Temperature Calibration

Temperature Calibration

Temperature Calibration equipment is quite varied to match the very varied types of methods for temperature measurement. Temperature is the most measured physical property so it is logical to invest in a calibration equipment.

Here is a general discussion on Temperature Calibration that also discusses ther technology behind different temperature probes (thermocouple RTD, and thermistor). Temperature Measurement and Calibration Application Note-What every instrument technician should know
 

Temperature Calibrators

Temperature Calibrators are used to troubleshoot and calibrate temperature probes and the instruments that are connected to the probes such as chart recorders, temperature controllers, and PLC's.

Source Only vs. Read/Source Calibrators

Source
The minimum function of a calibrator is to source a known thermocouple and/or RTD. That permits performing zero/span adjustments. In source mode testing of conditions that otherwise would be difficult or unsafe is possible. Imagine testing a reactor high temperature alarm. With a temperature calibrator it is safe to simulate a 90% full condition to test a high alarm warning and 95% to test the high high (HH) alarm. Another example is simulating a zero and span to confirm that a remote temperature controller is reading correctly.

Read
For troubleshooting problems, reading the signal coming from the sensor is necessary.

How many RTD and Thermocouple types is enough in a Calibrator?

There are many different thermocouple and RTD types of temperature probes. Does that mean your new calibrator needs to be able to handle all of them? Maybe, if you are a contractor you might come across several. Facilities with wide temperature ranges and applications might need the capability too.
 

Most Popular Temperature Probe Types

K T/C

J T/C

T T/C

PT-100 (0.00385 coefficient) RTD

PT-1000 (0.00385 coefficient) RTD

Thermocouple and RTD Basics

Thermocouples (T/C’s) generate a linear millivolt (mV) signal with temperature. They are two dissimilar wires soldered at the tip. RTD’s use a resistance principle. They are a thin coiled wire, usually platinum, that varies in resistance (ohms) with temperature. Thermocouples have a wider operating temperature range and better resistance to vibration and shock than RTD’s, but RTD’s are much more accurate and repeatable. RTD’s are 4-10 times more expensive. Type K, J, T, N, and E are known as base metal thermocouples because they are made from common metals and R, S, and B are made from Noble metals.
 

Thermocouple Types and Applications
Type	Wire Types	Selection Advice
K	Chromel/Alumel	Wide range. Inexpensive. Popular. Better at above 1000 °F (538 °C)
J	Iron/Constantan	Wide range. Inexpensive. Popular. Keep away from atmospheres that oxidize the iron. Shorter life above 1000 °F (538 °C)
T	Copper/Constantan	Typically best accuracy, especially at ambient and below freezing. But lowest max temp. Popular in Pharmaceutical industry.
N	Nicrosil/Nisil	For applications where Type K have shorter life. Better accuracy than K or J but less popular
E	Chromel/Constantan	Highest mV output. Medium temp range
R	Platinum/Platinum-13% Rhodium	High temp industrial applications above 1000 °F (538 °C)
S	Platinum/Platinum-10% Rhodium	High temp laboratory applications above 1000 °F (538 °C)
B	Platinum/30% Rhodium	Same as type R and S but higher max temp and lower mV output

Notice in the above thermocouple table, absent is the temperature range and accuracy. They vary so much by manufacturer that it is best to look at the specifications when selecting a temperature probe.
 

Infrared Calibrators / Blackbodies

Infrared Calibrators, also called Blackbodies, are reference standards used to calibrate Infrared Thermometers and Thermal Imaging Cameras . Even those IR Thermometers that cannot be adjusted benefit from testing to verify the consistency and validity of results. Virtually any instrument with a spot size diameter less than the cavity size can be calibrated. It is important not to have the IR thermometer too close to the target. This will cause the IR thermometer’s optics to heat excessively, which will cause false readings. It is also important to be not too far away. This will cause the target to not fill the IR thermometer’s spot size and will cause a false reading.

Advice for Selecting an Infrared Calibrator

• Make sure to know the desired temperature range
• Select a model with a target area larger than the spot size diameter of the instrument to be checked
• Decide between surface type or cavity type infrared calibrator

How does an Infrared Calibrator work?

The units use a plate that is heated. The plate is painted with a black paint with emissivity of 0.95. The temperature is controlled by a digital controller. The controller uses a precision platinum RTD as a sensor and controls the surface temperature. Models that achieve temperatures below ambient use a Peltier Thermoelectric Cooling system.

The IR calibrator is calibrated with an emissivity setting of 0.95. The IR calibrator has a variable emissivity adjustment that allows the user to vary their apparent emissivity from 0.90 to 1.00. This setting should match the IR thermometer's emissivity setting. It is best to use the emissivity setting of 0.95. However, some IR thermometers do not allow for an emissivity setting of 0.95. For these instruments, the calibrator's emissivity setting should be set to the IR thermometer's emissivity setting.
                                                                 
Every object with a temperature above absolute zero (0 Kelvin) radiates energy over a wide spectral band. For example, if a significant part of this energy is within the band of 400–700 nm, we can see that energy. This is the visible light band. This is the case with an electric stove burner at a temperature of 800°C. The burner will appear red or orange to the eye (red hot). That burner is also emitting energy at other wavelengths, which we cannot see. This includes wavelengths in the infrared portion of the electromagnetic spectrum.

An example of an object emitting energy at wavelengths we can see is the sun. By the same respect, if we are measuring an object at room temperature, (23°C), the peak wavelength is 9.8μm. The temperature corresponding to a peak wavelength at 8 μm is 192°F (89°C) and the temperature corresponding to a peak wavelength at 14 μm is −86°F (-66°C). This is one of the reasons the 8 – 14 μm is widely used in handheld IR thermometers.
IR thermometers take advantage of this peak wavelength phenomenon. They measure the amount of energy radiating from an object and calculate temperature based on this measured energy. In most handheld IR thermometers, the sensor and optical system measure IR energy in the 8-14μm band.

Emissivity is defined as the ratio of the energy emitted at a temperature to the energy emitted by a perfect blackbody at that same temperature. A perfect blackbody would have an emissivity of 1.0. However, in the real world there is no such thing as a perfect blackbody.

For example, if a perfect blackbody emits 10000 W/m2 at a given temperature and a material emits 5000 W/m2 at that same temperature, then the emissivity of that material is 0.5 or 50%. If another material emits 9500 W/m2 at that same temperature, it has an emissivity of 0.95.

It is important to note that for any opaque material, the ratio of energy reflected plus the ratio of energy transmitted is equal to 1.0 (this is known as Kirchhoff’s Law). Therefore, if a material’s emissivity is 0.95, the material reflects 5% of the energy radiated by objects facing it. By contrast, if an object has an emissivity of 0.50, the material reflects 50% of the energy radiated by objects facing it. This means this reflected energy can contribute to measurement accuracy. This is especially true when measuring materials with lower emissivity, and objects at lower temperatures.
A lack of knowledge of emissivity itself can contribute greatly to inaccuracy in IR temperature measurement. For an example, say we are measuring an object at 500°C. We assume it has an emissivity of 0.95. However, its emissivity is really 0.93. This would cause our 8-14 μm IR thermometer to read the temperature 6.7 degrees low, a – 6.7°C error in temperature measurement.

Emissivity, blackbodies and graybodies

Most people associate a blackbody calibration source with calibrating infrared thermometers. Although the word blackbody specifically refers to an ideal surface that emits and absorbs electromagnetic radiation with the maximum amount of power possible at a given temperature, many calibrators with non-ideal surfaces are also referred to as "blackbody calibrators." While an ideal surface would have an emissivity equal to 1.00, many of these "blackbody calibrators" have an emissivity of approximately 0.95 (better described as a “graybody”). A true blackbody calibration source would usually be a long cavity with a narrow opening. Unfortunately, the opening is usually too narrow to be useful for calibrating common infrared thermometers, which require a large target size for an accurate calibration. The advantage of a true blackbody calibration source is that the emissivity is precisely known. Whereas traditional flat plate calibrators have emissivities with uncertainties too large for meaningful calibrations of most thermometers.

Temperature Calibration Metrology Wells

Temperature Calibration Metrology Wells are used to compare a known set temperature with the measurement from the thermocouple, RTD, and liquid filled thermometer under test. The checking of temperature is vital in numerous processes. Temperature is the most widely measured variable, so having a temperature dry well or liquid bath for calibration work is easy to justify.

Considerations when selecting a Temperature Dry Well or Liquid Bath

• Lab or portable.
• Because temperature is measured from cryogenic to extremely high temperatures, they are made to cover these wide ranges. Selection starts with understanding the required temperature range.
• Desired accuracy. Rule of thumb for process instrumentation is to select calibration equipment with an accuracy of 4x better than the instrument being calibrated. Baths come supplied with a digital temperature controller. Look at its accuracy specification. If insufficient, you can purchase an external temperature probe and display with better accuracy and use it as the master meter. It will also be easier to send out for annual re-calibration, than a complete system.
• Watch the stability specification. The benefit of having a high accuracy temperature probe is lost if the dry well or liquid bath temperature is constantly fluctuating.
• While it is important to account for future needs, do not overly exaggerate the range. As the ranges widen, more complicated solutions are required. The following table compares liquid and dry block and various liquid bath mediums.

Technology Typical Temp. Range	Benefits and Drawbacks
Liquid Medium: Water 4-95°C (40-200°F)	Lowest initial cost Wide common temperature range Water is easy to cleanup, non-hazardous, practically no cost Accommodates odd size temperature probes Takes several hours to warm-up or cool-down and stabilize* Water baths evaporate a lot of water as temperature gets closer to boiling point, even with a lid or use of polypropylene spheres (i.e. ball blanket). Be prepared for refilling. Moisture from evaporation may corrode electronics over time
Liquid Medium: Alcohol -40°C/°F to Ambient	Water Bath may be possible to use with denatured ethanol (alcohol)  (check with manufacturer or TEquipment) Evaporation and volume of bath alcohol is concern not only for refilling but flammability Easy to cleanup Takes several hours to cool-down and stabilize
Liquid Medium: Glycol or Oil 0-300°C (32-400°F)	Higher cost than Water Bath and over time the most expensive because of continued replacement of bath medium as it oxidizes from the heat Typical bath mediums: glycol/water mix, mineral oil, Dow Silicone Oil, or polyalphaolefin (PAO) Glycol/Water mix typical range: 0-95°C (32-200°F). Oils intended for temperatures above water boiling Oil fumes must be well ventilated using laboratory hood Accommodates odd size temperature probes Takes several hours to warm up or cool-down and stabilize*
Dry Block -25 to 1200°C (-32 to 650°F)	Bench models have higher initial cost than liquid baths Portable versions possible No evaporation or fuming concerns. Hassle-free Requires inserts for known probe diameters and lengths, some supplied with unit, additional ones purchased separately Fastest warm-up and stabilization time at 20-60 minutes Blocks made of aluminum approximately until its melting point of 660°C (1200°F) . Above that temperature stainless steel is typically used.

Getting sub-ambient temperatures with temperature baths
To lower the bath temperature below ambient several different solutions are possible.
 

Thermoelectric cooling is based on a Peltier principle. It is purely electronic with no moving parts, except a cooling fan. Great benefit over refrigeration compressors, but thermoelectric cooling is very limited in cooling capacity (i.e. BTU’s of heat removal). Because of that limitation, it will only work in very small baths. The technology can be adapted to dry block or small volume liquid baths.

Dip Cooler (“Coldfinger”) is an external refrigerated chiller with a pump that circulates in a closed loop a cooling medium of denatured alcohol or glycol/water mix through a cooling coil that is immersed in the temperature calibration bath. A heat exchanger in the cooler has one side with the cooling medium and another with the refrigerant. Available from Techne.

Flow thru Cooler is a refrigerated chiller without a circulation pump. The a cooling medium of denatured alcohol or glycol/water mix is circulated in a closed loop by a pump in the temperature bath instead. Analogous to a drinking water fountain using city water pressure and no internal pump. A heat exchanger in the cooler has one side with the cooling medium and another with the refrigerant Available from Techne.

Built in refrigeration compressor(s). Having the compressors built into the bath offer the convenience of one instrument and avoid need for circulating cold fluid. Also colder temperatures are possible. A negative is that the combined instrument is larger and heavier, so more expensive to ship for repairs.

When purchasing a Cooler or bath with built in refrigeration compressors, make sure to select a model with the appropriate local voltage (110 VAC or 220 VAC). Refrigeration compressors are only available in one or the other voltage. If the wrong one is purchased, getting a transformer to step up or step down the voltage is expensive because of the amperage that compressors draw.