Exergen D-Series manual Background reflection errors

D-Series Handheld IR Scanners

3. User adjustment errors

A setting of emissivity = 0.9 on an IR “gun” from one manufacturer will not necessarily match that of another IR “gun” from another manufacturer. There are no standards set in the industry on the precise measurement and meaning of “emissivity”.

Also, Quality Assurance programs should not rely upon any instrument that allows users to alter the instrument settings and to let it display whatever the user wishes.

Effect of Ambient Temperature on Target Reading for 100 °F (38 °C) Target with .8 Emissivity

Target T (F)		4	16	27	38	(C)
110			Conventional Infrared			43
106
102	D-Series					38
98
94
90	20	40	60	80	100	32
0						120
			Ambient T (F)

4. Background reflection errors

Even if emissivity is constant (see #2), there are still errors induced by changing ambient temperatures. For example, with emissivity = 0.9, ambient reflections account for 10% of the signal that the IR “gun” will see. If ambient changes, the IR “gun” will display a different target temperature, even if the target remains at the same temperature.

Figure 3. D-Series remains accurate even if the ambient temperature varies, while conventional IR devices have considerable inaccuracies.

5. Contact errors

Thermocouples, RTDs, thermistors and other contact devices only measure their own temperature. They do not measure surface temperature. Published “Accuracy” specifications are for the probes only, not the surfaces they must measure. Users must guarantee that the probes are brought to the same temperature as the surface. Can you guarantee that your probes are brought to the same temperature as the targets to be measured?

6. Friction heating errors

For moving surfaces, a contact probe is prone to frictional heating. The size of the error is dependent on the rough- ness of the surface, the speed, the coating on the probe, and so on. It is impossible to control all the variables.

Time Comparison Between D-Series and Contact Thermocouple for Measuring a 500° F (260° C) Surface

500					Contact Error 260
400	D-Series					200
300						150
deg F						deg C
200				Contact Probes		90
100
0
0.001	0.01	0.1	1	10	100	1000
	Time from start of Measurement (sec)

Figure 4. D-Series measures surface temperature in a fraction of a second, while contact probes (thermocouples, RTD’s, thermistors, etc.) require many minutes to acheive equilibrium. Contact probes always have a residual error due to imperfect heat transfer from the surface to probe.

7. Heat sinking errors

For most non-metals, heat sinking errors can be quite large. The heat transfer rate of the metal leads required on “contact probes” conducts heat faster than the target material can replace, resulting in unknown and fairly large errors. In general, the less dense the target material, the larger the heat sinking error with a contact probe.

8. Time based errors

Contact temperature probes are slow. The temperature of a target can change more quickly than most probes, resulting in errors in real time measurement.

51 Water Street, Watertown, MA 02172 USA

800-422-3006 • 617-923-9900 • Fax 617-923-9911