Emissivity & it's Measurements

EMISSIVITY

The relative ability of a material's surface to emit heat by radiation is measured by its emissivity (ε). All objects at temperatures above absolute zero emit thermal radiation. However, for any particular wavelength and temperature. The ratio of energy radiated from an object's surface to the energy radiated from a blackbody at the same temperature and wavelength is known as emissivity.

 

EMISSIVITY (ε) = Radiant energy of an object surface at a given temperature

                       Radiant energy of a blackbody at the same temperature

 

The values of emissivity can range from 0 to 1 for perfect radiator (Blackbody) value is 1 while for perfect reflector (Whitebody) value is 0. Because they release a portion of their maximum possible blackbody radiation at a given temperature, most natural objects are classified as "graybodies."

Emissivity of Materials

The emissivity of a particular material depends on its specific chemical makeup and surface characteristics. Smooth, shiny surfaces, for example, tend to have higher reflectivity and thus, low emissivity. Materials including cloth, rubber, plastics, ceramics, water, and organic materials have high emissivity. The emissivity factor materials broadly categorized as metal and non- metal and its emissivity standard values illustrated

Table 1: Metal & Non- Metal Materials Emissivity

 

Metal	Emissivity	Non- Metal	Emissivity
Bare aluminum	0.02-0.4	Concrete (rough)	0.93-0.96
Gold	0.02-0.37	Glass	0.76-0.94
Copper	0.02-0.74	Wood	0.8-0.95
Lead	0.06-0.63	Carbon	0.96
Brass	0.03-0.61	Human skin	0.98
Nickel	0.05-0.46	Paper	0.7-0.95
Steel	0.07-0.85	Plastic	0.8-0.95
Tin	0.04-0.08	Rubber	0.86-0.94
Silver	0.01-0.07	Water	0.67-0.96
Zinc	0.02-0.28	Sand	0.76-0.9

Factors Affected the Emissivity of a Material

There are several factors which affected the emissivity of a material. We need to be aware of their effect on emissivity values

• Effect of Materials or Surface Condition

In the case of metallic materials, emissivity will decrease with polishing and increase with surface roughness. Metals which have been subject to a high temperature industrial process normally have a heavy oxide layer and have a high and stable emissivity values.

• Effect of Wavelength

Emissivity will usually vary with wavelength - for example, the emissivity of polished metals tends to decrease as wavelength becomes longer and the emissivity of roughness of metal tends to increases. Material properties usually depend on wavelength. Figure 1 the emissivity of metals usually decreases non-metals can show increases as well

Figure 1: Wavelength dependence of normal emissivity for different materials

• Effect  of Temperature

Emissivity will generally change with temperature if the surface properties of the material change, for example if coatings become tarnished or degraded, or for metals such as aluminium. Plancks law state that, the total energy radiated increases with temperature while the peak of the emission spectrum shifts to shorter wavelengths. The energy emitted at short  wavelengths increases more rapidly with temperature. Figure 2 illustrates some examples effect of Temperature on emissivity. Some materials show fairly strong variations, so it may be necessary for practical purposes to know whether the temperature of a process under IR observation will remain within a certain temperature interval such that the emissivity for this study can be considered constant 

Figure 2: Temperature dependence of emissivity for different materials

Emissivity Importance for Temperature Measurement

AST Pyrometer measure surface temperature of object by sensing the infrared radiation emitted by the object. The amount of radiated energy being radiated depends on the emissivity of object surface.

For an example many organic surface materials fall close to 0.95 on the emissivity scale. For this reason, pyrometer is pre-set at an emissivity of 0.95. However, object with much lower emissivity ratings, such as polished metal, will give false temperature readings. This will happen if the emissivity setting is not adjusted before the temperature reading taken

Methods to Find the Right Emissivity Setting

There are a few ways to find the right setting of emissivity to know the correct value of emissivity.

• Method 1: Emissivity Standard Tables

In most cases we often find the emissivity is different on the same material in different applications to know the sensible emissivity value first method suggested you to refer the standard emissivity table it will give you a rough estimation.

• Method 2: Compare Measurements with a Contact Probe

This method contains comparing the measurements from the AST Pyrometer with a Thermocouple or any another type of contact probe which provides accurate reading.Then measure the target material surface temperature with a thermocouple type contact probe. Make sure that the probemust be good thermal contact with the object, and note the thermocouple reading with the help of Tempsens Tempmet Digital Thermometer as illustrated figure 3.

Figure 3 : Method 2

In next step to remove the thermocouple and focused the pyrometer at the same point on the surface material. If you use this method, be sure the target temperature is the same in both locations Adjust the emissivity setting on the pyrometer until it measures the same temperature as the thermocouple. This is the emissivity setting to use

• Method 3: Compare Measurements with a Known Emissivity

In this method you can use masking tape, paint like matte black barbecue or coat a measurable area of the surface with a non-reflective coating. Ensure the area you paint is at least twice the size of the sensor’s measured spot size for the chosen measurement distance. Leave an uncoated area of the same size next to the painted area. Then by using an emissivity setting 0.92  as  illustrate in figure 4 measure a Figure 4

Figure 4: Method 3 setup

painted  area. Note the measure temperature reading with the help of AST pyrometer. Next step focused the AST pyrometer at the uncoated area and adjust the emissivity setting until it measures the same. This is the emissivity setting to use.

• Method 4: Test Emissivity

Power delivered to Black plate will be more  than Polish one because of Basic Black body radiation principle to maintain a specific  temperature  inside a closed enclosure. We maintain temperature for both plate is 100 ºC i.e. 373K 

Figure 5: Method 4 setup

 

Mathematical Formula:    (w1-w2)= (Eb-Εt)* ε *A *(T⁴s1-T⁴s2)

(29.5 ∗ 0.14 − 22.5 ∗ 0.11) = ( 1 − Εt) ∗ 5.6 ∗ 10−8 ∗ 0.08 ∗ 0.08 ∗ (3734 − 343.54)
1.65 = (1 − Εt) ∗ 1.95
0.84 = (1 −Εt )
Εt = 1 − 0.84 = 0.16

 

w1 Power Delivered to Black Plate (29.5 Volt and 0.14 A),

 w2 Power delivered to Polished  Aluminium  plate  (22.5Volt  and  0.11A),

 Eb Emissivity  of  Black  Body,

Εt Emissivity of Polished body, ε S-B Constant :  5.6 ∗ 10⁻⁸, A Area of the plate,

Ts1 Specific temperature i.e. 100 ºC and Ts2 Enclosure temperature i.e. 70 ºC

                                        

Features AST provides for Emissivity setting to measure accurate temperature

AST provide facility to set emissivity using software Infrasoft 1.0.2 as well as by External 4-20mA Input. For example, as illustrated in figure 6 when user feed emissivity value 0.82 in the emissivity  tab  it  measures 299 ºC temperature accurately. In  another  method  if  user  select Figure 6: Emissivity value Setting “Analog” as emissivity mode in software then emissivity can be set by external 4 - 20mA Input.