Non-contact Infraed Temperature Sensors/transmitters

[robh 12]

I am looking for a low cost, non-contact infared temerature sensor/transmitter. I have looked at some of the ones from Omega, Raytek (spendy!), and Mikron. Wondering what other people have used with good results. I will be reading the temerature on the surface of a plastic sheet that will be moved into a area between heaters. The sensor will remain in the heated area at about a distance of 8"-12" away from the plastic sheet (plastic moves as a result of being heated). I would like to remove the plastic when it gets to the optimal forming temp. Havent been told the optimal temp yet, but I know that it is well below 500 F. I am debating between 4-20 mA output or the thermocouple output. I will be using a MicroLogix 1200 as a controller. Thanks

[TimWilborne 108]

Have you looked at Honeywell. I used one of theirs about 4 years ago and had good sucess. I can't remember the part number. Check their site out and let me know if you are interested. If you are I will try to dig up the drawings. If I recall they were not that expensive.

[burnerman 4]

Try this http://www.landinst.com/infrared/index.htm

[panic mode 246]

i'm not sure about cost but i've seen infrared thermocouples like from Exergen (JR-...)

[DanW 41]

Land, Raytek and Ircon are the major players in infrared. They all have accurate stuff and have complete model lines that compete head to head with each other, generally model for model, all of which provide a conditioned industrial 4-20mA signal over some range of temperature. Exergen uses entirely different technology - and its output is characterized as a thermocouple, NOT as a linearized a 4-20mA signal. Hence Exergen plays in an entirely differnet ball park and their stuff costs a fraction of what the other stuff does. But it comes with several trade-offs. Exergen expects the receiving device (PLC AI, recorder, controller, indicator, whatever) to be capable of thermocouple input with cold junction compensation and T/C curve characterization. There is no emissivity adjustment on an Exergen, you MUST use a bias or offset value at the receiver to compensate for the target's emissivity. If I recall properly, plastic has a fairly high emissivity, but it isn't 1.00. You have to wire Exergen sensors with matching thermocouple extension wire, not copper wire. Exergen is also very subject to temperature variations of the body of the sensor itself. Heat the body 10° and the output will rise 10° even if the target temperature is constant. You'll need to shield it from drafts that could heat or cool the body. The major players use similar method for defining target size, a circular zone of a certain diameter and a certain distance from the lens. The target size varies with distance from the IR sensor, and is specified by a ration like 100:1, which would be a target size of 1" diameter at 100" from the lens. Their instruments produce a temperature reading using a value on the order of 98% or thereabouts, where the signal produced is indicative of 98% of the IR energy in the target zone, with only 2% contribution from the area outside the target zone. . However, Exergen uses a value of 50%, which I only discovered when I got really bizzarre readings under the assumption that their readings were 98% inside the target zone. The values made sense once it was made clear that the reading used only 50% of the energy inside the target and 50% outside the target. They don't advertise that little known fact.. The effect this has is critical if you have a cold zone adjacent to a hot zone and the instruments takes too much of a reading from the zone you're not aimed at. The major players do things like laser sighting but with Exergen you sight down the tube and do your best, which in many cases is adequate. I've had problems trying to connect industrial signal conditioners, like temp transmitters, to Exergen IR sensors because of impedance mismatches. Although the output is characterized as a thermocouple, it is not a plain junction thermocouple, its impedance is much higher. All that said, if the IR instrument is looking at a sheet of plastic, then you probably have a fairly sizeable area to work with, so you can probably aim it at a zone in the center, rather than along the edge, which could be a much colder or hotter region, affecting the ultimate temperature value. Regarless of which brand, all infrared is subject to error due to attenuation of the IR energy due to things like dust or oil coating the lens, or steam in the viewing path. Keeping the lens clean in industrial environments is always a consideration. I think the Exergen is suitable if you can - get an even temperature distribution across a target about twice target diameter they claim - maintain a stabilized sensor body temperature - can live wtih good repeatability, rather than absolute termperature readings for accuracy purposes. In other words if it reads xxx today, you need it to read xxx tomorrow under the same circumstancesm even if a Land or a Raytek unit tells you its yyy and it probably is at the that yyy temperature. Dan

[robh 12]

WOW great information. Thanks Dan.