Wika TR10-A Bedienungsanleitung PDF herunterladen (Seite 34)

3345267.12 01/2014 GB/D

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

The following formula is used for the calculation: T

max

= P

x R

+ T

max

Surface temperature (max. temperature at the probe tip)

= from the Zener barrier data sheet

= Thermal resistance [K/W]

= Temperature of the medium

Prerequisite is an ambient temperature T

amb

of -20 ... +40 °C.

Example

Resistance thermometer RTD

Diameter: 6 mm

Temperature of the medium T

= 150 °C

Supplied power: P

= 1150 mW

Temperature class T3 (200 °C) must not be exceeded

Thermal resistance [Rth in K/W] from table = 75 K/W

Self-heating: 1.15 W x 75 K/W = 86.25 K

max

= T

+ self-heating: 150 °C + 86.25 °C = 236.25 °C

The result shows, in this case, substantial self-heating at the probe tip.

As safety margin for type-examined instrument (for T6 to T3), an additional 5 °C must be

subtracted from the 200 °C; hence 195 °C would be permissible. This means that in this case

temperature class T3 is exceeded signicantly and therefore not permissible. An additional

thermowell could be used as a remedy.

Additional information

Temperature class for T3 = 200 °C

Safety margin for type-examined instruments (T6 to T3)

= 5 K

Safety clearance for type-examined instruments (T1 to T2)

= 10 K

1) EN/IEC 60 079-0: 2009 Section 26.5.1

9.3 Calculation example for the above-mentioned resistance thermometer with thermowell

RTD measuring insert Ø 6 mm without transmitter, built into a 3F design multi-part thermowell.

Thermal resistance [R

in K/W] from table = 37 K/W

Self-heating: 1.15 W x 37 K/W = 42.55 K

max

= T

+ self-heating: 150 °C + 42.55 °C = 192.55 °C

The result shows, in this case, substantial self-heating at the probe tip.

As safety margin for type-examined instrument (for T6 to T3), an additional 5 °C must be

subtracted from the 200 °C; hence 195 °C would be permissible. This means that in this case

temperature class T3 is not exceeded.

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

1 2 ... 29 30 31 32 33 34 35 36 37 38 39 ... 75 76

Kommentare zu diesen Handbüchern

Keine Kommentare

Wika TR10-A Bedienungsanleitung Seite 34

Kommentare zu diesen Handbüchern

Verwandte Produkte und Handbücher für Messgeräte Wika TR10-A