In hydraulic systems, the temperature is that one property that can tell a lot, and I mean A LOT! Machines are just like us, people - whenever we get sick - our temperature changes! A faulty component can be found through temperature measurement, the condition of a complete hydraulic system can be evaluated and monitored through monitoring temperature at key points, faults can be detected at early stages by monitoring the tank temperature alone - the list goes on... I will definitely elaborate on this topic in a separate article, but for now, it is sufficient to say that the importance of knowing the accurate temperature of hydraulic stuff makes temperature reading gear a must-have for us - hydraulic doctors.
In the old days, our "ancestors" used to apply contact thermometers for the purpose - which did the job fine, until the day they got replaced by the non-contact measuring devices - namely the infrared thermometers. Cheap and practical, these handheld devices make temperature reading a snap, or a piece of cake if you will... However, there is more to these gadgets than just "point and shoot", and so this post is aimed at providing a couple of tips on what these devices are and how to use them correctly to get valid readings.
First of all - the physics behind. All matter emits electromagnetic radiation from the surface when its temperature is above absolute zero, this radiation is called thermal radiation. Infrared thermometers can "see" this radiation and infer a body's surface temperature through measuring its intensity. The thing is - real life components emit this energy at different rates, even when they are at the same temperature. This quality of an object is called emissivity, and it is primarily defined by its surface. It is very important to take emissivity into account to get correct readings. The general rule is - shiny and polished surfaces (like new hydraulic fittings, hose terminals, or polished valve bodies) are poor emitters, therefore their temperature will read lower than real unless compensated for.
The second thing about shiny components is their reflectivity (which is relatively high, since they are shiny), due to which an inaccurate reading is possible when they reflect thermal radiation emitted by another component - in this case changing the aiming angle of the thermometer will read a different temperature, even when pointing at the same spot.
There are models of infrared thermometers that allow you to manually introduce the emissivity coefficient to make readings more precise. This is a nice feature, but I, personally, instead of adjusting my test gear for different surfaces, prefer to "adjust" the surfaces for my test gear. There are two ways to do this - one is to use dark masking tape, applied over the spot you want to read, and in this case, you'll have to wait a bit for the tape to heat up to the component's temperature, and the other way (my favorite) - is to use a black matte spray to "decorate" the area you wish to measure.
Advantages of the latter are - a) you always get the same finish, which means stable readings, and b) you clearly mark the spots where you took the readings - which means that next time the temperature will be read at the same spot. Disadvantages - well, not all machine owners will like you to graffiti their equipment, even when it is for their own good, so you'd better ask them before you do...
If you think that this is not that important - take a look at the pictures to the left (I had to blank out the background, sorry about that...). As you can see, the readings between the plain shiny and painted surfaces differ by more than twenty degrees, which is a lot.
Here is a nice article on infrared thermometers for those who want a little more detail.
Infrared thermometers are an essential tool in hydraulic equipment diagnostics, and I never leave home without one. Every mechanic working in this field should have one and know how to use it.