RTD and a wick
20 RTD
20 sondes
dewith
typeWick
RTD
Frequency
(# of probes)
Fréquence
Fréquence
(nbre
sondes)
6
5
Slower Drying
(less venting)
Faster Drying
(more venting)
4
3
35%
60%
5%
2
1
<=-10
-9
-8
-7
-6
-5
-4
-3
-2
-1
0
1
2
3
4
5
6
7
8
9
>=10
0
Écart
de lecture
Reading
offset (°F)
(°F)
24
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Calibration Checks
1) Wet-bulbs using a RTD and a wick
a) Same methods dry-bulb RTD can be used if you
remove the wick
b) In operation a portable wet-bulb
o
o
25
Allows to check the cable and junctions
Allows to check water and wick
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Calibration Checks
1) Wet-bulbs using a RTD and a wick
b) In operation with a portable wet-bulb
26
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Results from Kiln Audits
Electronic Wet-Bulbs
Electronic
Wet-Bulbs
2020
sondes
électroniques
Frequency
(# of probes)
Fréquence
Fréquence
(nbre
sondes)
7
6
Slower Drying
(less venting)
Faster Drying
(more venting)
5
4
3
20%
75%
5%
2
1
<=-10
-9
-8
-7
-6
-5
-4
-3
-2
-1
0
1
2
3
4
5
6
7
8
9
>=10
0
Écart
de lecture
Reading
offset (°F)
(°F)
27
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Calibration Checks
2) Electronic Wet-Bulbs
a) In operation with a portable wet-bulb
o
o
28
Allows to check the cable and junctions
If readings are not acceptable, the probe must be send to the supplier
for calibration = expensive
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Results from Kiln Audits
Paper Wafer Electrical Resistance
9
8
7
6
5
4
3
2
1
0
Slower Drying
(less venting)
4%
Faster Drying
(more venting)
63%
33%
<=-10
-9
-8
-7
-6
-5
-4
-3
-2
-1
0
1
2
3
4
5
6
7
8
9
>=10
Frequency
(# of sondes)
probes)
Fréquence
Fréquence
(nbre
PaperdeWafer
27 20
sondes
type Sensors
buvard
Reading
offset (% EMC)
Écart de lecture
(% d'humidité
d'équilibre)
29
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Results from Kiln Audits
Impact on wet-bulb temperature of the variation of EMC of 1%
Dry-Bulb
(°F)
30
Wet-Bulb Depression Equilibrium
MC
(°F)
(°F)
(%EMC)
Variation of Wet-Bulb
Temperature in
Function of EMC
190
186
4
16.0
1°F / %HE
190
180
10
11.0
2°F / %HE
190
174
16
9.0
4°F / %HE
190
160
30
6.0
8°F / %HE
190
135
55
3.0
12°F / %HE
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Calibration Checks
3) Paper Wafer Electrical Resistance
a) By applying a known electrical resistance
between connectors
o
If readings are not acceptable, the service technician adjust a
potentiometer on controller I/O cards
b) In operation with a portable wet-bulb
31
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Presentation Plan
1) Types of temperature sensors in kilns
2) Results of calibration checks from kiln audits
3) Calibration checks methods
4) Conclusions
32
© 2015 FPInnovations. All rights reserved. Copying and redistribution prohibited. ® FPInnovations, its marks and logos are trademarks of FPInnovations.
Conclusions
In average, one sensor out of three in a dry kiln is
outside its accuracy specification
This tells that calibration check frequency and/or calibration
check methodology and/or calibration methodology has to be
reconsidered by many of you
Reading Offsets are not normally distributed along
Normal Law. Two hypothesis to consider:
▫ Aging of sensors, cables, junctions, etc.
or
▫ In operation, it is harder to detect:
-
33
RTDs, Transistor Sensors or Electronic Wet-Bulbs that under-estimate
temperature
Paper Wafer Sensors that over-estimate EMC
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