Calibration Checks of Dry-and Wet-Bulbs - ESF...1) Example of DryBulb Sensors Calibration Check 150...

Marc Savard, Wood Drying Specialist

New England Kiln Drying Association

2015 Spring Meeting

May 14

Observations From Kiln Audits

Calibration Checks of Dry- and

Wet-Bulbs

© 2015 FPInnovations. All rights reserved. Copying and redistribution prohibited. ® FPInnovations, its marks and logos are trademarks of FPInnovations.

Presentation Plan

1) Types of temperature sensors in kilns

2) Results of calibration checks from kiln audits

3) Calibration checks methods

4) Conclusions

2


Main types of temperature probes in kilns

1) Dry-Bulb Temperature

▫ Resistance Temperature Device (RTD) Sensors

▫ Transistor Sensors

2) Air Humidity

▫ Dry- and Wet-Bulbs Psychrometer

▫ Electronic Dry- and Wet-Bulbs or Electronic RH Sensors

▫ Paper Wafer Electric Resistance Sensors

3


Dry-Bulb Temperature Sensors

1) RTD Sensors

4

� RTD = « Resistance Temperature Device »

� Based on the fact that the electrical resistance of

platinum is dependent of its temperature

Copyright © Sechoir Mec


2) Transistor Sensors

5

� Used mainly by European kiln controllers

▫ 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)

Dry-Bulb Temperature Sensors


Air Humidity Sensors

1) Dry- and Wet-Bulbs Psychrometer

6

� 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

© 2015 FPInnovations. All rights reserved. Copying and redistribution prohibited. ® FPInnovations, its marks and logos are trademarks of FPInnovations. 7

1) Dry- and Wet-Bulbs Psychrometer


� The temperature difference between the dry bulb and

wet bulb thermometers is a function of air relative

humidity


2) Electronic Dry- and Wet-Bulbs Temperature Sensors

8

� VAISALA brand are the most used for dry kilns

� Doesn’t require water in the kiln

� Expensive



3) Paper Wafer Electrical Resistance

9

� Used mainly by European

kiln controllers

▫ 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



Presentation Plan




4) Conclusions

10


FPInnovations Kiln Audit Program

11

� Standardized Audits (Keep It Simple and Short)

1. Kiln Tightness

2. Mechanical Components

3. Controller (calibration check)

4. Airflow

5. Temperature Distribution


Results from Kiln Audits

12

1) Example of Dry-Bulb Sensors Calibration Check

150

155

160

165

170

175

180

185

190

23.0 23.5 24.0 24.5 25.0 25.5 26.0

Température de l'air (ºF) .

Temps de séchage (heures)

Vérification de la calibration des bulbes secs

Séchoir # 1 Station 1

RTD 1 RTD 4

RTD 5 RTD 8

Dry-Bulb Temperature (°F)

Recording Time (hours)

Kiln #1



150

155

160

165

170

175

180

185

190

23.0 23.5 24.0 24.5 25.0 25.5 26.0



Vérification de la calibration des bulbes secs

Séchoir # 1 Station 1

Ts6 RTD 1

Ts12 RTD 4

Ts4 RTD 5

Ts10 RTD 8



Kiln #1





190

195

200

205

210

215

220

19.0 19.1 19.2 19.3 19.4 19.5 19.6 19.7 19.8 19.9 20.0


Temps (heures)

KILN # 2

B12

RTD #12



Kiln #1


1) Example of Wet-Bulb Sensors Calibration Check


WB 17 WB 18

WB FPI WB FPI

WB SP

150

155

160

165

170

175

180

185

190

23.4 23.6 23.8 24.0 24.2 24.4 24.6 24.8 25.0 25.2 25.4

Température humide (ºF) .


Vérification de la calibration des bulbes humides

Séchoir # 1 Station B

TH11 Th12

BH17 BH18

TH Cons.

Wet-Bulb Temperature (°F)


Kiln #1


1) RTD Sensors

16

� PT100

▫ 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)



� RTD Sensors

17


0

5

10

15

20

25

30

<=-

10 -9 -8 -7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9

>=1

0

Fré

qu

en

ce

Écart de lecture (°F)

99 sondes RTD

Fréquence (nbresondes)

Faster Drying

(more heating)

Slower Drying

(less heating)

64% 4%32%

Frequency (# of probes)

Reading offset (°F)

99 RTD Probes


Calibration Checks

18

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

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

1) RTD Sensors

Calibration Checks


c) RTD dry-wells▫ Only the sensing part of the probe is heated

1) RTD Sensors

Calibration Checks


d) In operation with a thermocouple▫ Allows to check the cable and junctions

Make sure your

thermocouple meter has

been calibrated lately!

Calibration Checks

1) RTD Sensors


0

2

4

6

8

10

12

14

16

<=-

10 -9 -8 -7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9

>=1

0

Fré

qu

en

ce


26 sondes transistors


Faster Drying

(more heating)

Slower Drying

(less heating)

� Transistor Sensors

22

65%


8%27%



26 Transistor Probes


2) Transistor Sensors

23

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!

Calibration Checks


� Wet-bulb using a RTD and a wick

24

0

1

2

3

4

5

6

<=-

10 -9 -8 -7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9

>=1

0

Fré

qu

en

ce


20 sondes de type RTD

Fréquence(nbresondes)

Slower Drying

(less venting)

Faster Drying

(more venting)

5%35%


60%



20 RTD with Wick


1) Wet-bulbs using a RTD and a wick

25

a) Same methods dry-bulb RTD can be used if you

remove the wick

b) In operation a portable wet-bulbo Allows to check the cable and junctions

o Allows to check water and wick

Calibration Checks


b) In operation with a portable wet-bulb

1) Wet-bulbs using a RTD and a wick

Calibration Checks


� Electronic Wet-Bulbs

27

0

1

2

3

4

5

6

7

<=-

10 -9 -8 -7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9

>=1

0

Fré

qu

en

ce


20 sondes électroniques

Fréquence(nbresondes)

Slower Drying

(less venting)

Faster Drying

(more venting)

75%



20 Electronic Wet-Bulbs

5%20%



2) Electronic Wet-Bulbs

28

a) In operation with a portable wet-bulbo Allows to check the cable and junctions

o If readings are not acceptable, the probe must be send to the supplier

for calibration = expensive

Calibration Checks


� Paper Wafer Electrical Resistance

29

0

1

2

3

4

5

6

7

8

9

<=-

10 -9 -8 -7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9

>=1

0

Fré

qu

en

ce

Écart de lecture (% d'humidité d'équilibre)

27 sondes de type buvard


Slower Drying

(less venting)

Faster Drying

(more venting)


Reading offset (% EMC)

20 Paper Wafer Sensors

63% 33%4%



� Impact on wet-bulb temperature of the variation of EMC of 1%

30

Dry-Bulb

(°F)

Wet-Bulb

(°F)

Depression

(°F)

Equilibrium

MC

(%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



a) By applying a known electrical resistance

between connectorso If readings are not acceptable, the service technician adjust a

potentiometer on controller I/O cards

b) In operation with a portable wet-bulb

3) Paper Wafer Electrical Resistance

Calibration Checks


Presentation Plan




4) Conclusions

32


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:

- RTDs, Transistor Sensors or Electronic Wet-Bulbs that under-estimate

temperature

- Paper Wafer Sensors that over-estimate EMC

33

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Calibration Checks of Dry-and Wet-Bulbs - ESF...1) Example of DryBulb Sensors Calibration Check 150...

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