Thread: A Word of Caution !
01-18-2000, 11:20 AM #1TIManFirehouse.com Guest
A Word of Caution !
I have had a couple of issues resurface lately that can be directly related to firefighter safety so I thought I should pass them along :
I received an email from a user who wanted to know why their batteries were only lasting for 20-30 minutes versus 90 minutes. I explained there could be a number of reasons why a battery will not provide a charge for its advertised period of time. These can apply to any manufacturer’s batteries.
Short Charge Due to “Memory” Development – Batteries such as NiCAD (Nickel Cadmium) can develop a “memory”. When they are not discharged all the way down, they will start to hold a charge only to the level or amount that they were discharged to. This usually takes a number of discharge / charge cycles to occur, but we have found some cases where this has occurred with as few as 10-15 cycles. It is important if you use NiCAD batteries that you discharge them all the way before recharging them or that you use a ‘battery conditioner” which will discharge the battery automatically before recharging. NiMH (Nickel Metal Hydride) and Lithium Ion batteries do not have the memory problems that NiCADs batteries do.
Exposure to Temperature Extremes – Exposing a battery to a temperature extreme, hot or cold, can effect the chemistry of the battery causing it to loose its charge. In some cases the time of operation will be decreased in others the charge will be lost completely. The exact temperature depends on the battery chemistry and the insulating properties of the battery shell. Thermal Imagers that have internal batteries will also receive some insulation from the casing around the battery. It is important that you conduct some field trials to determine how well your batteries handle temperature extremes both off of the unit (in someone’s pocket) and attached to the thermal imager while it is operating. In some cases the manufacturer can provide you with a time versus temperature chart to show you at what points your operating times will become adversely affected. It is winter out there (in some places) so remember to keep your batteries warm since temperatures below 35 F will rapidly deplete your operating times.
I received an email which asked why Bullard was not offering an on screen temperature display. I explained that there was to much of a potential for error so we felt it was better to leave it off rather than have it on and providing bad information. The writer replied they had received literature with a unit they were evaluating and the sales rep had confirmed that the temperature display was accurate to within +/- 1 F. I explained that there was a possibility of that being correct, but only under a very specific & controlled set of circumstances. Under the conditions faced by firefighters there was no way that claim was true. There are at least 6 factors or variables that can affect the accuracy of a non-contact temperature measurement. These include : emissivity, atmospheric attenuation or augmentation, surface anomalies, FOV or area measured, background energy, and optical transmission. There is little or nothing that firefighters can or would do to control these, so the resulting measurements can be off from a few degrees to few hundred degrees. Example of an error due to incorrect emissivity ; a shiny metal surface is measured with the correct emissivity value of .05 and displays an accurate measurement of 133 F, however when measured with an emissivity value of .95 (the value preset in most thermal imagers) it returns a reading of 83 F. This is a reading that is 50 degrees below what it should be, and could be a very dangerous error to someone making a decision in a Haz Mat scenario.
PLEASE, if you have a unit with a temperature display, evaluate how it is being used. Make sure the operators understand this potential for error so they do not end up making a potentially life threatening decision based on bad information.
I invite any comments from Mr. Little or Mr. Clynne since both of their units have these onscreen temperature displays.
Hope these comments will keep someone from ending up in the middle of bad situation with a dead battery, or making a bad decision based on bad information.
If anyone knows of any other potential problems or safety issues please post them here so that our brothers and sisters do not have to learn the hard way.
Good Luck, Be Safe,
[This message has been edited by TIMan (edited January 18, 2000).]
01-18-2000, 04:39 PM #2S. CookFirehouse.com Guest
RE: Battery Life
We were have TIC battery life problems, some lasting around 15 minutes.
We solved the problems with an ACT charger from Advanced Charging Tech. http://www.actcharge.com/ . We got 2 single chargers with the discharge cycle option and our problems have gone away. We were able to recover (for lack of a better term) the short batteries.
01-24-2000, 01:14 AM #3FireOpticFirehouse.com Guest
TIMan is dead on in his references to battery issues and temperature measurement systems.
With regard to battery life, Nickel metal Hydride batteries do not "like" to be charged below 32F, or above 60F. Many charging circuits will alter the charging current during these conditions to minimize the bad effects which can result, such as reduced life and low total charge. Also, most batteries are actually made of several "cells" connected in series. If one of these cells fails (due to shorting or overheating as an example) the battery will not be able to hold a complete charge, but this will manifest itself as a low starting voltage or shorter battery life.
With regard to temperature measurement, do not believe any salesman or representative who tells you his TIC has the ability to accurately measure temperature 100% of the time. TIMan is completely accurate in his assessment of the limitations of temperature measurement systems. There are currently two types of systems, pyrometers and through the lens radiometric measurement systems.
Pyrometers will average the energy contained within a conical "beam" which is located roughly at the center of the image (ISG Talisman, ISI Vision 3). Through the lens measurement systems convert the energy being measured by a given "pixel" to a pre-calibrated temperature value (ICC FireOptic). Both systems are dramatically affected by all of the factors TIMan mentioned, but the biggest is probably the emissivity of the object being measured. Emissivity is a factor, ranging from zero to one, and is an indicator of how much radiant thermal energy an object gives off compared to an ideal "blackbody" at the same temperature. Most objects around us, such as painted surfaces, bricks, plastics, wood, skin (even snow) have an emissivity somewhere between 0.85 and 0.95. Other objects, such as glass, chrome plating and many shiny surfaces will have much lower emissivities. In the real world, dark cars have a higher emissivity than lighter ones, that's why they will get hotter during the summer. Most TICs have their emissivities set to a high number, ususally between 0.85 and 0.95. With this wide potential variation in emissivity, any non-contact temperature measurement system (including the inexpensive handheld "laser" units) are only good for measuring relative temperature between objects of the same material, finish and texture. A good example of this would be "measuring" the temperature of the ballasts in a bank of fluorescent lights. If the temperature indicator for 5 light fixtures shows 97F, 101F, 95F, 145F and 99F, it is apparent which one is the likely candidate for overheating. This does not mean that the average temperature of a light is around 98F, it simply indicates that one of the four fixtures is much hotter than the others.
Also, the emissivity of an object changes significantly with increasing temperature so beware of using any non-contact measurement system used above 300 to 400F.
Above all, NEVER make a life threatening risk assessment based upon a non-contact temperature measurement.
TICs are just a tool, and like all tools, they can and do fail.
President - ICC
01-31-2000, 01:20 AM #4dalittleFirehouse.com Guest
First, I want to apologize for taking so long to post this remark to TIMAN’s comments on temperature measurement systems in infrared imagers.
TIMAN is theoretically correct. All kinds of things affect the readings of a direct temperature device or non-contact thermometer. However, in the firefighting application, the only significant factor we must consider, if that, is the emissivity of the object. In firefighting, it is not important to distinguish temperature differences so tight that even the wind speed and direction must be considered.
Infrared cameras used for PPM and process control applications (in industry) measure temperature deltas so tight that you should, technically, consider all these variables.
These considerations are input variables into the camera that weigh the temperature
readout, given some known relationship. However, we must understand that, even in
industrial applications, trained thermographers rarely have the data to enter into their cameras when shooting a target for a radiometric measurement. For the most part, its more trouble than its worth, and insignificant. HOWEVER, emissivity is a factor that must be considered.
I will not redefine emissivity, TIMAN and Mr. Clynne have done a good job explaining
the concept in the posts above this one.
FOR FIREFIGHTING, lets consider why we should consider the need for non-contact
First, let’s understand how an infrared camera works. In basics, every time a camera
scans the target scene, the camera’s sensor looks through the lens and each pixel records
IR energy (a pseudo-temperature) value. The camera then examines each temperature recorded and finds the coldest temperature. It assigns the color BLACK to that temperature. Then, each temperature gradation above that is assigned a shade of gray. In BST, there are 256 shades of gray. Now, here’s the important part: the temperatures above the 255th shade of gray are assigned “peak white” - the highest gray scale the camera can create. ALL the temperatures above 255 shades are peak white.
With that in mind, lets examine the following questions.
1. The hottest object in the screen is white, correct? -- Wrong.
2. White objects are hot, right? -- Not necessarily, they are hot only relative to other stuff in the scene. (and for heaven’s sake, for this discussion, lets ignore reverse polarity.)
3. An object that is 500 degrees in one scene can be just as “white” as one that is 45 degrees in another, correct? - YES.
Lets look at this scenario: Imagine you are doing overhaul and you see a white spot that
resembles the IR image of extension on the screen. You assume there is fire in there and
have the wallboard pulled. You find out that there’s nothing in there, it was merely an
area that, at one time had heat, but now, only residuals remain. What you saw was
residual of where a fire was - not where there is active fire. It would help to know if that spot was only 82 degrees or 182 degrees, right?
Manufacturers with significant IR expertise know the effects of emissivity and the many
other factors on non-contact temperature measurement. The concept is not new to us. We
choose to employ this technology because it is a useful tool. We also preset our
emissivity value in a firefighting camera at 0.95 because, on average, it gives the best
compromise. (We did not want firefighters to have the ability to adjust the emissivity
value.) We understand that infrared imaging can be a very confusing topic. Keep it as
simple as possible and introduce only the factors whose effects are significant in the
scenarios that the equipment was intended to be applied to.
Remember, we are fighting fires, not analyzing the integrity of a computer chip, or trying to determine the useful life of a bearing. For those applications, I’d not buy a Bullard, ISG or Scott, or whatever, I’d get an Agema 590. Its an awesome camera - but it has all those adjustments.
And, as far as specifying resolution and repeatability, +/-1 degree quoted by ISI and ISG, these numbers are correct as specified given the emissivity presets of .95 for ISG K90 Talisman, and .98 for ISI Vision3. What’s the difference between .95 and .98? Basically none.
Really, training is key here. Know what the equipment does, and know its limitations.
And, above all, always remember that the you make decisions, not the equipment.
Cameras and direct temperature readouts, overlays and transmitters are merely tools in
your arsenal. Use them only as they are intended to be used.
Director, North Amer. Opers.
ISG Thermal Systems USA, Inc.
02-16-2000, 12:39 AM #5McNiecefafeFirehouse.com Guest
I have used many different kinds of cameras during evaluations. The most startling result came when a particular camera with a temperature probe was transmitting to a monitor. A single cup of ice water had a temperature of 157 degrees. Does a transmitter skew the temperature probe? If a cup of ice water was 157 degrees, I wonder what a live fire would be?
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