The pros and cons of bulb thermometers

 Bulb thermometers are perhaps the most ancient technology you could find for reliable and precise temperature measurents.

How does a bulb thermometer works?

Easy.

It is made of tube of small diameter (capillar) were a fluid is enclosed. The trick is that the fluid has a physical property that allows it to expand with temperature changes. These changes are so remarkable that you can actually see through the capillar how the mercury rises from the bulb.

In other words, the bulb thermometer relies on a thermal fluid that remains liquid over a given range of temperature measurement and with a reasonable thermal expansion coefficient.

Some advantages of bulb thermometers

• Training is not needed (little, in fact)

• It does not need electric energy in order to work

• No batteries needed

• It can be portable

• There are handheld versions

• It is cheap

• Different, coloured, fluids can be used to ease readings

• No further calibration is required

Some disadvantages of bulb thermometers

• Hard to read

• Numbers can be too small

• Precision is not trustable due to the scale (human intervention)

• It needs to be in contact with the fluid/object to be measured

• It should be installed in a place with very low or zero vibrations

• It may be broken easily (even the ones made of plastic)

• Data can not be transferred from the instrument to a controller or another device

• Not suitable for closed loops

• It shall be installed in well illuminated places

• Scale can be erased after long time of manipulation

• Slow time response

Some interesting stuff on bulb thermometers

• Commonly used for lab applications

• Different fluids can be used to extend the range of measurement

• These can be bought even in a pharmacy

• Some working fluids can be toxic (like mercury)

• These can be used as test/reference instrument when compared with other technologies

• Poor eyesight can be a trouble

Any question? Write in the comments and I shall try to help.

Other stuff of interest

• LE01 - AC and DC voltage measurement and continuity test
• LE 02 - Start and stop push button installation 24V DC
• Some examples of temperature instruments
• Minor losses  - Formulas
• What is a process variable?
• What are the most important process variables?
• Time dependence of process variables
• A list of process variables

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Ildebrando.