Articles by Exergen President Francesco Pompei Ph.D.
- Through the Looking Glass: Alices Quest for Emissivity
- A case for the unpowered IR thermocouple: I&CS, Special Report on Temperature Control.
- A Certified Emissivity Blackbody for Calibrating Infrared Thermometers: Sensors On-line.
- Aspects of increased production speeds via IR control of product internal temperature: Electricity + Control.
- Inside & Out: Increasing Production Speeds via IR Control of Product Internal Temperature: Intech/ISA.
- Hunting Mighty Milliwatts: 7×24 Exchange.
Links to Helpful Sites
INTRODUCTION TO INFRARED THERMOMETERS by Omega Engineering
What is an IRt/c?
The IRt/c is a patented Infrared Thermocouple. It is a self powered sensor that emulates a thermocouple within specified temperature ranges and provides a predictable and repeatable signal outside that specified range. With programmable input devices, the mV signal output tables are available to download. The infrared technology allows you to measure product temperature without touching the product. It also allows for a very fast reading (~ 100 msec response time).
Will the IRt/c be wired like a thermocouple?
Yes, the red wire is negative just like a t/c. But, you must pay attention to leakage current issues. As the IRt/c has higher impedance than your conventional t/c. (3 – 16 kohms compared to <100 ohms), it is recommended to use input devices with leakage current < 10 nA. If that is not possible, then the input device, should be one that allows enough offset to compensate for leakage current. As a general rule, the smaller the offset the better.
What is the standard cable length of an IRt/c?
3 feet of thermocouple wire (J, K, etc.) is the standard length. Other cable lengths are available for an additional charge, with Minimum Order Quantity requirements.
How do I select which IRt/c is suitable for my application?
What you need to know are the following:
Distance between target and sensor
Target temperature range
Ambient temperature range (where sensor is located)
|AMBIENT TEMPERATURE RANGE||RECOMMENDED MODELS|
|32 - 160F (0 - 70C)||IRt/c.01, IRt/c.03|
|32 - 185F (0 - 85C)||IRt/c, IRt/c.1X, IRt/c.SV|
|32 - 240F (0-115C)||IRt/c.3X, IRt/c.3SV|
|32 - 400F (0 - 204C)||IRt/c.5, IRt/c.10|
|32 - 700F (0 - 371C)||IRt/c, IRt/c.1X, IRt/c.3X with CJK-1 air cooling jacket|
|32 - 1000F (0 - 538C)||IRt/c, IRt/c.1X, IRt/c.3X with CJK-1 water cooling jacket|
Since the IRt/c is designed to emulate a conventional t/c over a specific temperature range, the target temperature range must be selected from the selection table below:
So the complete part is: IRt/c – MODEL DESIGNATION (*) – THERMOCOUPLE TYPE (**, J, K, etc) – TEMPERATURE RANGE
HOW DO I CALIBRATE MY IRt/c?
There are two things that need to be calibrated for with the IRt/c.
- LEAKAGE CURRENT – The IRt/c has higher impedance than the conventional t/c. And every input device has a certain amount of leakage current. The recommended leakage current for an input device is < 10 nA. If it is higher, than an offset will be needed. To calibrate for leakage current, blind the sensor (cover the sensing eye with foil). Record the temperature. Then short out the input (wire the + and – together) to the input device. This will give you the cold junction input temperature. The difference in temperature is your offset. Adjust in the input device by using the OFFSET, CAL LOW, or ZERO function.
- EMISSIVITY – A target's ability to emit energy compared to a blackbody is called emissivity. The IRt/c is calibrated on targets with an emissivity of 0.9, and in ambient blocks that simulate real world conditions. Every target will have a different emissivity, so a calibration must take place to compensate for emissivity. First thing to do is measure the target's true temperature with a reliable reference (Exergen's D or DX Series thermometer on a non-metal or coated metal target). Record that temperature. At the same time aim the IRt/c at the target. Adjust the input device by using the GAIN, CAL HIGH, or SPAN in the input device to match the reference.
This is what's called a 2 POINT CALIBRATION. If your input device only has the capability to do just the OFFSET, you should do a ONE POINT CALIBRATION at the target's control temperature.
What makes the IRt/c better than other Infrared Temperature Sensors?
The main features of the patented IRt/c are it's:
Self powered – The World's only self powered infrared temperature sensor
Instrinsically safe – With its self powered feature, there is no energy stored, which makes it intrinsically safe
Calibrated to real world conditions for real world performance accuracy (see Tech Notes 89 and 90) – Other infrared sensors are calibrated to blackbody sources, with no compensation for ambient temperature errors, the IRt/c compensates for changes in ambient conditions
Can I measure a spot that is 0.1″ with a 3:1 sensor at 0.3″?
No, each IRt/c has a minimum spot size, the minimum spot size of each sensor is listed below. You can not measure a target smaller than the minimum spot size.
|MODEL||MINIMUM SPOT SIZE||UP TO DISTANCE (inches)|
|IRt/c.2/15ACF||0.11 x 0.35"||1.7"|
|IRt/c.2/15ALF||0.12 x 0.28"||4.1"|
What happens when my target is smaller than my spot size?
The IRt/c will measure the average amount of energy within that spot, including the background. So if the background is a lower temperature, the sensor will read lower. If the background and the spot size is constant, then the error can be calibrated out for, using the same calibration method as used for emissivity.
How do I measure a low emissivity target with an IRt/c?
Ideally, you want to condition the surface to be measured (paint, coat, oxidize, lubricate, etc). That will increase the surface's emissivity and give the sensor more energy to measure. If that can not be done, there are adjustable LoE models that can be used at higher temperatures (starting at about 300F). If it is a low temperature, low emissivity target, a precalibrated model with an input device that can adjust for a high gain can be used, and a reflective shield between the sensor and target helps.