Total immersion vs partial immersion = total confusion?

[David Lyga]

This discussion infers that either tube or stem analog thermometers are being used, not digital ones.

Thermometers are somewhat devious in that they tend not to fully disclose how their secrets can be really successfully harvested.

I just learned (!) after 50 years in the darkroom (!) that there are two varieties of thermometers: total immersion and partial immersion. The ‘immersion’ part refers to how much of the thermometer must be inserted into the liquid in order to achieve a healthy semblance of precision. (First, I will qualify a bit here: precision refers to the repeatability of data; i.e., even if the thermometer of ‘off’ a bit, it will always be ‘off’ by the same amount. Accuracy, on the other hand, refers to the conformance with established governmental standards and leaves no leeway for readings that do not adhere to such.)

How many are already aware of these pitfalls? Do you simply take a ‘total immersion’ thermometer and stick a bit of it into a liquid and let it go at that? One knows that liquid in a given environment is usually a couple of degrees colder than the air surrounding that liquid. (I would ascribe this temperature discrepancy to the fact that the liquid is always evaporating, but please correct me if that assessment is wrong.) Thus, what becomes the most precise reading in times when it is impractical to immerse the complete total immersion thermometer into the liquid? There are formulas to figure this out but we do not usually wish to bother with that.

One the other hand, there are ‘partial immersion’ thermometers which require only partial immersion into the liquid (usually determined by a guide line on the thermometer). However, they are not considered to be as accurate as the total immersion type.

Take, for example, the expensive Kodak Process Thermometer: is it OK to dip this only partially into a liquid? I ask this because it is a very long thermometer and usually immersing this complete thermometer into liquid is either difficult or nearly impossible to do.

Dial or stem thermometers also pose another problem: How must of that stem must be in the liquid? – David Lyga

 

[Gerald C Koch]

Mercury and alcohol thermometers usually have a line near the base and below the calibrations to indicate the immersion point. This line usually goes completely around the thermometer so it cannot be mistaken for anything else. It is important that the liquid being measured not be below OR above this line for the most accurate reading. Really don't know the rule for dial thermometers. But then they are not all that accurate to start with.

 

[AgX]

Basically I'm aware of this issue. But I have seen a lot of thermometers that got no mark at all indicating a immersion depth.

I just checked the catalog of a manufacturer of lab thermometers:

-) those with no mark are total immersion ones

-) those for partial immersion have a mark

 

[Pioneer]

I use an old dial thermometer and dunk it in without covering the head. It is repeatable in that I can pull it out of the water, wipe it off, dunk it back in and get the same reading within a degree. I'm not sure I need much more precision than that.

The only other thermometer I have any experience with is a bulb thermometer. With those the bulb is placed where you want to take the temperature. It may or may not matter whether it is immersed, as long as the bulb itself is in the location you want.

The way I understand the difference is that a bulb thermometer can be used to measure temperature variance throughout a column of liquid, or inside a baby's rectum, depending on where the bulb is placed. A dial thermometer is not so good at this but will give you pretty good idea of the average temperature along the stem of the thermometer.

Of course I could be totally wrong but the interesting part is that film developing doesn't seem to require that high a degree of accuracy...or precision.

 

[Mainecoonmaniac]

As with people thermometers there are two. Oral and rectal. The difference? The taste

 

[Sirius Glass]

I thought that this was a thread about baptism but then I read the OP and I agree with Gerald C Koch. I hope that is does not become a religious discussion about thermometers.

 

[Pioneer]

As with people thermometers there are two. Oral and rectal. The difference? The taste

But which one is total immersion??

 

[ic-racer]

Partial or full immersion, the difference is either 'none' or so small it does not register as a change on the printed scale of any of my thermometers. What are my measuring anyway? In B&W a fraction of a degree this way or that does not make much difference. If you are doing color without a processor, a fraction of a degree off will be masked by larger temperature or time errors.

 

[Sirius Glass]

As with people thermometers there are two. Oral and rectal. The difference? The taste

This is hard to swallow!