By Ray Peacock
At the risk of being insensitive by saying this sounds like a ‘Duh” moment let’s not. Let’s explore just what it means and why it is significant, especially in the case of temperature sensors.
The fact is, non-contact temperatures sensors, and similar non contact sensors for size and shape are so-called because they measure a certain property by not contacting the object itself. Aside from the obvious side benefits of not causing any changes to the object being measured, non contact temperature sensors have some significant benefits and accompanying advantages over contact temperature sensors.
As an aside, for just a moment realize that both contact and non contact temperature sensors do not exactly measure anything, their indication of a temperature value is inferred from the combination of several factors.
In the case of contact temperature sensors, some analog, physical property of the sensor changes due to the sensor being in contact with the object of measurement. For example, a liquid-in-glass thermometer has the length of a thin column of colored liquid alongside a temperature scale changes as the sensor and object remain in contact.
The change is not immediate and the two must remain in contact for a sufficiently long period of time to say that the column length has stopped changing and the liquid column is said to be “in equilibrium” with the temperature of the object being measured. That is, to say, they are at the same temperature because there is no heat flow between them, or it is so small as to be neglected.
The length of the liquid column has been previously calibrated to the scale on its side to read in commonly used units of temperature.
This example points out the key factors in inferring the temperature of an object by a contact sensor:
(1) they must be in contact long enough for the two to have
(2) no heat flow between them, or be in thermal equilibrium with each other.
A non contact temperature sensor, on the other hand needs no contact, but other factors enter into the inference of the temperature of the object for the sensor’s response to the object.
The most common type of non-contact temperature sensor is a single waveband radiation thermometer, or “IR thermometer”. It is basically a radiation receiver, or radiometer, that has been calibrated in terms of the temperature of a reference source of thermal radiation, most often a Blackbody simulator.
These are optical devices that use either or both mirrors or lenses to collect thermal radiation from a designed optical field of view and focus that radiation onto a sensing element or transducer. The transducer converts the thermal energy of the radiation received from an object into a physical response, either a small electrical signal or something else; electrical signals are most common.
If the object is a reference blackbody source that completely fills the optical field of view of the sensor, one can calibrate its electrical output versus the temperature of the blackbody and thus generate a calibration response for it.
In use, if one aims the non contact sensor at an object that fills its optical field of view, its electrical response is an indication of the object’s temperature, not necessarily a complete or accurate measurement. Some additional factors need to be considered, such as the “non-blackbodyness” of the objects, whether there are effects from any of the intervening media that could attenuate or possibly enhance the amount of radiation received by the sensor.
It sounds complicated, and in truth, doing it from first principles, it is. However, this technology was discovered more than 100 years ago.The technology is very mature and has evolved into an engineering discipline rather than a research program.
The fact is: non-contact temperature sensors exist in a wide range of devices with many capabilities. They have, as promised above, some significant advantages over contact sensors.
First of all, they do not have to be, and in fact could be seriously damaged, if they reached the temperature of the object being measured, especially if it was very hot.
At temperatures above the melting point of most thermocouples and other high temperature sensing products, say above about 1700 °C (3092 °F), there are not very many ways to measure temperature at all. So, non-contact temperature sensors have a real advantage at the hot end of the temperature scale.
Since they do not have to be in thermal equilibrium, with the object being measured, that means there is no inherent time delay in getting a temperature measurement with a non-contact sensor.
The only time limit lies in the sensor’s own time response properties and that of the electronics to with they are very often connected. Sub-second temperature measurements are not only possible, they are usually very common.
Sub-millisecond response times, however, are less common and not found among the garden variety IR Thermometers one can buy for less than about $1000 USD!