Some helpful information on ASTM and other glass laboratory thermometers(taken from
http://www.icllabs.com/thergenlinfo.html
)
All ASTM and other glass laboratory thermometers can be classified into 2 general groups - those designed and fabricated for total immersion and those designed and fabricated for partial immersion. If you use glass thermometers, it is essential that you understand the difference, and how each type of thermometer is used!
Most laboratory errors in temperature measurement result from incorrect usage (immersion) of the thermometer!Total immersion thermometers are designed with scales which indicate actual temperature when the bulb and the entire liquid column are exposed to the temperature being measured. In practice, a short length of liquid column (usually one inch or less) is permitted to extend above the surface of the liquid being measured to permit reading of the thermometer.
Most total immersion thermometers can also be used in a condition of complete immersion, wherein the entire thermometer is exposed to the temperature being measured, as with a thermometer inside a freezer.
Partial immersion thermometers are designed to indicate the actual temperature when a specified portion of its stem is exposed to the temperature being measured.
How can I know the difference?Partial immersion thermometers will have 1) an inscription on the reverse of the thermometer stating the required immersion depth - ie 76MM IMM or similar marking, and 2) in most cases, a "ring" is inscribed on the stem of the thermometer indicating the depth to which the stem should be submerged.
http://www.icllabs.com/pics/sepblank.JPG
On this line drawing, there is a "ring" inscribed on the thermometer just above the contraction chamber. If one views the reverse of the thermometer, the inscription 125MM IMM (or as appropriate) should appear. Accordingly, partial immersion thermometers are easy to identify.
Total immersion thermometers are sometimes a little trickier to identify. Some of the better manufacturers are inscribing TOTAL or TOTAL IMMERSION on the reverse of the thermometer, but regrettably this is not an industry-wide practice. The photo below is of an ASTM 112C thermometer, which is designed for total immersion. There is no "ring" inscribed, of course, and there is no "TOTAL IMMERSION" marking on the reverse.
If there is no inscription on the reverse indicating immersion, you should assume the thermometer is designed for total immersion.
http://www.icllabs.com/pics/112c.gif
What's the difference in use? As explained above, the partial immersion thermometer is immersed in the liquid being measured up to the line, or ring. The total immersion thermometer must be immersed to within approximately one inch of where the mercury column resides at the temperature being measured.
So what happens if the total immersion thermometer is not immersed to the depth it should be?You will have an erroneous temperature reading. The amount of the error depends upon what the temperature is that you are measuring, and how much of the thermometer stem that should be immersed is outside the liquid you are measuring. An extreme example: you have a -1/201C thermometer, 24 inches in length, total immersion, and you are testing the liquid in a beaker on a hotplate. Only about 2 inches of the thermometer is in the liquid. The thermometer indicates 190C. How much error do we have? Almost 5 degrees C. The liquid in the beaker is this amount hotter than the thermometer indicates.
I have this expensive, calibrated, total immersion thermometer I bought recently, similar to the one in the example above. I need it for applications similar to that described. Is this useless to me?No, it may not be the best thermometer for your application, but you can use it. However, you'll need to calculate an 'emergent stem correction', which we will explain below.
