Engine jacket water temp.

[Lucky Dog]

I have two electrical temp. gauges and one murphy mech. gauge with tattle-tale on each Detroit all of which, of course, read differently. Since water temp. is really hot in the Gulf this time of year, I shoot the boss where the senders screw in with a IR thermometer. I'll limit the question to the mech. gauge since it is supposedly the best. The IR reading is about 10 degrees higher than the gauge reading. Normally this is no big deal but on really hot days in the summer (like yesterday), the IR reading gets to 195-200 degrees which is uncomfortably close to my limit.

Now the question - I wonder if the IR reading is close to the actual jacket water temp. or is the boss temp. influenced higher by its proximity (right above) the 500 degree exhaust elbow? Is there a more appropriate place to shoot the temp.?

[MikeP]

I shoot the temp at the thermostat housings - this is the same place where the mechanical gauge sensors are located on our 8V71TIs.

I also find that my IR gun reads a bit higher than my two mechanical water temp gauges. I believe that the mech gauges are more accurate because their sensors are IN the water flow. They also both agree within a degree or two. Remember - The engine's temp specs are based on water temperature, not the surface of the block or any other exterior item. Since an IR gun can only measure the temperature of the surface it is aimed at, it can't actually measure the temp of the water in the engine anyway.

Also, typical IR guns are just like camera light meters - they are calibrated to an average. Just as camera light meters are set to "see" an 18% gray (which almost nothing in the world is), most IR guns are fixed to read a surface emmisivity of .95.

So if the gun isn't looking at something that has an emissivity of .95, the reading will not be correct. I found a table that contains the following info for emissivity:
Painted surfaces vary from .74 to .96. Stainless steel varies from .10 to .80. Non-oxidized unpainted metals vary from .02 to .21. (I just took these three as an example.) This is a pretty darned wide range of variation which means, for example, that the temp a typical IR gun will read off stainless steel will never be the correct surface temp of the SS!

IR guns are also very dependent on distance. To be as accurate as possible, they should be held not more than a couple of inches from the object to be measured. As the range increases, the cone of coverage increase dramatically and the gun will be reading a much larger area than is intended which causes inaccuracy. The laser does NOT define the measuring area of the cone - it is just a conveniant aiming device.

Really good (multi thousand $) IR temp measuring devices have user-adjustable/user calibrated emmisivity and they project an actual coverage cone so you can see precisely what is covered. But I don't think we'll be buying many of those.

IR guns are really handy and very versatile. I like them because they are quick and repeatable so it's easy to see if something has changed - to me that's their real value.

[Lucky Dog]

Thanks, Mike, for your very informative thoughts. I was kinda thinking along those lines but I don't like to take chances with those temps. I let 'em get away from me once and learned the hard lesson - a couple of boat bucks for a cracked head.

[Genesis]

The mechanical gauges should be correct.

If they're not, REPLACE THEM. They're cheap and you really need one GAUGE on each engine for oil pressure and water temperature that IS correct - always.

BTW, "never exceed" on a Detroit is 195F, because somewhere between 200 and 205 you'll be buying a cylinder head.