By Nick Stefano

 mailto:nick@nac.net

In my last article I compared my experiences with the different types of instrument shelters versus radiation shields. I will now compare liquid in glass thermometers to digital temperature systems. Please do not take umbrage with this article if you are a fan of one thermometer or another. I am only reporting what I have encountered and with what I am confident with.

For years the old standard were the liquid in glass thermometers. I mention thermometers (plural) because there are two that observers read from. Many observers still use these thermometers. First, the minimum liquid-in-glass thermometer is mounted horizontally on a Townsend support, which holds both the minimum and maximum thermometers. The minimum thermometer is alcohol filled with one-degree etchings on the stem. Most alcohol minimum thermometers are colored to make them easy to read. Alcohol is used in the minimum thermometer because it freezes at a much lower temperature then the minus 38°F freezing point of mercury. A barbell shaped index determines the minimum temperature achieved. The thermometer is mounted on an angle (about 10 degrees) the lower point being the bulb end. As the temperature falls it drags down the index to the lowest point reached. Once the temperature rises the index remains at the lowest point. The index is reset by turning the thermometer clockwise while it is still mounted to the Townsend support until the index slides "down" to the current temperature. The Townsend support is designed to allow the observer to reset both minimum and maximum liquid in glass thermometers without releasing the thermometers from their mount. Incidentally, when taking the low reading on the thermometer you MUST READ THE RIGHT SIDE OF THE BARBELL SHAPED INDEX.

I have encountered several problems with the minimum thermometer. A major one is the occasional separation of the alcohol. This gives you an inaccurate reading of both the current temperature and the minimum. One must frequently look very closely at the liquid in the stem constantly to make sure there is no separation. Another problem related to this same situation is that sometimes a very small air bubble develops in the bulb. If this occurs, and it is sometimes difficult to notice, the temperature will be inaccurate. The only way to remove the bubble is to apply dry ice to the bulb. This will pull down the liquid into the bulb and the bubble disappears. Separations can usually be fixed by connecting a strong string to the opposite end from the bulb on the thermometer and whirling it around in a circle. The centrifugal force usually will bring the liquid together again. Another way and much more dangerous approach is to apply heat to the bulb. As the temperature rises the liquid tends to re-connect. The obvious danger here is applying too much heat and causing the liquid to explode past the end of the instrument. I have done this several times.

Another issue is with the accuracy of the minimum thermometers. I recommend a third standard mercury thermometer juxtaposed next to the minimum and maximum thermometers just to make sure that the true current temperature is the same on all instruments. Also, every so often the minimum thermometer should be tested by placing it in a thermos with crushed ice and water. When you submerge the bulb end of the thermometer into the slush, you "should" get a 32°F reading on the thermometer. If you don’t then you know that you have a problem that needs attention.

Overall, this thermometer is a good one when it is in perfect calibration. The obvious concerns are the little things that happen and if not paid close attention too will effect the true readings.

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The mercury filled maximum liquid-in-glass thermometer has been by far the most accurate and reliable of all the thermometers I use. It is also mounted on the Townsend support in a horizontal fashion. Unlike the minimum thermometer, the bulb sits at the highest point. This instrument works just like the thermometer used to take a temperature when you suspect a fever. A constriction near the bulb allows the mercury to rise past in one direction as the temperature rises yet, will not allow the mercury to return once the temperature falls. The maximum temperature reached can be determined by simply looking at the highest point that the mercury attained. To reset the temperature, one must either release the set-lock on the Townsend support, which frees the thermometer's movement and whirling it on the mount in either direction. The centrifugal force will cause the mercury to reset past the constriction and stop at the current temperature.

Another way to reset the thermometer is to simply remove it completely from the Townsend support and whirling it or "snapping" it down so the mercury rests past the constriction. When you remount it, make sure ALL the support mount locks are turned tight.

Liquid-in-glass maximum thermometers have been my favorite because of the lack of any problems I’ve had with them and due to their consistent accuracy. One rare problem that I experienced was a "weak" constriction which allowed the mercury to return to the bulb rather easily without swirling. Both the minimum and maximum liquid thermometers are analog readouts so they must be read carefully.

 DIGITAL READOUT THERMOMETERS.

With technology comes change. One of the biggest changes I have seen over the years is the advancement from analog readouts to that of digital ones. The digital thermometer has some nice advantages over the analog. First, you can not make a mistake reading it. It simply says it is 72.6. There is no parallax (encountered with the LIG instruments if you don’t read them "head on’) and since it is reading from a probe attached to a cable run into your home, there is no need to walk out to the instrument shelter in all types of inclement weather to retrieve a reading.

A problem I have encountered with the electronic instruments concerns determining their accuracy. Since these systems are housed in a radiation shield it is impossible to check the probe for accuracy without removing the probe from their shields, which are usually the "beehive" type. They make it a bit difficult to remove the probe, but once it is out it can be tested in the aforementioned ice bath. Most systems, at least the National Weather Service’s Maximum-Minimum Temperature System has NO calibration device to correct the problem if one is observed. The MMTS system supplied to me is one full degree off at 32°F, a most important temperature. It reads too low. Don’t worry, I make the corrections. Even though the new NWS tolerance for accuracy has been lowered to within two degrees. This is too liberal for this observer. Can’t calibrate it either.

Another issue to be addressed here is not only the accuracy at 32°F but whether the accuracy is a linear across all potential temperatures. By this I mean that the instrument can be accurate at 32° but may not be at lower or higher temperatures. At one time I owned a digital thermometer that was three degrees too high at 90 degrees. At the lower end the instrument showed increasingly too low temperatures, and was three degrees too low at 0°F. All of this, yet it was right on the mark at 32°F!

One physical problem with this system occurs when the digital display is not fully operating. This creates a problem because your eight can look like a nine; your seven becomes a one and so on. To catch this, use common sense and be in tune with the actual air temperature. The MMTS gets its power source from AC. Some other manufacturers get their supply from batteries. Unless you have a battery backup on your AC digital system, you can lose data if there is a power black out. That can never happen with analog liquid-in-glass thermometers.

Fortunately, there are several excellent companies out there whom provide quality digital thermometers whose accuracy appears not to waver over time. Please be careful before you purchase an electronic thermometer. Ask many questions and check out their track record.

As we come up with new ideas, new solutions and more sophisticated ways of measuring temperatures, without a doubt I am sure that the quality of this instrumentation will improve to even greater levels than we have seen over the last decade.

Nicholas Stefano

890 Greenville Road

Wantage, New Jersey 07461

973-875-9103