Appendix C: Sensor Packaging and Installation
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176 Appendices Appendix C: Sensor Packaging and Installation Appendix C: Sensor Packaging and Installation Installation General installation considerations Once you have selected a sensor and it has been calibrated by Lake Shore, some potential Even with a properly installed temperature difficulties in obtaining accurate temperature measurements are still ahead. The proper sensor, poor thermal design of the overall installation of a cryogenic temperature sensor can be a difficult task. The sensor must be apparatus can produce measurement errors. mounted in such a way so as to measure the temperature of the object accurately without interfering with the experiment. If improperly installed, the temperature measured by the sensor Temperature gradients may have little relation to the actual temperature of the object being measured. Most temperature measurements are made on the assumption that the area of interest Figure 1—Typical sensor installation on a mechanical refrigerator is isothermal. In many setups this may not be the case. The positions of all system elements—the sample, sensor(s), and the temperature sources—must be carefully examined to determine the expected heat flow patterns in the system. Any heat flow between the sample and sensor, for example, will create an unwanted temperature gradient. System elements should be positioned to avoid this problem. Optical source radiation An often overlooked source of heat flow is simple thermal or blackbody radiation. Neither the sensor nor the sample should be in the line of sight of any surface that is at a significantly different temperature. This error source is commonly eliminated by installing a radiation shield around the sample and sensor, either by wrapping super-insulation (aluminized Mylar®) around the area, or Figure 1 shows a typical sensor installation on a mechanical refrigerator. Note the additional through the installation of a temperature- length of lead wire wrapped around the refrigerator stages to minimize thermal conductance controlled aluminum or copper shield (see along the leads. If the optical radiation load through the window is large, the sample Figure 1). temperature will not necessarily be the same as that of the sensor in the block. A sensor placed in more intimate contact with the sample may be required. 2-lead versus 4-lead measurement 4-lead measurements are recommended for all sensors. 2-lead measurements can be performed with diode sensors with a small increase in uncertainty. Refer to Appendix E: Temperature Measurement, for a detailed discussion. Lake Shore Cryotronics, Inc. | t. 614.891.2244 | f. 614.818.1600 | e. [email protected] | www.lakeshore.com Appendix C: Sensor Packaging and Installation Appendices 177 High temperature effects Table 1—Wire heat-sinking length required to thermally anchor to a heat sink at temperature T to bring the temperature of the wire to within 1 mK of T Below room temperature, the primary effect lower of using dissimilar materials bonded together Heat-sinking length (mm) for wire sizes in sensing elements or packages is stress- Tupper Tlower 0.21 mm2 (24 0.032 mm2 0.013 mm2 0.005 mm2 induced by different expansion coefficients. (K) (K) AWG) (32 AWG) (36 AWG) (40 AWG) Above room temperature, additional problems Copper 300 80 160 57 33 19 can occur. Alloying, diffusion (Kirkendahl 300 4 688 233 138 80 voids), chemical reactions, and corrosion Phosphor bronze 300 80 32 11 6 4 (especially in the presence of moisture and chlorine) accelerate as the temperature 300 4 38 13 7 4 increases. These factors can cause Manganin 300 80 21 4 4 2 catastrophic failure with time, or a shift in 300 4 20 7 4 2 the sensor calibration. Completely accurate 304 SS 300 80 17 6 3 2 de-rating data for all situations that could be 300 4 14 5 3 2 encountered is impossible to compile. Note: values are calculated assuming wires are in a vacuum environment, and the thermal conductivity of the adhesive is given by the fit to the thermal conductivity of VGE-7031 varnish Conduction (lead attachment) Another source of heat flow that is often There are a number of ways in which sensor neglected is conduction through the electrical leads can be properly anchored, with the TIP: maintain electrical isolation leads that run between the sensor and the choice usually determined by the needs To maintain good electrical isolation over many and constraints of the particular application. thermal cycles, a single layer of cigarette paper can ambient environment. 32- or 36-gauge, low be varnished to the thermal anchor first, and the wire thermal conductivity wire such as phosphor Longer leads may be wound directly around then wound over the paper and varnished down. The bronze or manganin is used to alleviate this a sensor adaptor or another anchor adjacent actual sensor leads are then soldered to this thermally problem. These leads must also be thermally to the sample and varnished into place. The anchored lead wire after the sensor body is mounted. varnish serves two purposes: it physically For a more permanent installation, replace the VGE- anchored at several successive temperature 7031 varnish with a suitable epoxy such as Stycast® points between ambient temperature and the holds the leads in place, and it increases 2850-FT. sensor. Performing a 4-lead measurement will the contact surface area between the wire overcome the high lead resistance. and the sample, or sample holder. VGE-7031 varnish is widely used as a low-temperature Caution: varnish can cause crazing The physical mounting of the leads of a sensor adhesive and can be easily removed with of Formvar® insulation. is as important as the mounting of the sensor methanol. As long as the leads are electrically One can make a separate thermal anchor to itself. Thermal contact to the active element insulated with an enamel-type coating, such which the thermometer leads are attached. A ® in a cryogenic sensor occurs both through the as Formvar (see caution note) or polyimide, typical technique for producing a physically sensor body and the electrical leads. In fact, the varnished-down leads provide a suitable compact anchor uses small gauge wire for some sensors (e.g., germanium resistance thermal anchor (thermal short) to their (32 AWG) insulated with Formvar®, polyimide, thermometers) the primary thermal contact is surroundings. Leads with heavy insulation, or a similar coating. The wire is wound around ® through the leads. For accurate temperature such as Teflon , minimize the potential for the sample in a bifilar manner or onto a readings, the sensor and its leads must be making a thermal short to the surroundings, separate bobbin and bonded with varnish. For anchored so they are at the same temperature resulting in more thermal conduction down most applications, a bonded length of 5 cm as the sample being measured. Table 1 shows the leads into the sensing element. Resulting to 10 cm provides a sufficient thermal anchor typical heat sinking lengths. temperature measurement errors can be unless poor practices elsewhere in the system significant. permit excessive heat leaks down the leads. Copper wire may require several meters for heat sinking. Lake Shore Cryotronics, Inc. | t. 614.891.2244 | f. 614.818.1600 | e. [email protected] | www.lakeshore.com 178 Appendices Appendix C: Sensor Packaging and Installation Cryogenic accessories for installation What you may need: Cryogenic wire Wire Phosphor bronze Cryogenic wire is different from normal wire due to its low thermal conductivity and high Manganin electrical resistivity. The most common types of cryogenic wire are phosphor bronze and Nichrome Copper manganin. Phosphor bronze is a nonmagnetic copper alloy. Manganin wire has a lower thermal 1 Constantan conductivity (a factor of about /3) and higher resistivity compared to phosphor bronze wire. Stainless steel coaxial cable Both are readily available in small gauges ranging from 32 to 42 AWG. Either polyimide or Solders polyvinyl formal (Formvar®) is used to insulate the wires. The polyimide is a resin with a 220 °C 60/40 lead/tin thermal rating. It has exceptional resistance to chemical solvents and toxic heat. It also is 90/10 lead/tin unaffected by exposure to varnish solvent. Formvar® is a vinyl acetate resin rated at 105 °C. Silver Ostalloy® 158 (Wood’s metal) It has excellent mechanical properties such as abrasion resistance and flexibility. The film will Indium-silver withstand excessive elongation without rupture when stressed during winding. Formvar® has Indium a tendency to craze upon contact with solvents such as toluol, naptha, and xylol. It should be Fluxes given an annealing preheat prior to varnish application. The Formvar® insulation can be removed RMA mechanically or chemically during terminal preparation. Keep Clean flux Stay Clean flux Stay Silv® flux Phosphor bronze wire is readily available in multifilar form with 2 or 4 wires. In bifilar form, the wires are twisted to minimize noise pickup. In quadfilar form, the wires are either straight or 2 Insulating materials twisted pairs twisted together. The latter form is most useful for standard 4-lead measurements. Ceramics Masking tape The wires are bonded together for ease in heat sinking while the twisting helps minimize noise Polyester tape pickup. Straight Quad-Lead™ wire can be bonded together with the help of VGE-7031 varnish. Kapton® films Teflon® tape The bonding agent is soluble in alcohol. Heat shrink tubing G-10 Other types of common cryogenic wires include nichrome wire, which has a very high electrical Mylar® (polyester film) Fiberglass sleeving resistivity making it excellent for heater windings. Ultra miniature flexible coaxial cables with Epoxies 304 stainless steel or copper conductors are available for providing shielded leads when VGE-7031 varnish Stycast® 2850 FT epoxy necessary. For low resistance, heavy duty lead wires and multifilar silver-plated twisted copper Cigarette paper wire are available.