Frequently asked questions and answers
Total immersion thermometers are designed to indicate temperatures correctly when just that portion of the thermometer containing the liquid is exposed to the temperature being measured. For example, if a bath temperature is 50°C, the thermometer would need to be immersed to the 50°C line, and then pulled out approximately 3/4" to attach a scope or to read by eye.
Partial Immersion thermometers are designed to indicate temperatures correctly when the bulb and a specified part of the stem (for example 76 mm) are exposed to the temperatures being measured.
Be aware that total immersion thermometers are generally more accurate than partial immersion thermometers of similar range and design. Partial immersion thermometers will show variances in temperature due to emergent stem temperature differences. Therefore, whenever possible, one should choose a total immersion thermometer. If the bath is too shallow or total immersion is otherwise impractical, emergent stem temperature corrections should be known and measured to reduce variances in repeatability.
What are the liquids used in the thermometers and are they all of the same accuracy?
The liquids in glass thermometers vary depending on range, use and accuracy requirements. The two most common liquids are metallic mercury and various organic liquids.
Mercury is a metallic element which is liquid at room temperature. It is the most reliable and repeatable of the thermometer liquids. It has a usable range from -40 to 500°C and has a linear coefficient of expansion. When mixed to form an amalgam with thallium the useable range is expanded down to approximately -58°C. When mercury is mixed with gallium, the resulting amalgam has an expanded range up to approximately 650°C.
Common organic liquids are mineral spirits, kerosene, toluene, and more recently, certain citrus extract based solvents. All organic liquids need to be dyed. The common colors are red, blue and green.
While most of the liquids used today are less toxic than mercury, organic liquids wet the sides of the bore causing repeatability problems. Also, they do not have linear coefficients of expansion. Therefore, unless they are etched stem with several calibration points, they will have severe accuracy limitations. Organic liquid thermometers are also very sensitive to changes in emergent stem temperatures.
By jacketing the thermometer in teflon (a group of fluoropolymers made and sold under this tradename by E.I Dupont) the thermometer is made more resilient to bumps and jars. It is not unbreakable. However, if the thermometer breaks the liquid will be retained within the teflon jacket, therefore stopping contamination wherever the thermometer was being used.
Encapsulating the thermometer in teflon does not change the calibration of the thermometer, but it does increase the lag time. Teflon encapsulated thermometers should not be used in timed tests.
It is also important NOT to teflon encapsulate certified standards. A teflon encapsulated standard will not give precision results due to non-repeatable and non-linear changes in the teflon each time the instrument is heated.
SAMA stands for the Scientific Apparatus Makers Association, of which MILLER & WEBER, INC. is a long time member. SAMA thermometers are manufactured to the specifications of the joint American National Standard ANSI/SAMA Z236.1-1983
ASTM stands for the American Society for Testing & Materials. All thermometers bearing the ASTM mark meet the requirements of ASTM Standard Specification E-1. MILLER & WEBER, INC. has members sitting on the committee (E-20.05) responsible for this standard.
Yes. However, a completely calibrated and certified reference standard with calibrated ice point either in its main scale or as an auxiliary scale, requires only one complete calibration in its lifetime. The need for re-calibration of a "properly manufactured" liquid-in-glass thermometer is due to the gradual relaxation of residual mechanical strains in the glass that have a significant effect on the volume of the bulb. Using current NIST methods, it is possible to avoid the usual requirement for complete re-calibration of the instrument by the re-calibration of a single previously calibrated temperature. The re-calibration of a single point provides a reliable indication of the effect of this change in volume and provides a means for the accurate adjustment of the remainder of the scale. The most convenient reference temperature to achieve in the laboratory is the ice point. This can be done in-house with a minimum investment in equipment, or it can be sent to us.
Thermometers without ice points need to be completely re-calibrated a minimum of once each year.
How you re-join a separation depends on where the separation occurs.
Separation At The Upper Portion Of The Mercury Column:
Most well constructed thermometers contain an expansion chamber at the extreme top of the capillary. This chamber allows for the overflow of mercury should the thermometer be overheated. If the instrument does not have scale divisions over 250°C it serves as a means of rejoining this type of separation. While holding the instrument in a vertical position, slowly heat the bulb until the separated segments and a portion of the main column enter the chamber. Never heat the bulb directly over an open flame and be sure the heating medium has a flash point above the highest temperature graduated on the thermometer. Do not allow the chamber to be more than half to three quarters full, otherwise the bulb may break due to excessive pressure. The nitrogen pressure will force a rejoining of the mercury. If necessary tap the thermometer gently. After the mercury is re-joined, continue to hold the thermometer vertically, and examine the column as it cools and retracts to be sure it is intact.
Separation In The Contraction Chamber:
Many thermometers contain scale ranges which begin well above ambient temperature. The thermometers have a contraction chamber or enlargement which prevents the mercury from entering the bulb at ambient temperatures. Heavy jarring of the instrument or thermal shock can cause a separation in this chamber. If the separated mercury is in the form of a speck or small amount, invert the thermometer and gently tap it against the palm of the other hand. This will cause a larger separation, adding additional volume and weight to the separated portion. The thermometer should then be righted and only the tip of the bulb should be dipped in ice water or salted ice water. Remove from the water and gently tap the bulb on a desk blotter and allow the separated portion to fall to the bottom of the chamber and re-join the mercury reservoir. Allow thermometer to warm to room temperature and check to be sure all separations have been recovered.
Separation In The Lower Portion Of The Column:
The general procedure for joining this more difficult (and less frequent) type of separation is to cool the bulb only to a temperature sufficient to retract all the mercury into the bulb. A slow and careful return to ambient temperature will return an intact column. Caution must be taken when the range is such that at the freezing point of mercury (-38.8°C) some mercury still remains in the capillary. If the thermometer is returned to ambient temperatures rapidly after freezing, there is a chance the bulb will crack. This breakage is caused by the mercury thawing in the capillary more slowly than the mercury in the bulb, creating a blockage of the expanding mercury. To avoid this, care must be taken to allow the mercury in the bulb to liquify at the same rate as the mercury in the capillary. To accomplish this, carefully place only the tip of the bulb into the cooling mixture (for example dry ice and alcohol). Hold the instrument vertical until all of the mercury is in the bulb, then allow the instrument to come back to ambient temperature while still holding the instrument vertically in the air. Do not jar the thermometer or hold it at an angle. Gas bubbles could develop in the bulb and cause inaccuracies. If gas bubbles form in the bulb, return it to us for repair.
Organic Liquid Separations:
The liquid in organic filled thermometers can often be rejoined by using gentle centrifugal force. Swinging the thermometer in a slow arc causes the liquid to be pushed towards the bulb. Do not snap or shake like a fever or maximum registering thermometer. Also, for small separations at the top of the column, tapping the instrument against a finger or hand may break the contact of the liquid against the capillary wall and allow it to drain down into the main column.
If centrifugal force does not work, it is possible to heat the thermometer until the liquid reaches the expansion chamber at the top of the thermometer similarly as with the mercury thermometers above. Be very cautious when using a heating method with organic liquid thermometers. If the bulb breaks, the liquid inside is flammable.
My maximum registering thermometer looks separated and won't shake all the way down. Is it defective? Does this thermometer work differently from a regular thermometer?
No, maximum thermometers will normally look separated below the thermometer's constriction. Capillary constriction allows mercury to rise when heated, but prevents it from receding until shaken down. To shake down, grasp thermometer near the top and with swinging motion with a snap, shake mercury below temperature to be measured. Thermometer cannot be shaken down below room temperature. Allow sufficient time for thermometer to attain maximum reading. Remove from heat and allow to cool to room temperature before reading. Errors may result if reading is taken before thermometer is cooled. A vacuum exists above the mercury column and the readings must be made with the thermometer in an upright position. PLEASE NOTE: APPEARANCE OF MAXIMUM REGISTERING THERMOMETER IS DIFFERENT THAN A REGULAR THERMOMETER. CONSTRICTION APPEARS TO LOOK LIKE A MERCURY SEPARATION AND WHEN INVERTED THE MERCURY WILL RUN UP AND DOWN. THIS IS NORMAL.
Our calibration services take between one and two weeks. This is because our Certification process meets the strict quality controls of ISO Guide 25 and ASTM E-77 (as well as other quality standards).
Each instrument, either ours or yours, is given a thorough visual inspection before it is tested. It is inspected to make sure that there are no residual strains in the glass. The lines and numbers are checked for uniformity and placement and the capillary clearances are checked. The mercury or other fluid is then drawn down into the bulb (out of the capillary) and the capillary is checked for foreign matter and irregularity. The liquid is also checked for moisture. AFTER THIS INSPECTION THE THERMOMETER MUST RECOVER AT ROOM TEMPERATURE FOR A MINIMUM OF 72 HOURS.
Only after these 72 hours can the proper temperature testing be done. The instrument is tested in state-of-the-art baths against NIST traceable master standards at the test points specified either by a recognized standard or by our customer.
By having the thermometer ice point calibrated during the initial complete calibration of the thermometer you can avoid having annual re-calibration of the entire thermometer.