Show me how this thing works.
Show me how this thing works.
Will it make me rich? Will it make me wise?
Will it give my empty life a little bit of meaning?
Will it make the lions lay down with the lambs?
Will it cure this rush? Will it give us cash?
WIll it do all this without a serious side effect?

("Show Me How This Thing Works," by Cracker, 2009)

 

YOU ... DOING THE WORK OF A THERMOCOUPLE 

Thermocouple Calibration Tables

The linear relationship of each specific calibration of thermocouple can be represented as a table of data.

A Type J calibration table provided by Reo Temp Instruments can be accessed via this link:

Reo Temp's Type J Calibration Table

Instructions on how to read the table are below.

The columns of millivolt data that correlate to temperatures in the table are the same data that are plotted to generate the mV-to-temperature relationship for a Type J thermocouple shown in the graph to the right.

Okay, a temperature conversion had to be performed because the graph is in degrees Fahrenheit and the table is in degrees Centigrade/Celsius ... but otherwise the data used for both are the same.

Note the structure of the Type J Table:

• The temperatures listed downward in the leftmost and rightmost columns show the same temperature and are also listed in increments of  "every 10 °C"  (-210, -200, -190 ...).
• The top line shows increments of (negative) one degree Centigrade/Centigrade, reading from the left to the right, from 0 to 10 (0,-1,-2 …-10).
• All the data between the top line and side columns are millivoltage readings for a Type J thermocouple.

For example, the steps to find the millivoltage reading that represents -175 deg C is done stepwise:

• Find the -170 on the left hand side ...Aha! There it is! ...5th temperature down from the top!
• Then mosey over to the "-5" column and the output in millivolts at this junction is listed as  -7.265 millivolts.

Conclusion: a -7.265 millivolt reading infers a temperature of -175 deg C.

DIY! Do It Yourself!

Try these other examples to make certain that you understand how to read the chart:

What is the millivolt output observed at 60 °C?
You can answer this question using the above table.

What is the millivolt output observed at 523 °C?
To answer this question you must access the Reo Temp's Type J Calibration Table link.

In all the examples above, a temperature was given and the correlating millivoltage was determined.

In the real world, the millivoltage is generated as output from the heated junction of the thermocouple ... and the temperature is then correlated to this output.

 

Determine a Temperature from a mVolt Output

In the real world, a pair of Type J multi-point thermocouples could be spaced as shown in the reactor graphic shown at the right.

The two multipoint thermocouple bundles measure the temperature of the inlet, middle, and outlet of the catalyst bed that has been loaded into the reactor.

The data that appears in the Reo Temp's Type J Calibration Table is exactly the same data that would be used by an electronic device to correlate the mV output from the separate thermocouples into catalyst bed temperatures.

 

 

 

Another important tidbit of information is that the reference temperature of the thermocouples is 20 °C.

The above statement means that both wires at the reference junction are being held at 20 °C.

Otherwise stated, the reference junction temperature is being maintained at temperature that is 20 °C higher than 0 °C.

Ergo, both of the Type J reference junction wires are going to be impacted by the temperature increase from 0 to 20°C.

The voltage that the reference wires generate at their junction box must be added to the voltage output generated above the reference temperature ...

(hey! that's the voltage output generated from the measuring side junction of the thermocouple!)

Say what?

Well, instead of explaining it to Fred again how about I just show you how it works?

 

 

How  This Thermocouple Thing Works

The voltmeter at the top part of the catalyst bed indicates an output of 26.201 millivolts.

What is the temperature at the catalyst bed inlet?

 

The voltmeter at the bottom of the catalyst bed indicates an output of  20.882 millivolts.

What is the temperature of the catalyst bed outlet?

 

PTOA Readers and Students who are reading the PTOA Segments in the intended sequential order already learned in PTOA Segment #37 entitled "Endo Vs. Exo ... All Greek to Me!" how to determine if the reactions taking place in a reactor are endothermic or exothermic.

Are the reactions taking place in this reactor endothermic or exothermic  ... and why should the Control Board Operator care either way?

 

ANSWERS:

According to the Table, a 20.0 °C reference temperature will give a millivolt output of 1.019.

Determine the Catalyst Bed Inlet Temperature:

Adding the reference millivoltage to the catalyst bed inlet voltage output yields a total millivoltage output of:

1.019 + 26.201 = 27.22 mV

Access and scan Reo Temp's Type J Calibration Table.

27.22 mV correlates to 497 °C (927 °F).

Conclusion: Inlet catalyst bed temperature is 497 °C (927 °F).

 

Determine the Catalyst Bed Outlet Temperature:

Adding the reference millivoltage to the catalyst bed outlet voltage output yields a total millivoltage output of:

1.019 + 20.882 = 21.901

Scan the table once more.

21.90 mV correlates to 401 °C (754 °F)

Conclusion:  Outlet catalyst bed temperature is 401 °C (754 °F).

 

Determine if the Reaction inside the Reactor is Endothermic or Exothermic

Outlet Temp - Inlet Temp =

401 deg C - 497 deg C = -96 °C

Catalyst surface provides the surface and favorable conditions that encourage a chemical reaction to take place.

Conclusion:The reactions in this reactor are endothermic!

Why does the Process Operator care?

As the reactor keeps doing its job converting feedstock into product, coke will lay down on the surface of the catalyst and the temperature differential will decrease (meaning become less negative).

The loss of temperature differential between the top and bottom of the catalyst bed will hint that the reactor needs to be shutdown and regenerated or refilled.

 

LET'S GET REALLY REAL

The Beauty of Electronic Measurement

Of course in the real world hand-held volt meters are not used to collect millivolt readings.

The beauty of electronic temperature measurement via thermocouple is that the generated mV output signal can be amplified and converted into a standard 4-20 mA signal.

The standard signal can then be transmitted and ultimately "brought into the board" in a format that the Control Board Operator can easily understand and monitor, trend, and even set high or low temperature alarm limits on.

Any PTOA Reader or Student that does not understand the above paragraph needs to revisit THIS PTOA SEGMENT and continue reading the next ten PTOA Segments in sequential order. 

Heck, if you are that far behind take the time to catch up completely! You will be glad that you did! Remember ... nobody at the PTOA is watching you catch up! Take all the time that you need to understand each PTOA Segment.

And, of course ... in the real world ...

the exposed-junction style of thermocouple shown to the right would not last long in the reactor.

The best style of thermocouple probe for reactor inlet and outlet temperature measurement is the focus of the next PTOA Segment.

TAKE HOME MESSAGES: PTOA Readers and Students learned how to convert a mV output reading from a thermocouple into an inferred temperature measurement.

Each type of Thermocouple (J, K, etc) has a unique calibration table.

When using a calibration table, make certain that it is for the type of thermocouple that is being used.

Just like a calibration graph, the calibration table reveals the linear "temperature-versus-mV generated" relationship for the thermocouple; the table just has all the data in columnar format.

When determining a temperature from a calibration table, the mV output must be corrected for the reference temperature.

Many thanks to Reo Temp for allowing instructional use a Type J Thermocouple Calibration Table.

©2016 PTOA Segment 00108
PTOA Process Variable Temperature Focus Study Area
PTOA Process Industry Automation Focus Study Area

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