What is a filled bulb temperature sensor and how it work?
To
measure temperature filled-bulb systems exploit the principle of fluid
expansion. If a fluid is enclosed in a sealed system and then heated, the molecules
in that fluid will exert greater pressure on the walls of the enclosing
vessel. By measuring this pressure, and/or by allowing the fluid to expand
under constant pressure, we may infer the temperature of the fluid.
Class
I and Class V systems use a liquid fill fluid (class V is mercury). Here, the volumetric expansion of the liquid drives an indicating mechanism to show
temperature:
Class III systems use a gas fill fluid instead of liquid. Here, the change in pressure with temperature (as described by the Ideal Gas Law) allows us to sense the bulb’s temperature:
A fundamentally different class of filled-bulb system is Class II, which uses
a volatile liquid/vapor combination to generate a temperature-dependent fluid
expansion:
Class
II systems do have one notable idiosyncrasy, though: they have a tendency to
switch from Class IIA to Class IIB when the temperature of the sensing bulb
crosses the ambient temperature at the indicator. Simply put, the liquid tends
to seek the colder portion of a Class II system while the vapor tends to seek
the warmer portion. This causes problems when the indicator and sensing bulb exchange
identities as warmer/colder. The rush of liquid up (or down) the capillary
tubing as the system tries to reach a new equilibrium causes intermittent
measurement errors. Class II filled-bulb systems designed to operate in either
IIA or IIB mode are classified as IIC.
One
calibration problem common to all systems with liquid-filled capillary tubes is
an offset in temperature measurement due to hydrostatic pressure (or suction)
resulting from a difference in height between the measurement bulb and the
indicator. This represents a “zero” shift in calibration, which may be
permanently offset by a “zero” adjustment at the time of installation. Class
III (gas-filled) and Class IIB (vapor-filled) systems, of course, suffer no
such problem because there is no liquid in the capillary tube to generate pressure due to height.
A
photograph of a pneumatic temperature transmitter using a filled bulb as the
sensing element appears here:
Instead
of directly actuating a pointer mechanism, the fluid pressure in this
instrument actuates a self-balancing pneumatic mechanism to produce a 3 to 15
PSI air pressure signal representing process temperature.
Filled-bulb
temperature sensors are seldom used in industrial applications anymore, chiefly
due to the superiority of electrical sensors. The only significant advantage
filled-bulb sensors hold over electrical sensors is not needing electricity1 to
function, but this is usually not a serious consideration within a modern
industrial facility.
Note:
It is possible to build self-powered thermocouple temperature indicators, where
an analog meter movement is driven by the electrical energy a thermocouple
sensing junction output. Here, no external electrical power source is required!
However, the accuracy of self-powered thermocouple systems is poor, as is the
ability to measure small temperature ranges.
List of Prominent Manufacturers: Baumer, Dwyer, Endress+Hauser, Fisher, Herz, Kobold, Labom, Precision Mass, Schmierer, Trafag, WIKA
Follow to get new posts and updates about the blog. Appreciate your feedback!
No comments:
Post a Comment