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Appendix 2. Thermal Elements
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A2-1
IM 02M04B01-01EN
Appendix 2. Thermal Elements
A2.1 General
(1) Principle of Operation
A sealed assembly of a bulb, capillary tubing, and
a helical measuring element is filled with a liquid
(T1A).
As the temperature at the bulb changes, the
element either coils or uncoils due to a volume
change. The movement of the element causes a
corresponding movement of the pointer.
FA0201.ai
T1A element
CAUTION
Use a bulb well of model TW1 or TW3 when
the bulb cannot be tapped directly due to the
mechanical vibration reason or when strict
sealing is required.
Element Sizes
Class X mm
J mm
U mm
Y mm
Bushing
T1A 33 to
169
*1
457
(Standard)
150 to 3000
(Optional)
(X + 50) to
(X + J – 100)
9.5
R1/2 male
or
R3/4 male
*1:
Affected by measuring span
(2) Common Bulb Types
FA0202.ai
Plain Bulb With Bendable Extension
Fixed Union Assembly
Bendable bulb
extension “J”
Sensitive
length “X”
“Y”
“Y”
Jam nut
Bulb
extension
Bushing
Sensitive
length “X”
Insertion length “U”
FA0203.ai
Adjustable Union Assembly
Bushing
Bulb extension or
connecting tubing.
Unscrew jam nut to remove bulb.
To vary bulb immersion,
loosen packing nut and
slide tubing.
(3) Installation Precautions
Never - in shipment, storage, or service - subject
the bulb to temperatures above the range of the
instrument, unless overrange protection is provided.
Underrange protection is usually furnished when
0% of measuring range is above 25°C.
The thermal system must not be crushed, cut, or
twisted. If any part is damaged, the assembly (bulb,
tubing, and helical element) must be returned to
Yokogawa for repair.
(4) Typical Temperature Installation
FA0204.ai
Loosely coil excess tubing
and secure in place.
Avoid extremes of
temperature in
locating tubing.
Fasten down tubing with
clips, staples, or tape.
Do not crush tubing.
