IR temperature oddity

I have one of those IR temperature measuring guns where you point its laser at a surface and it displays the surface temperature. Trying to check my cars a/c efficiency via the face vents, it was showing -4 to

-8C and possibly even colder. Obviously it is wrong - it cannot reach such low temperatures, so I added a thin strip of metal in front of the vent, but got similar figures. What is going on?

Is it the draft from the blower upsetting the reading or what? I have checked the accuracy and it is spot on.

Reply to
Harry Bloomfield
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When I used one of those things at work it had to have the emissivity of the target material set to the correct value. Not doing so could drastically affect the results. Could this be the problem here?

Reply to
Davey

The refrigerant can probably get below that temp. There is a shut off switch that usually stops the heat exchanger getting below freezing. Maybe its busted?

Reply to
dennis

Unlikely, I have seen similar too low temperatures on my last two cars, it has to be something to do with the way these IR things work.

Reply to
Harry Bloomfield

Can't measure air temperature with it - emissivity is too low.

You will need to use something in the air flow which will quickly take on the air temperature, such as a piece of paper, or the vent grille. Many metals also don't work, such as copper, and most metals with polished finishes.

Reply to
Andrew Gabriel

Can't say it helps at all, but skin is quite good.

Reply to
polygonum

Basically it's the wrong instrument for the job.

I use cheap "weather station thermometers for air and liquid temperature

here is an external/internal example that I have modified for external/external for my fridge freezer.

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I was surprised to find that the two channels had differing ranges and if the only the external channel would read down to freezer temperatures.

The two thermistors were electrically interchangeable, I encapsulated the internal bead thermistor by wrapping it in self amalgamating tape, and dipping it in two part epoxy adhesive. It was interesting to see the temperature shoot up as it hardened.

Reply to
Graham.

Why do you think the air flow can't be that cold? The air con won't be able to make make the interior of the car that cold due to the thermal mass and solar heating. Remember to bring the interior temp down it has to pump out heat faster than it's coming in from the thermal mass and solar heating. You ain't going to do that in any sensible time unless the air coming in is pretty cold,

If the bit of metal (unpolished?) or the grills in the air flow where all showing rouoghly the same temp that is pretty much going to be the temp.

The evaporator on air con units can get well below freezing and freeze the condensate. That is not good for effciency as ice is a good insulator. It probably depends on the particular make/model of car if the system has the abilty to defrost the evaporator.

Unlike aircon used to keep kit cool in all outside airtemp/humidity conditions, car makers may assume that the aircon won't be on unless the outside air temp is fairly high and thus able to keep the evaporator free of ice.

Reply to
Dave Liquorice

Except for the ones that use the aircon to provide dry air in the Winter for demisting, but I would expect them to put the evaporator after the heater in the circuit.

Reply to
John Williamson

The manual on both the cars I drive states that the evaporator will turn off before it freezes. You really don't want ice forming and it doesn't need to if the heat exchanger has been sized correctly.

Reply to
dennis

Stick a thermometer in the grill and check against the ir thingo.

Reply to
F Murtz

That is not a problem - I am sure it is misreading, because the climate control would not allow it to run to such a low temperature, but my question was why it read so low? It was pointing at the air vent, or back back the air vent and I even tried it on some other material in the path of the vent. Could it have been affected by the air flow, normally they are used on static items.

Reply to
Harry Bloomfield

I doubt very much if your IR thermometer is sensing air temperature. The way to find out what the air temperature actually *is* is to put thermistor or a traditional thermometer in the air flow. And I think that it could be below zero.

Reply to
newshound

OK, I'll give that a go.

Reply to
Harry Bloomfield

American car adverts. sometimes used to refer to 'beer-chilling' air conditioning in their cars. And we know that Americans only like cold beer, warm Bud tastes absolutely disgusting.

Reply to
Davey

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Enjoy.

Reply to
Huge

Not seen that crowd before.

Reply to
Davey

I was thinking more of the resultant reduction in RH rather than ground temperature effect.

Yes, theoretically, the temperature of space is around the -270 deg C mark (3 deg K - IIRC, the temperature of the CMB).

I've just had another go but, although the sky is mostly clear, there are some whispy clouds around suggesting an elevated moisture content in the clear parts borne out by my only getting a -17 deg C reading.

The ground shows 12 ded C whilst the wall of the kitchen extension (brick) gave a reading of 13.6 which (by pure chance imho - I don't believe the repeatability is much better than a deg or two) exactly matched the external sensor reading of the weather station remote.

I was hoping to get a reading from thick cloud cover which I was expecting to show a much warmer reading not too different from a ground temperature reading as I recall the last time I was able to check this.

Reply to
Johny B Good

Do the specs. of the IR device give any target distance requirements or limitations?

Reply to
Davey

I bought this from a flea market stallholder known as "Maplin Man" on account most of his stuff was Maplin Returns that he'd bought at a disposal auction. This was a good 2 or 3 years ago and I can't recall whether it was a loose item or came with any packaging/documentary materials. If it did, I haven't seen it/them in a long time now.

The best I can offer is info off of the stickers. With any such device, there is no range limit as such, just a spot size (diameter) to distance ratio which, on this unit is given as 8:1 (with example distances and spot diameters of the capture area).

This IR sensor, like most of its breed, includes a sub mW class 2 red laser pointer which, nominally shines on the centre point of the capture area by way of a pointing guide. The reading being sensed by the much larger area of the "Spot" which, for a distance of 16 metres, works out to just a little over 6 square metres. This represents a reasonably small portion of the sky by which to sample the temperature of the greenhouse gasses in the upper atmosphere on a cold dry winter's night.

Reply to
Johny B Good

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