Wika TR10-A Manuel d'utilisateur télécharger pdf (Page 32)

3345267.12 01/2014 GB/D

32 WIKA operating instructions RTD and TC, intrinsically safe designs (Ex i)

9. Calculation examples for self-heating at the sensor/thermowell tip

The self-heating at the sensor tip or thermowell tip depends upon the sensor type (TC/RTD),

the probe diameter, the thermowell design and the power supplied to the sensor in the event of

a failure. The table below shows the possible combinations. The table shows that when a failure

occurs, thermocouples produce much less self-heating than resistance thermometers.

Thermal resistance [R

in K/W]

Sensor Probe Ø in mm

2.0-

< 3.0

3.0-

< 6.0

6 - 8 3.0 -

6.0

0.5-

< 1.5

1.5-

< 3.0

3.0-

< 6.0

6.0-

12.0

Sensor type RTD RTD RTD RTD TC TC TC TC

without thermowell 245 110 75 225 105 60 20 5

with multi-part thermowell

(straight and tapered)

(e.g. TW22, TW35, TW40, TW45, etc.)

135 60 37 - - - 11 2.5

with thermowell - machined from solid

material (straight and tapered)

(e.g. TW10, TW15, TW20, TW25, TW30,

TW50, TW55, TW60, etc.)

50 22 16 - - - 4 1

Special thermowell – EN 14597 - - 33 - - - - 2.5

Tx55 (tubular holder) - 110 75 225 - - 20 5

Built into a blind bore

(minimum wall thickness 5 mm)

50 22 16 45 22 13 4 1

1) surface-sensitive

When using multiple sensors and simultaneous operation, the sum of the individual powers must

not exceed the value of the maximum permissible power. The maximum permissible power must

be limited to 1.5 W maximum. This must be guaranteed by the plant operator.

9.1 Calculation example for RTD measuring point with thermowell

Use at the partition wall to Zone 0: Calculate the maximum permissible temperature T

max

at the

thermowell tip for the following combination:

RTD measuring insert Ø 6 mm with built-in model T32.1S head-mounted transmitter, tted into

a Design 3F multi-part thermowell . Power supply is, for example, via a Model KFD2-STC4-EX1

transducer power supply (WIKA article No. 2341268).

max

is obtained by adding the temperature of the medium and the self-heating. The self-heating

of the thermowell tip depends on the supplied power P

of the transmitter and the thermal

resistance R

The following formula is used for the calculation: T

max

= P

x R

+ T

max

= Surface temperature (max. temperature at the thermowell tip)

= from transmitter data sheet

= Thermal resistance [K/W]

= Temperature of the medium

Prerequisite is an ambient temperature T

amb

of -20 ... +40 °C.

9. Calculation examples for self-heating at the sensor/thermowell tip

1 2 ... 27 28 29 30 31 32 33 34 35 36 37 ... 75 76

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Wika TR10-A Manuel d'utilisateur Page 32

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