Accurate 4-point RTD temperature measurements: CherryTemp device calibration

Temperature Calibration Importance

Guaranteeing temperature accuracy at the exact spot where the biological experiment is taking place is one important challenge for current life sciences research. Cherry Biotech has provided since its establishment strong expertise in temperature control and has been committed to develop the best products guaranteeing the reliability of scientific results. This has been accomplished by precisely controlling the experimental parameters which depend in our devices. During the setup of each CherryTemp system, prior to its shipping to our customers, a calibration process involving state of the art microdevices is used. Two microprobes are integrated in a calibration chip which replicates the properties of our disposables. These 50-nanometer thick miniaturized sensors are placed at the specific position of the biological sample to guarantee that the correct target temperature is reached at this precise spot. The probes provide temperature and time resolutions of 0.01°C and 0.1 second respectively. This application note is intended to present the available temperature measurement methods and to show the relevance in our system of the aforementioned technique.

Temperature Sensors

There are many techniques for measuring temperature in practice and their accuracy depends on a number of factors which include calibration against the absolute temperature scale, thermal disturbance due to the sensing method, and limitations of the transducer output monitoring system. Depending on the nature of the temperature sensors used for the intended purpose they can be classified as contact or contactless [1].

Non-contact monitoring systems observe the object or medium of interest remotely. In these devices, convection and radiation phenomena are generally used to monitor temperature.

On the other hand, contact transducers perform invasive measurements and are required to be in physical contact with the sensed object, or the medium of interest, being subjected to an intrinsic observer effect. In general, these monitoring systems use conduction as a temperature transducing element and are used indistinctly in solids, liquids or gases over a wide range of temperatures. The use of invasive instrumentation involves a disturbance due to the difference existing between the temperature being measured and that which would exist in the absence of the sensor.

Non-contact Temperature Sensors

Contactless temperature probes have the advantage to be generally robust for measurements in extreme conditions where the sensor can be degraded during its use by temperature itself. This category includes the use of techniques such as infrared thermography [2], refractive index methods [3,4], absorption and emission spectroscopy [5,6], line reversal method [7], spontaneous Rayleigh [8] and Raman scattering [9], coherent anti-Stokes Raman scattering [10], degenerative four wave mixing [11], laser-induced fluorescence [12], speckle methods [13] and acoustic thermography [14]. These methods provide temperature measurements in ranges which include minimums of -269°C and maximums of 2700°C in some cases. Details on the listed methods can be found in the provided references.

In order to meet the range of temperatures in which life sciences experiments are generally performed, and in which the CherryTemp device operates, it is important to underline the capacity of infrared and acoustic thermographs. These devices can provide measurements between -40°C and 2000°C or -269°C and 1800°C respectively. Having both a relatively high cost, infrared thermography is the only one, of the two techniques, also providing a usable, reliable and commercial sensing method with an accuracy up to ±2°C and a sensitivity of 0.1°C.

The principal criteria to be considered in the selection of an individual infrared temperature measurement system include the temperature range, atmospheric conditions, spectral sensitivity range, optical signal strength, desired signal level, maximum acceptable noise, cooling constraints, the spectral pass band, field of view, the resolution, speed of response, stability, the reference standard, geometry, and cost [2].

Contact Temperature Sensors

Invasive or semi-invasive temperature probes involve the contact of a physical part of the measuring system with the medium of interest.

Semi-invasive methods rely in remote monitoring a temperature sensitive coating. These heat sensitive materials can be thermochromic liquid crystals [15], thermographic phosphors [16] and heat sensitive paints [17]. In these cases, temperature variation generates a color or fluorescence change that can be quantified externally. Thermochromic liquid crystals can provide measurements between -40°C and 283°C with an accuracy of ±1°C and a sensitivity of 0.1°C while thermographic phosphors can be used to target temperature ranges between -250°C and 2000°C with an accuracy of up to ±0.1% and a sensitivity of 0.05°C. Sensitive paints are not reversible and have a limited use.

On the other hand, invasive temperature measurement techniques involve placing the physical sensor on or within the measured plane or space. The number of methods and devices belonging to this category is very extensive. The most important are: thermal expansion -based techniques [18,19,20], thermocouples [21,22], resistance temperature devices (RTD) [23,24], thermistors [25], junction semiconductor devices [26], fiber optic probes [27], capacitance thermometers [28,29], noise thermometers [30], quartz thermometers [31], and magnetic thermometers [32,33]. Most of these invasive temperature measurement techniques either have a limited set of applications or are only able to reach accuracies which are worse, or of the same range, than non-invasive or semi-invasive methods. However, thermistors, RTDs and semiconductor devices provide measures with accuracies that can reach ±0.01°C in the first two cases and  ±0.1°C in the case of semiconductor probes. Furthermore, these devices display very good sensitivities and, in the case of platinum-made RTDs probes, are relatively unreactive. In fact, standard platinum resistance thermometers are used to define temperature international standards between -259°C and 961°C with an accuracy lower than ±0.007°C in carefully controlled laboratory conditions [34].

RTDs are the most accurate temperature measuring probes currently available for the afore mentioned range. These sensors are based upon the principle that the resistance of a conductor is related to its temperature. This is because the motion of free electrons and of atomic lattice vibrations are temperature dependent. As a result, the measurement of the electrical resistance would be analogous to the measurement of the temperature in the metal.

Planar Temperature Measurements

Measurements of temperature at a surface are not easy to perform. A probe placed in contact with a surface will alter the temperature being measured by itself. Furthermore, the interface existing between the sensor and the surface being measured will be also a critical element for the intended purpose. Limitations do exist to make the measurement both practicable and accurate.

A pointed probe could be very convenient for usability purposes but would fail to get correct measurements due to its limited contact area. A long thermocouple wire laid on the surface would make much better measurements but limitations could arise from the practical point of view. Probes with a contact pad can facilitate the measurement with a reasonable shape but the contact material should be as highly thermally conductive as possible while providing intimate contact with the surface. In general, the best that can be done is to attach sensors in direct contact with the object where the temperature can conveniently be measured, covering only a small area. The objective is to maximize contact while minimizing interference on the measurement. Furthermore, when measuring biological samples, the intimate contact of the probe with the sample can lead to degradation of the sensor as well as destruction of the sample.

For these reasons, Cherry Biotech uses a calibration procedure based in planar platinum probes integrated in the sample-containing substrate of the system which is not present during the biological experimentation. The design of these microprobes, in combination with the associated electronic instrumentation, permits a 4-point, also named 4-wire, RTD temperature measurement. This method eliminates lead wire resistance uncertainties.

4-Points measurement Resistance Temperature Devices (RTD)

The four points RTD devices have two leads connected to the two ends of the temperature sensing electrical resistance (see Figure 1). These are simultaneously connected to a constant current supply and to a voltmeter with a high input impedance of at least 100MΩ.

 

To understand the measure principle, it is important to notice that due to the high input impedance of the voltmeter a negligible amount of current will circulate through its wires. As a result, the voltage measurement will reflect the voltage at the RTD very accurately avoiding lead wires voltage loses. By the same principle, practically all the current will circulate through the RTD guaranteeing a well-known value for this parameter. The result is a very accurate measurement of the electrical resistance, or alternatively impedance, of the RTD. This implies temperature measurement accuracies and sensitivities only dependent on the existing characterization of the conductor thermal and electrical properties, the intrinsic relation of these properties, and the quality of the electronic instrumentation.

Cherry Biotech calibration technology uses state of the art microfluidic chips which integrate planar platinum microelectrodes with a thickness of 50nm. The chips are the same that are used by the CherryTemp device but with the addition of the 4-points RTD temperature probe in the surface of the disposable glass slide where the biological sample is placed. To further enhance the procedure, Cherry Biotech integrates two 4-points temperature microsensors in each calibration chip (see picture of the probes in Figure 2).

 

The size of the devices is compatible with the dimensions of the chamber that holds the biological experiment. The full system allows unmatched control over the sample temperature.

Figure 1: Scheme of a 4-points resistance temperature detector (RTD) circuit. The measured RTD electrical resistance will be measured with negligible loading and lead-wire resistance errors.

Figure 1: Scheme of a 4-points resistance temperature detector (RTD) circuit. The measured RTD electrical resistance will be measured with negligible loading and lead-wire resistance errors.

Figure 2: Picture of the calibration glass slide containing 2 parallel 4-points micro RTD probes.

Figure 2: Picture of the calibration glass slide containing 2 parallel 4-points micro RTD probes.

Conclusion

Miniaturized RTD are currently the best temperature probes for guaranteeing temperature accuracy in our system and giving the best outcome to our customers. It has no rival in terms of suitability to measure at the place of the biological sample as well as of method accuracy and sensitivity.  Integrated planar platinum miniaturized RTDs, combined with 4-points measurement instrumentation, provide a robust method to calibrate Cherry Biotech’ devices displaying an accuracy of 0.01°C inside the liquid sample and in contact with the biological specimen.

CherryTemp heater/cooler for live-cell imaging

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Stable & Precise : Long term stability


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