Good Calorimetry articles...

Calorimetry is an important subject for replicators. I propose that people give reference and comments on some valuable articles or books...

I just found an article from LANL about calorimetry typically for RTG and fission, describing the various methods.

http://www.lanl.gov/orgs/n/n1/panda/10. Calorimetry.pdf
I see mostly heat flow calorimeters, passive or servo-mode (à la McKubre), air bath, water bath, big-size, analysing various problems...
Many points are linked to nuclear technology, but there is probably much to learn.

Someone very competent have cited those documents about the problem of thermocouple in Hot Hydrogen

http://www.temperature-house.c…/the-green-rot-phenomenon

Quote from MICC ltd

The Green Rot Phenomenon 24/09/2015

MICC Ltd recommendations on green rot phenomenon exhibited in Type K thermocouples used in heat treatment applications with hydrogen atmosphere and operating at high temperatures exceeding 900 °C

Issue

Thermocouples recovered from a customer site were exhibiting large negative drift, open circuit and upon inspection had become very brittle, in places broken and exhibited so-called “green rot”.
The application was an annealing furnace operating at 1050 °C with a hydrogen atmosphere.

Green rot corrosion

Green rot is a type of corrosion that occurs on the positive leg of a type K thermocouple (and type E) if two conditions are met:-

• application temperature of approximately 800 - 1100 °C and
• the thermocouple is exposed to the low concentration of oxygen environment (for example, steam, reducing or cyclically reducing and oxidising atmospheres, or stagnant atmosphere within a protection tube)

The name “green rot” is derived from the greenish shimmering colour forming on the positive leg.

The cause of it is the selective oxidation of chromium, and related to it, chromium depletion in the surrounding area, which occurs in the NiCr alloy of the positive leg of the thermocouple. The negative leg material is not affected.

The consequence of this change in chemical composition of the positive leg of the thermocouple is a drift of the measured value caused by decreasing thermoelectric voltage. This effect is accelerated if there is a shortage of oxygen (reducing atmosphere), since a complete oxide layer, which would protect it from further oxidation of the chromium, cannot be formed on the surface of the thermocouple.

The problem sets in slowly as the wire degrades. If it is not discovered in time, measuring errors of tens of degrees can occur. In extreme cases, assuming the wire does not break, negative measurement errors of 50°C or more may be found. The thermocouple calibration is permanently disrupted by this phenomenon.

In addition to the negative measurement errors the thermocouple wire will exhibit brittleness and eventual failure.

Recommendations

In order to provide a solution for the customer the following solutions were recommended (set out in order of cost, technical solution and ease of implementation)

• keeping of the dewpoint in hydrogen or cracked ammonia atmospheres below -42℃
• changing to the use of type N for the thermocouple pair. The type N (NiCrSi-NiSi) thermocouple has an advantage in this regard due to its Silicon content. A more robust protective oxide layer forms on its surface under the same conditions due to the silicon..
• the use of Type K thermocouples which incorporate sacrificial titanium wires to delay the reaction
• introduction of additional oxygen into the Type K protection tube through the positive overpressure from a suitable air supply
• introduction of an inert atmosphere into the Type K protection tube through the positive overpressure from a suitable inert gas supply
• the use of Type S thermocouple but it will require a ceramic sheath

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http://www.gayesco.com/pdf_files/GrainGrowth.pdf

Quote from Howard Hilborn

GRAIN GROWTH IN TYPE K THERMOCOUPLES
By Howard Hilborn

Grain growth in metals is a well-documented phenomenon; however, its cause, effect and cure with respect to type K
thermocouples seems little-known to most thermocouple users.

In all cases, grain growth is time and temperature dependent; most often, in fact, time at elevated temperatures is such an
overriding factor that little else can be changed to generate a positive result.

However, some steps can be taken to solve the problem for temperatures normally seen by type K thermocouples.
Exposure to temperatures above 1200°F for extended periods of time will cause extensive grain growth in a type K
thermocouple when conventional sheath material is used. We have seen cases where the cross-section of the Alumel
wire was covered by two to three grains. When this occurs, the material can easily break at the grain boundary, resulting
in thermocouple failure.

Test results indicate that failure rate is difficult to accurately predict. In one test case, for example, 10% of the samples
failed in three months, 60% in six months, and 72% in 11 months of exposure to temperatures in the 2000°F to 2100°F
range. All samples studied were ¼” O.D., 310 S/S sheathed type K single-element thermocouples, all were tested under
identical conditions, and all exhibited extensive grain growth (two to five grains in cross-section). While those that failed
did so at different times, some never failed at all, although they exhibited the same conditions as those that did fail.
Though there has been some speculation as to why certain thermocouples did not fail (when for all practical purposes they
should have), no definitive conclusions were reached; suffice it to say, they simply exceeded themselves. All of the
failures took place in the Alumel wire, as it is more subject to grain growth than the Chromel wire.

The coefficient of expansion differential between the Alumel wire and the sheath is the biggest controllable contributor
to thermocouple failure from grain growth. In fact, if the coefficient of expansion of the sheath is identical to that of the
thermocouple wire, grain growth is either drastically reduced or even completely eliminated. 310 S/S is commonly used
as a thermocouple sheath at elevated temperatures, and it withstands the environment well; however, the expansion
differential is extreme. Inconel more closely approximates the coefficient of expansion of Alumel and works well at
high temperature as long as there is no sulphur in the process, as sulphur bearing atmospheres cause rapid failure of
Inconel due to a phenomenon known as “green rot”.

In the past, the best choice of sheath material for minimizing grain growth was Inconel; however, recent developments in
sheath materials have given us new options. New duplex alloys will soon be commercially available that very closely
match the coefficient of expansion of Alumel and give superior resistance to temperatures as high as 2400°F.
Preliminary tests of type K thermocouples with this sheath material have shown an increased resistance (by a factor of
eight to 10 times) to grain growth of the Alumel wire.

A typical application in which grain growth causes failures is in fired-process furnaces where, while the temperature being
measured is the tubeskin temperature, the thermocouple is often exposed to process gas temperatures that far exceed those
tubeskin temperatures. In such situations, proper routing can play a major role in extending thermocouple life. By
keeping the thermocouple sheath in contact with the furnace tubes, the tubes are used as heat sink, thereby keeping the
thermocouples much cooler than the surrounding gas temperatures. By using this method of thermocouple routing, life in
excess of six years has been achieved in situations that would otherwise have seen failure in less than a year, in the
majority of thermocouples.

The ultimate solution at this time is to convert to type N thermocouples and use the new duplex alloy sheath material
when it becomes available. This combination has shown the ability not only to completely eliminate grain growth, but
also has the interesting side effect of enhancing electromotive force (EMF) stability… but that’s a subject best saved for
another paper.

The words Chromel-Alumel are registered trademarks of the Hoskins Manufacturing Company

Inconel is a registered trademark of Inco Alloys International

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