Another little project that I have started.
I already own some Black Body thermal reference sources for my thermal camera work but they are relatively rare so I buy any 'bargains' I find. This lead me to become the owner of a very nice quality unit that I will detail in this Post. Before I do that though, a little about Black Body thermal reference sources and why they are needed.
The Black Body Thermal Reference source
Thermal cameras and IR thermometers are factory calibrated to provide a measurement accuracy of around +-2C or 2% when correctly used. This calibration holds for many years with decent quality cameras and thermometers but it is a good idea to check the equipment for accuracy at least every year. In some cases, every time it is used ! (Lab work) Calibration can be effected by component aging and contamination of optical blocks. It should not be blindly assumed that if a thermal camera is functioning that it is still making accurate measurements within its original specifications. There are various simple ways to check a thermal camera or IR thermometer for reasonable accuracy. I will not cover that topic fully here but two that come to mind are the 'melting ice bath' for 0C and the 'Boiling Water bath' for 100C. Thermocouples attached to a suitable ambient temperature metal plate can also be used but errors are possible.
The 'industry approved' method for checking the performance and accuracy of a thermal camera or IR thermometer is to use a purpose designed Black Body thermal reference unit. Such units contain a Black Body thermal emission plate or chamber that is driven by a suitable heating or cooling system. The heating and cooling system is designed to provide very accurate temperatures at the emission plate and this is why these units are classed as an accurate temperature reference. The Black Body emission plate of chamber is often coated in a special high emissivity paint that appears matt black in the optical domain. This surface is delicate and should not be touched with hard objects or fingers ! The emissivity value of the coating on the emission plate or chamber interior is stated by the Manufacturer in the specifications and is often 0.98. It is important to know the emissivity of the emission plate for accurate measurements but it need not be 1.0, 0.98 or even 0.96, so long as it is a decent value, known value, and a relatively flat response across the operating temperature range of the Black Body reference source. Some reference sources use plain flat emission plates, whilst others use the superior concentric ring format. The raised concentric rings help to avoid thermal reflections causing measurement errors. Any surface with an emissivity value of less than 1.0 can produce thermal reflections.
So what is the common construction of a Black Body thermal reference source ?
There are several designs for thermal camera Black Body reference sources in use. Some are cheaper to produce than others and some provide greater accuracy of temperature. You tend to get what you pay for with these units as they are specialist and so all are relatively expensive. Expect to pay around US$800 for a cheap one and many thousands of US$ for the better quality units. I did say they were expensive !
The form that the heater or cooler takes in a Black Body thermal reference takes dictates the complexity of the design. A few common types are detailed below:
1. Heater element
In this design, the Black Body plate or chamber is directly heated by a suitably constructed heating element and cooling is by natural radiation, convection and conduction paths. It cannot produce temperatures below ambient. The heater is controlled by a closed loop analogue or digital temperature controller that employs Fixed DC, Variable DC or PWM as the drive for the heater. The temperature sensor is normally a Thermocouple, Thermistor or RTD.
2. Heater element and fan cooler
Similar to the 'heater only' design, this Black Body uses a suitable constructed heating element that directly heats the plate or chamber. The design also incorporates a fan that can pass cooling ambient air over the plate or chamber in order to reduce their temperature more quickly when required. The fan is not normally part of the temperature stabilisation process. The heater is controlled by a closed loop analogue or digital temperature controller that employs Fixed DC, Variable DC or PWM as the drive for the heater. The temperature sensor is normally a Thermocouple, Thermistor or RTD.
3. Hot air blower utilising heater and blower fan unit
In this design the emission plate of chamber is heated to the required temperature by the flow of air over its rear/outer surface. Some designs employ air paths within the plate of chamber walls. The air is heated above ambient temperature by a motorised turbine / heater combination. Such is commonly found in hair dryers. The temperature of the air exiting the blower assembly and the emission plate/chamber is monitored by the heater controller. The drive to the heater is adjusted to provide the required reference temperature. Depending upon the design, the heater is driven by DC, PWM or Triac chopped AC. The fan speed may be constant or variable to suit the air path design and Delta T response time required.
4. Water jacket with temperature controlled reservoir utilising a heater or refrigerator
A 'Wet' Black Body reference source is normally intended to be static and not portable. It comprises an emission plate or chamber that is directly coupled to a relatively small water chamber that either sits behind it (plate) or around it (chamber).
The water chamber has inlet and outlet ports that permit water to be pumped through it at the desired rate. The water is part of a closed loop system that incorporates a main (large) water reservoir, water heaters and/or coolers, a water pump and a small reservoir located at the emission plate or chamber, as already described.
The water in the main reservoir is temperature controlled using heaters and/or coolers to maintain the required temperature. The heated or cooled water is then pumped around a closed loop water circuit that includes the small reservoir and so heats or cools the Black Body emitter. The system has a high thermal capacity and slow Delta T rate. This makes it an accurate, but slow to respond system that is often used at a set temperature without the need to change that temperature quickly. The water in the main reservoir is normally heated by a conventional heating element and drive circuitry from the temperature controller. The temperature controller often monitors the temperature of the emission surface, main reservoir and even in the path to the small reservoir. It can control the temperature of the main reservoir and the flow rate of water around the closed circuit. Where water cooling is required, this may be provided by a Peltier cooler or conventional mechanical refrigerator. The Peltier can act directly upon the main water reservoir metal casing, whereas the refrigerator tends to operate using a cooling circuit coil within the reservoir. A motorised stirrer may also be included in the main reservoir to equalise the temperature throughout its volume of water. With cooled main reservoirs it is sometimes necessary to use a mixture of glycol and water to prevent freezing at sub Zero C temperatures.
There are versions of the 'wet' Black Body reference that have the emission plate or chamber integrated into the side of a single large reservoir. This removes the need for the small reservoir, interconnecting hoses and pump. The single reservoir system can suffer from a vertical temperature gradient across the emission plate. For this reason a water stirrer is desirable to ensure the thermal layering within the reservoir remains disturbed and the temperature throughout the water relatively well distributed.
It will come as no surprise that 'wet' Black Body thermal references are intended for Lab use and are very expensive.
5. Peltier combined heater and cooler controlled by accurate temperature controller
The Peltier heating and cooling element is a very useful tool when considering how to create heating or cooling at low loads. Their power consumption can be a significant drawback however. The Peltier element has a lot to offer the designer of Black Body thermal reference however.
1. It can heat or cool a surface depending upon the polarity of its supply
2. It is solid state with no moving parts
3. It can change its surface temperature quickly, only limited by the thermal load placed upon it.
4. It is relatively easy to drive with either variable DC or PWM drive types.
5. It is relatively compact but it does require a heat-sink and sometimes a fan.
6. It is relatively long lived and robust if treated properly
7. Relatively inexpensive these days.
A Peltier element is capable of producing a Delta T across its two plates in excess of 70C. Operating at very high Delta T does reduce its life however. Special 'high temperature' versions are available however. With an Ambient temperature of 20C, a Peltier element system can easily produce a surface temperature of -20C to +80C and even -45C to +100C if auxiliary cooling and heating is employed.
The Peltier element is usually driven with either a varying DC voltage, DC Current or a PWM signal. They should not be driven by simple low frequency switched DC if long life is desired. Slow DC switching causes thermal shock within the device. They require a decent controller in order to be used as a thermal generator for a Black Body thermal reference as their temperature needs to be closely controlled to prevent unacceptable levels of overshoot in the system. A PWM controller is the favoured choice in modern microprocessor controlled Black Body Thermal References. Analogue closed loop control systems are manufactured for controlling Peltier elements and these can be very accurate if designed correctly. The controller for a Peltier element differs to that used for a simple resistive heater in that it is often capable of reversing the polarity of its output drive to permit the Peltier to both heat and cool a surface. The peltier element based Black Body thermal reference is therefore able to produce temperatures above and below ambient. A very useful feature. When working at very low temperatures, consideration must be given to condensation production and icing of the Black Body emitter surfaces.
When considering the design of a Block Body reference source, the Peltier element certainly ticks a lot of boxes ad is a well understood technology. Its one major drawback is efficiency at less than 6%. The elements have limited load capacity and high current draw normally measured in Amperes ! It is not uncommon to drive a common Peltier element with 6A at 12V. Power hungry beasts ! Sadly this hunger for power also effects how well the elements can cool. At some point in the cooling process the self generated heat within the Peltier element starts to degrade the cooling performance. It is then time to consider using multiple Peltier elements in a 'Stack' to reduce the drive needed on each element. Such is not normally needed for a Black Body Reference source however so I will not expand on the issues related with Peltier stacking.
The temperature sensor used by the Peltier controller may be any type that offers decent accuracy in this application. It is common to find Thermistors and RTD's such as the PT100 and PT1000. A combination of well designed Black Body emitter, Peltier elements and Peltier controller can produce an excellent Black Body reference source accurate to within 0.1C of setting. At these levels of accuracy, air currents across the emitters surface even have an effect ! This may be controlled using appropriate housings and tubes to limit such air movement contamination. Operating the emitter plate at sub Zero C temperatures can necessitate the use of Aluminium coupling tubes to the camera that are flushed with Nitrogen gas to prevent condensation and frost issues. That is a whole other topic however.
The Peltier element based Black Body Thermal Reference can be a very nice item to have in your thermal camera test equipment arsenal
6. Ambient temperature reference with accurate thermometer
An Ambient temperature reference source is the simplest of references for use with thermal cameras and IR thermometers. It's thermal emission plate of chamber is totally passive. The only active components used in such a reference are within the thermometers that provides the user with an accurate temperature reading from the emission plates surface. The emission plate sits at whatever the ambient temperature of its location may be. These are best used in an environment that has reasonably stable ambient temperature. Operation is simple. The thermometer is switched on and allowed to stabilise. The sensor on the rear of the emission plate of chamber provides a good level of accuracy that the user notes. The emission plate of chamber has a specified emissivity that is entered into the camera or IR thermometer. The camera or IR thermometer is then aimed at the emission plate of chamber and the reading noted.
If the reading is not within the expected tolerance then it s likely that the camera or IR thermometer requires attention or calibration. It does not get much simpler than this type of reference source. Point of failure are limited to the thermometer module and degradation of the emission surface. For this reason, such easily portable check sources are quite popular in some industries. Some also contain ports in the emission plate/chamber for insertion of conventional fluid based thermometers to provide further confidence in their ambient temperature reading.
Such a simple check source is eminently suitable for DIY construction. The required parts are available cheaply and construction is simple. I detailed such a project in another Post some time ago.
The disadvantage of the Ambient temperature check source is that it can only provide a single point of temperature reference as opposed to the wide range of temperatures available on other thermal reference sources. It cannot truly be used as a calibration check source as a result.
Fraser