# NASA TM-110327 "Recommended Strain Gage Application Procedures for Various Langley Research Center Balances and Test Articles" (T. C. Moore Sr., LaRC, March 1997) - Notes, Part 1 (PDF pages 1-47) Reader scope: PDF pp. 1-47 (printed pp. 1-41). PDF pp. 48-94 are covered by a second set of notes (part 2). Those pages hold the rest of Class IV, Class V, and Addenda 1-4: gauge matching, the M-Bond 450 and M-Bond 200 bulletins, and the flame-spray photographs. **Page numbering:** printed page N = PDF page N + 6. Below, citations read "p.N (PDF N+6)". **Figures:** PDF pp. 1-47 contain **no figures, tables, or photos**. Every page is typed text (checked against the page images; the photographs are all in the addenda, PDF p.78 onward). The OCR text layer is good. Fixes to the OCR: "EA-g34" = Hysol **EA-934**; "+/-.O05mVN/A 100F" = **+/-0.005 mV/V per 100 deg F**; "C-891113-A" really is the custom Micro-Measurements gauge part number (confirmed on the image of PDF 12). --- ## 1. Page-by-page map (PDF 1-47) | PDF | Printed | Content | |---|---|---| | 1 | - | Cover: TM-110327, March 1997 | | 2 | - | Blank | | 3-5 | i-iii | Table of contents, list of addenda, list of photos (Photos 1-15 belong to Addendum 4, the high-temperature flame-spray gauge, printed pp. 72-86) | | 6 | - | Blank | | 7 | 1 | Abstract (LaRC uses more than 10,000 gauges per year), introduction, explanation of "Application Class / Installation Type", and **general notes**. The procedures are for *new* installations, not regaging. Surface-prep materials "subject to change". **"Room temperature" means a part and air temperature of 72-80 deg F with RH never above 40%** | | 8-11 | 2-5 | Index of procedures. Class I = balances (printed 6-15). Class II = transducers (16-28). Class III = composites (29-40). Class IV = above 700 deg F (41-49). Class V = unusual cases (50-58) | | **12-16** | **6-10** | **Class I Type 1: standard LaRC 6-component balances, 0-200 deg F. THE PROCEDURE MOST RELEVANT TO US** | | 16-22 | 10-16 | Class I Type 2: cryogenic balances (NTF), -275 to 150 deg F | | 22-26 | 16-20 | Class II Type 1: LaRC transducers, 0-200 deg F (nearly identical to Class I Type 1) | | 26-31 | 20-25 | Class II Type 2: cryogenic transducers | | 31-34 | 25-28 | Class II Type 3: transducers at 200-450 deg F (M-Bond 450, RTV 3140 coating) | | 34 | 28 | Class II Type 4: spaceflight transducers (no procedure given; use Types 1-3 as a guide) | | 35-37 | 29-31 | Class III Type 1: Gr/Ep composites at room temperature (GA-2 or M-Bond 200, vacuum-pad clamping) | | 37-39 | 31-33 | Class III Type 2: Gr/Ep from 250 deg F down to -452 deg F (M-610 at 40 psi, 250 deg F) | | 39-41 | 33-35 | Class III Type 3: SiC-coated carbon composites at room temperature | | 41 | 35 | Class III Type 4: metal-matrix composites at room temperature (Al/Ti matrix oxidizes quickly, so gauge right after prep; M-Bond 200 for short-term tests) | | 41-43 | 35-37 | Class III Type 5: metal-matrix composites from -320 to 400 deg F | | 43-44 | 37-38 | Class III Type 6: metal-matrix composites from 400 to 700 deg F (custom procedure only) | | 44-46 | 38-40 | Class III Type 7: adhesive/polymer composites at room temperature | | 46 | 40 | Summary for composites: review the test parameters with the engineer first | | 47 | 41 | Class IV intro and Type 1: LARC-CKA1-1B high-temperature compensated gauge on Inconel/TMC, steps 1-3 (rest in part 2) | --- ## 2. Class I Type 1: Standard LaRC balance (0-200 deg F). Full detail (p.6-10, PDF 12-16) Apparent-strain correction is made over **80-180 deg F**. Balances that will run at 200-300 deg F should follow Class II Type 3 instead (p.6, PDF 12). ### 2.1 Gauging materials (p.6, PDF 12) 1. Gauge: chosen by the designer. The usual choice is **Micro-Measurements custom C-891113-A**. The cryogenic version is C-891113-B (p.10). 2. Adhesive: **M-BOND 610** (Micro-Measurements), a solvent-thinned, heat-cure epoxy. PRTs are bonded with **Hysol EA-934**. 3. Wiring terminals: **MM CPF-25C, CPF-38C, or CPF-50C**, sized to the balance. 4. Insulation pads under the apparent-strain and zero-correction wires: etched 1-mil **Kapton**. 5. Gauge-to-terminal jumper: **AWG 40 silver-clad copper**. 6. Apparent-strain (temperature-zero) correction wire: **nickel**, length set by the correction needed. 7. Zero-offset correction wire: **Manganin** (Driver-Harris). 8. Intra-bridge (gauge to gauge) and inter-bridge wire: **AWG 30 (7/38), 32 (7/40), or 36 (7/44)**, 7-strand silver-clad copper with **Teflon** insulation. **Small balances may need solid AWG 40 or 42.** 9-10. Exit leads and connector: set by the designer or facility. 11. Solder: **MM 361A**. 12. Thermocouples: type **J, AWG 36**, Teflon-insulated. 13. PRTs: **HY-CAL EL-700T**. 14. Protective coating: **Gagekote #8** (JP Technologies). 15. Sleeving over exit leads: nylon or fiberglass. ### 2.2 Surface preparation (p.6-7, PDF 12-13) 1. Degrease the whole balance by vapor-degreaser rinse with **ENSOLV** (Envirotech). 2. Inspect under the microscope for surface flaws, cracked beams, and sharp edges. 3. Rinse with pure alcohol. 4. Mask around the gauge areas. 5. **Micro-sandblast the gauge areas with 50-micron Al2O3.** 6. Remove the masking and blow off the grit with dry shop air. 7. Repeat the ENSOLV and alcohol rinses. **Start installing gauges immediately afterwards.** ### 2.3 Gauge installation and cure (p.7, PDF 13) 1. Clean the gauges, the pre-trimmed terminals, and the pads with alcohol and dry them. Lay them **upside down on a clean glass plate**. 2. Brush **one coat of M-610 on the gauge undersides AND one on the balance surface**. 3. **Air-dry at least 10 min** at room temperature, defined as at least 72 deg F with RH at or below 40%. 4. Place the parts at their marked locations and cover them with **thin pressure-sensitive Teflon tape**. 5. Put **silicone-rubber pads** over the tape. 6. Clamp at a **constant, uniform 60 psi** with a suitable clamp. 7. Heat in an oven to **340 deg F at about 6 deg F/min**. 8. **Hold 1 h at 340 deg F**, then cool by opening the oven door. 9. Unclamp. Check under the microscope for position, alignment, glue-line voids, and foreign matter. **Position tolerance is typically +/-0.005 in from the drawing.** Replace any doubtful gauge. 10. **Several cure cycles are needed** to gauge every face. Re-sandblast and re-clean each later area just before its cycle. 11. **Post-cure: ramp at the same rate to 340 deg F, hold 2 h, cool, and inspect again.** ### 2.4 Wiring (p.7-8, PDF 13-14). Given as "good practice", not a fixed procedure 1. On a worksheet, record each gauge's resistance **to 0.01 ohm at the gauge solder dots**. 2. Remove oxide from the CPF terminals with a **hard rubber eraser**. 3. Solder the AWG 40 jumpers from the gauge dots to the terminals, following the NASA soldering handbook **NHB 5300.4(3A-1)**. 4. Remove flux with ENSOLV and a soft brush. 5. Re-measure each gauge **at its terminals**. This checks the jumpers and the solder joints. 6. **Insulation test for the wire itself:** submerge the wire in tap water and measure leakage to ground. It must be **> 10,000 Mohm (10 Gohm)**. 7. Strip and tin the intra- and inter-bridge wires carefully, inspecting for nicked strands before tinning. 8. **Make intra-bridge wires equal in length and resistance** wherever possible. 9. **Keep wires snug against the balance surface along their whole length. Never let wires span open areas of the balance. On lightly loaded balances, make sure the wires carry no part of the load on the measuring beams. Later, do not let the moisture coating bond wires to load-carrying beams.** 10. Plan the routing beforehand. **Never route a wire over an active grid or over the gauge jumpers.** 11. Clean off all solder residue. ### 2.5 Initial electrical checks (p.8, PDF 14) 1. Leakage to ground on every bridge must be **> 10,000 Mohm**, measured with a tester that applies **15 VDC (at 20 Mohm)**. 2. Record the electrical zeros **at the excitation that will be used in service**. Each zero should be **within +/-0.050 mV/V of the value calculated from the four measured gauge resistances**. This is harder to meet on small balances, where lightly loaded beams and fine wiring affect the zero. 3. The allowed zero tolerance is on the drawing. **The typical allowance is +/-0.4 mV/V.** 4. **Shunt each gauge mechanically** while watching the zero, to confirm the bridge is intact. The cryogenic section adds: do not use sharp probes, which dent the solder joints (p.12-13). 5. Hand-load each component to check its sensitivity. Also check that the zero returns after loading and unloading. ### 2.6 Elevated-temperature soak (p.8, PDF 14) Soak the fully wired balance at **200 deg F for 4 h**. After it cools, re-check the zeros. **A shift greater than 0.010 mV/V means there is a problem to investigate.** ### 2.7 Apparent-strain (thermal zero) correction (p.9, PDF 15) 1. Place the balance horizontally in the oven with **-N facing up, supported only at the non-metric end**. 2. Cover it with a **non-contacting enclosure** so no airflow reaches it directly. 3. Tape a thermocouple with **aluminum tape** to the top of the aft shoulder, or use the balance's own aft TC/PRT. 4. Use the same excitation as the load calibration, and let the setup stabilize. 5-6. Start recording output against temperature at an 80 deg F setpoint. Wait until **drift is under 0.005 mV per 5 VDC per 5 min** and the balance is within **+/-2 deg F of 80 deg F**. 7. Set the oven to 180 deg F and heat to within 5 deg F of it, **at no more than 6 deg F/min**. 8. **Soak at least 15 min**; large balances need longer. 9. Cool back to within +/-2 deg F of the step 6 temperature. 10. Read each component's output at 180 deg F against the **return-to-room zero**. 11. **Add nickel wire to the appropriate bridge arm** to cancel apparent strain. **The typical tolerance is +/-0.005 mV/V per 100 deg F.** 12. Repeat the run. 13. Apply the moisture coating (section 2.8) and run a check. 14. Make another run for repeatability: **within +/-0.003 mV/V of the previous run over the whole temperature excursion**. That run becomes the final one. Give a final microscope inspection, then send the balance to calibration. Water-cooled balances get one more run with the cooling jacket fitted. ### 2.8 Moisture-proofing (p.10, PDF 16) Applied after the apparent-strain run of step 12: 1. Flush with ENSOLV, then pure alcohol. 2. **Warm the balance to about 100 deg F.** While the surface is still warm (90-100 deg F), brush **Gagekote #8** over the gauges, jumpers, terminals, and the surrounding area. Coat thickness: **0.005-0.007 in**. 3. **Air-dry at least 20 min**, then brush a second coat. Then go back to step 13 above. --- ## 3. Class I Type 2: Cryogenic balance (p.10-16, PDF 16-22). Differences that matter or that are useful ideas - Gauges are **matched for apparent strain** before installation (Addendum 1, see part 2). Matched gauges are cataloged and kept in individual pockets of a gauge organizer (p.11). - The apparent-strain correction wire is **AWG 40 copper** rather than nickel. **"Axial differential compensation sensors"** of nickel/alloy 99 (Driver-Harris) correct thermal-gradient "loop error" on the axial bridge (p.10, 13-14). - **Before wiring, mask each gauge and lightly micro-sandblast around the gauges and terminals** so the moisture coating bonds better (p.12, PDF 18). Also used in Class II Types 2 and 3. - Hardware: dry-GN2 purge of the sting at about **4.5 SLPM** during thermal runs; Loctite 222 on the connector screws; Dow-Corning RTV-3140 between connector and sting; a custom TFE convection shield (p.10, 13). - Thermal runs: three PRTs (front, middle, rear) must agree within 3 deg F. Start at 70-75 deg F. Drift criterion is **< 0.005 mV per 5 VDC per 3 min**. Go to -270 deg F. Bridge power off at 0 deg F during warm-up (p.13-14). - Coating is **M-COAT B**, 2 coats, **0.002-0.003 in** cured, each **air-dried 2 h**, applied **only at 75 deg F or above with RH under 40%**. It goes over solder joints and bare wire only, **never over the active grids**. After coating, the zero must be within **0.050 mV per 5 VDC** of the pre-coat zero and leakage still above 10 Gohm (p.14, PDF 20). - Final verification: hot (150 deg F) and cold (-270 deg F) cycles, repeated until consecutive runs agree (p.14-15). - **Final checks before calibration (p.15, PDF 21):** - Record the final zeros. - **Wet-brush leakage test:** flood the gauge and solder areas with tap water using a soft artist's brush and measure leakage to ground. - **Wet-brush "water effect on signal" test:** no spurious signal greater than **0.075 mV at 8 VDC** excitation (about 0.0094 mV/V). The 8 V is used only for this deliberately harsh test. - Record appearance under the microscope. - Keep a **balance log book** recording every deviation, problem, and discussion with the designer (p.15). ## 4. Class II: Transducers (p.16-28, PDF 22-34) - **Type 1 (0-200 deg F)** is essentially the Class I Type 1 procedure, word for word. Same materials, M-610 cure at **340 deg F / 1 h at 60 psi** plus a **2 h post-cure**, Gagekote #8 coating, 200 deg F / 4 h soak, and the same zero, leakage, and apparent-strain limits. One change: the transducer sits in the oven so its "load beams expand without constraint", and the TC goes as close to the beam as possible (p.18). - **Type 2 (cryogenic)** mirrors Class I Type 2. It uses a type T AWG 30 TC (p.20-25). - **Type 3 (200-450 deg F)** (p.25-28, PDF 31-34): - Gauge **J5K-06-S104P-350/DP**; **M-BOND 450** per MM bulletin (Addendum 2); solder **570-28R**. - Clamp at **50 psi** under CHR type C Teflon tape instead of the MJG-2 Mylar tape. Post-cure at **50 deg F above the maximum operating temperature**. - Soak at the maximum operating temperature for 2 h. - Coat with **RTV 3140, 0.025-0.035 in thick**, brushed on warm and cured overnight. - **Type 4 (spaceflight):** no procedure; each part gets its own (p.28). ## 5. Class III: Composites (p.29-40, PDF 35-46). Points useful to a hobbyist - **Room-temperature-cure option (Types 1, 3, 7):** - Use **M-Bond GA-2** (a room-temperature or mildly heated epoxy) or **M-Bond 200** (cyanoacrylate). - **M-Bond 200 is allowed ONLY when every test is finished within 6 weeks of gauging.** For long-term or repeated testing, use the epoxy (p.30, 34, 39). This is the key reliability warning against a cyanoacrylate-bonded balance. - GA-2 cure: **4 h at 100 deg F or 2 h at 125 deg F**; if the part can't be heated, **overnight at 80 deg F or above**. Type 7: 6 h at 100 deg F, 4 h at 125 deg F, or overnight at 72 deg F or above with RH at or below 40%. - Clamp with a **vacuum pad**. - **Hinge-taping technique (p.30, PDF 36):** 1. Mark alignment lines on the surface matching the gauge's alignment marks. 2. Lay the gauge face-down on glass and apply **CHR type C Teflon tape** to its top. 3. Transfer it to the part and align it. 4. **Fold it back** on the tape hinge so the underside faces up. 5. Brush adhesive on the gauge and on the surface. 6. When the adhesive's viscosity is right, fold the gauge back down and clamp. This works with hand tools and is the practical way to place gauges inside a narrow window. - Hand-sanding alternative to micro-blasting: **220-grit SiC paper** (p.29) or 280 grit (p.32). - Gauge-wiring sequence for lead wires (p.30, PDF 36): 1. Remove oxide from the pads with an eraser. 2. Check the gauge resistance against its spec. 3. Tin the pads. 4. Strip and inspect the leads. 5. Tin the leads. 6. **Add a mechanical strain-relief loop near the gauge.** 7. Secure the leads to the part, then solder. 8. Remove the flux. 9. Check total resistance. 10. Leakage must be above 10 Gohm. Lead wire is typically a 3-conductor AWG 30 stranded silver-plated flat ribbon. - **Type 2 (Gr/Ep, -452 to 250 deg F):** M-610 at **40 psi, 250 deg F for 3 h minimum, then a 275 deg F / 2 h post-cure** (p.32, PDF 38). This shows that **M-610 can be cured well below 340 deg F** with a longer hold. Coarse surfaces get an EA-934NA basecoat cured 4 h at 275 deg F (p.31). - Coatings: - **M-Coat A** goes on solder joints and exposed wire only, as oxidation protection in a lab environment (p.31). - Coatings stiffen thin parts: "**reinforcing effect on thin surfaces**" is listed as a factor when choosing a coating (p.33, PDF 39). This is relevant to our 0.025 in plates. - **Type 4 (Al/Ti metal matrix, p.35):** aluminum oxidizes fast, so **gauge immediately after surface prep**. - **Type 5 (p.35-37):** - **Clean by scrubbing with M-Prep Neutralizer 5A** (ammonia-based). - Solder: 361A up to 300 deg F, **430-20S** up to 400 deg F. - Coating: GE RTV-159 about 0.030 in up to 400 deg F, or thin M-Coat B for cryogenic only. **Dry-N2 purge can replace a moisture barrier.** - Post-cure 350 or 450 deg F for 2 h, depending on test temperature. ## 6. Class IV Type 1 start (p.41, PDF 47) High-temperature (up to 1500 deg F) **LARC-CKA1-1B** half-bridge compensated gauge (JP Technologies) on Inconel 100 and Beta 21S TMC. It uses an Al2O3 flame-spray (Rokide) or plasma-spray basecoat, a Nextel 312 thermal blanket, Hoskins 875 AWG 25 leads, and Inconel strapping. Steps: 1. Degrease with MM FTF-1. 2. Mask an open area of **0.6 x 0.7 in** with 3M #64 fiberglass tape. 3. Micro-blast with 50-micron Al2O3. Thermal cycling to at or above the maximum temperature improves first-cycle data. **Not relevant to our balance.** Continued in part 2. --- ## 7. Condensed installation procedure for OUR balance (7075-T651, 12 x 350-ohm STC-13, 3 full bridges, 5 V) This is based on Class I Type 1 (p.6-10). Hobbyist substitutions are marked **[SUB]**. **Environment:** work at **72-80 deg F, RH at or below 40%** (p.1, PDF 7). [SUB] Use a small dehumidified room or an air-conditioned day. Do not bond on humid days. **A. Gauges and materials** - Gauges: MM **EA-13 or CEA-13 series, 350 ohm, 0.062 in grid** (for example 062AP or 062UW), from **one lot**. - Before installation, measure every gauge to 0.01 ohm with a 4-wire meter and **pair the closest values into each bridge**. This is a simplified form of LaRC's matching (p.11; Addendum 1). - Choose **pre-attached-lead or open-tab gauges with tabs pointing AWAY from the flexure root.** - Adhesive: **M-Bond 610** (closest to LaRC) or, as a simpler alternative, **M-Bond 600 or AE-10**. - **Do NOT use M-Bond 200 for a balance meant to be used over many months.** LaRC limits it to tests completed within 6 weeks (p.30). - Terminals: CPF-25C (the smallest LaRC uses) or MM CPF/CEG solder-terminal pads. - Wire: - Jumpers: AWG 36-40 solid copper (MM 134-AWP or 134-AQP). - Intra-bridge: AWG 32-36 stranded Teflon (MM 326-DTV or 330-FTE type). - Exit lead: shielded 4- to 6-conductor cable. - Solder: MM 361A-20R (or ordinary 63/37 rosin-core). Flux remover or isopropyl alcohol. - Coating: [SUB] **MM M-Coat A** (air-dry polyurethane) or **M-Coat D**, or Gagekote #8 if you can get it. Two thin coats. **B. Surface preparation (p.6-7)** 1. Deburr. **Break every sharp edge near a gauge.** 2. Degrease. [SUB] Use **MM CSM-2 degreaser or acetone** instead of the ENSOLV vapor degreaser. 3. Inspect under a loupe or stereo microscope for tool marks and cracks, especially at the 0.025 in plate roots. 4. Mask with tape. Abrade the gauge area. [SUB] Instead of 50-micron Al2O3 micro-blasting, use **wet-abrade with M-Prep Conditioner A (mild phosphoric acid) on 320-400 grit SiC paper**, then cross-hatch lightly. Use a hand micro-blaster (airbrush-type sand eraser) if available. - **Inside the 0.75 in axial window, use folded paper on a thin stick or a strip of abrasive cloth. Do not gouge or thin the 0.025 in plates.** Every 0.001 in removed changes the plate bending strain by about 8%. 5. Neutralize with **M-Prep Neutralizer 5A** (LaRC uses this on aluminum-matrix parts, p.36), then wipe with alcohol. 6. **Bond immediately.** Aluminum oxidizes quickly (p.35). **C. Layout** - Burnish layout lines lightly, outside the grid area. - LaRC holds **+/-0.005 in position** (p.7). - On the necks: - Two gauges sit side by side on a 0.50 in face. Center them symmetrically about the neck centerline so side-force bending cancels. - Place them at the same x-station top and bottom. - Put the solder terminals on the shoulders, **off the 0.60 in flexing neck**. **D. Bond and cure (p.7)** 1. Clean gauges and terminals and lay them face-down on glass. Pick up each with Teflon tape. Use the **tape-hinge method** (p.30) to position it, fold it back, apply adhesive, and fold it down. 2. Brush M-610 on the gauge and on the part. Let it **dry 10 min or more**. Lay the gauge down. 3. Build the stack: Teflon tape, then a silicone-rubber pad, then a backing block, then a clamp. - Pressure: **LaRC uses 60 psi** (p.7). [SUB] Use a spring clamp or MM GT-14 spring clamp, sized as pressure x pad area. For example, a 0.25 x 0.35 in pad needs about 5 lbf. 4. Cure. LaRC's schedule is **ramp at 6 deg F/min to 340 deg F, hold 1 h, cool in the oven, then post-cure 340 deg F for 2 h** (p.7). - [SUB] Use a **toaster or convection oven** with an independent thermocouple taped to the balance. - **See the 7075 temper flag (section 8, item 4)** before using 340 deg F. 5. Remove the clamp and inspect under magnification for voids, edge lift, and alignment. 6. Re-prep each later face just before bonding it (p.7, step 10). 7. Cure-cycle plan: - Cycle 1: neck tops. - Cycle 2: neck bottoms. - Cycle 3: both axial plates together (see section 8). - Then one post-cure for everything. **E. Wiring (p.7-8)** 1. Record each gauge's resistance at the gauge to 0.01 ohm. 2. Remove oxide from the terminals with an eraser. 3. Solder the jumpers (gauge to terminal), leaving a **small strain-relief loop**. 4. Remove the flux. Re-measure at the terminals. 5. **Complete each bridge on the balance**, using equal-length intra-bridge wires (p.8). Only 4 wires per bridge, or 6 with remote sense, then leave the metric part. 6. **Wires must lie flat in grooves or be tacked to non-flexing surfaces. They must never span the axial flexure gap or a neck as a free wire, and must never carry load** (p.8, item 9). 7. **Never route wire over a grid or a jumper** (p.8, item 10). 8. Clean all residue. **F. Electrical checks (p.8)** 1. Leakage to ground: **> 10 Gohm at 15 VDC**. - [SUB] A 15 V supply in series with the bridge-to-balance path and a DMM on its 10 Mohm voltage range. The DMM reading V = 15 x 10M/(10M + R_leak). **R_leak = 10 Gohm gives about 15 mV.** A reading below about 15 mV passes. - A hobby insulation tester also works. 2. Zero at 5 V excitation: - It should be within **+/-0.05 mV/V of the value calculated from the measured arm resistances**. - Overall zero within **+/-0.4 mV/V** (typical). Trim a large offset with Manganin wire or in the amplifier or software. 3. **Mechanical shunt check of each gauge:** press near the grid with a blunt tool, not a sharp probe (p.12-13), and watch for a sign-consistent response. 4. Hand-load each component (N, A, m). Check sensitivity and return to zero. **G. Stabilization and thermal checks (p.8-9)** 1. Soak **200 deg F for 4 h**, wired (p.8). - **Zero shift must be at or below 0.010 mV/V.** That is about 1% of our roughly 1 mV/V full scale. A larger shift means a bad bond or joint. - [SUB] Use the same oven, holding temperature below the solder and wire-insulation limits. 2. Apparent-strain run: - Place the balance -N up, supported at the sting end, inside a **cardboard or foil enclosure**. - Run 80 to 180 deg F (or 75 to 130 deg F, which better spans tunnel heating), ramping **at no more than 6 deg F/min** with a **15 min or longer soak**. - Target **0.005 mV/V per 100 deg F or less**. - [SUB] Rather than nickel-wire trimming, **mount a thermistor or RTD on the balance and correct the residual apparent strain in software**. STC-13 gauges on 7075 (CTE about 13 ppm/deg F) give a small residual. 3. Repeat the run. Two consecutive runs should agree **within 0.003 mV/V** (p.9). **H. Moisture-proofing (p.10)** 1. Clean with alcohol. 2. **Warm the part to about 100 deg F.** Brush **2 thin coats, 20 min or more apart.** - Use Gagekote #8 at 0.005-0.007 in total, or M-Coat A. - On the axial plates, keep the coat **thin and identical on both plates**, and **do not let it bridge a plate to a rail or a wire to a beam** (p.8, item 9; p.33). 3. Re-check: zero shift from coating **at or below 0.05 mV per 5 V** (p.14), and leakage still above 10 Gohm. 4. **Wet-brush test** (p.15): brush tap water over the gauges. - Leakage must stay above 10 Gohm. - Spurious output must be **at or below 0.075 mV at 8 V excitation** (about 0.0094 mV/V; with our 5 V, keep it at or below about 0.047 mV). 5. Re-run the apparent-strain check with the coating on. **I. Documentation:** keep a log book (p.15). Record: - Gauge lot and resistances. - Cure cycles and temperatures. - Zeros at every stage. - Leakage readings. - Deviations. --- ## 8. Flags that affect our geometry and design (balance_sizing.py) 1. **Clamping inside the 0.75 in axial window.** LaRC cures under **60 psi through silicone pads** (p.7). The 0.025 in x 0.75 in x 0.75 in plates are very flexible: k_ax is low, and a 5 lbf clamp load on one plate face would bend or yield it. - **Clamp the two inner-face gauges in one self-reacting stack:** plate A, gauge, Teflon, rubber pad, rigid spacer, rubber pad, Teflon, gauge, plate B, with **backing blocks on the outer faces of both plates**, squeezed by one small clamp. - Window budget: 2 x (tape ~0.003 in + rubber pad ~0.06-0.10 in) + spacer. **0.75 in is enough for the pads, but tweezers, tape-hinge handling, a soldering tip, and an inspection loupe all need access.** Plan for **0.75 in as a practical minimum**. Do not shrink the window. Consider making the window open on both sides in y, with no side webs, for access. - Alternatively, **put the axial gauges on the OUTER faces** of the plates, which gives free access. Grid strain at a given distance from the root is the same in magnitude with the sign reversed, so bridge arms just swap. 2. **Gauge near the plate root (g_off = 0.06 in).** - A 0.062 in grid has backing that extends past the grid end. The end-tab margin is typically about 0.03-0.05 in (check the MM data sheet for the chosen pattern). The grid center therefore cannot sit closer than about 0.06-0.08 in plus the root-fillet radius. - The backing must lie flat, not up the fillet, and the **tabs must point away from the root**. - With +/-0.005 in placement tolerance (p.7) and a steep strain gradient near the root, expect bridge-to-bridge sensitivity spread. Calibration absorbs it, but the **plate-to-plate mismatch also drives the N-to-A interaction** that the script estimates at 2% mismatch. - Consider moving g_off to about 0.08-0.10 in and recomputing t_p and strain. 3. **Terminal and wire space.** - CPF terminals and jumpers need room **off the flexing elements** (p.6-8). The 0.60 in necks have no room for terminals next to a gauge of about 0.2 in backing length; put the terminals on the shoulders or rails. - The forward-neck bridge wires must cross the axial flexure to reach the sting. **LaRC forbids spanning open areas and letting wires carry load** (p.8). For our 2 lbf full-scale axial force on a compliant flexure, that matters a lot. - Provide a **wire channel or groove along the side of the axial section** and use the finest practical wire (AWG 36-40) with a slack loop. Complete the forward bridge locally so only 4-6 wires cross. - Expect wire-crossing hysteresis to show up in the A calibration. 4. **Heat cure versus 7075-T651 temper.** - LaRC's M-610 schedule is **340 deg F x 1 h per cycle plus a 2 h post-cure** (p.7). With 3 or more cycles, that is about 5 h or more at 340 deg F. - 7075-T6 is aged at about 250 deg F. Repeated exposure at 340 deg F begins **over-aging, moving toward T7x, which lowers yield strength** (an engineering judgment, not stated in this document; LaRC balances are usually maraging or PH steel). - Use **the lower-temperature, longer-hold M-610 schedule**. This document shows M-610 cured at **250 deg F for 3 h at 40 psi, with a 275 deg F post-cure** (p.32, PDF 38). Also see the MM M-610 bulletin. - Or use a room-temperature or low-temperature epoxy (AE-10, M-Bond 600 at its lower schedule). - In either case, **re-check the stress margins in balance_sizing.py** with a derated yield strength if 340 deg F is used. 5. **Coating stiffness on thin plates.** A heavy RTV coat (0.025-0.035 in, p.28) would be comparable to the 0.025 in plate thickness and would add stiffness and hysteresis. **Use only a thin coat (0.005 in or less) and keep it identical on both plates** (p.33 "reinforcing effect on thin surfaces"). 6. **Zero and thermal budgets versus our signal.** - Our full-scale output is about 1 mV/V (5 mV at 5 V). LaRC's typical **+/-0.4 mV/V** initial zero is up to 40% of full scale, so check the amplifier offset range. - The soak criterion of **0.010 mV/V** is about 1% FS. - The apparent-strain tolerance of **0.005 mV/V per 100 deg F** gives about 0.1% FS for a 20 deg F tunnel temperature rise. Our 0.05% FS resolution target needs a **balance temperature sensor plus software correction**, or a nickel-wire trim. 7. **Excitation.** LaRC sets the zero and thermal checks at the service excitation (p.8). Our 5 V on 350 ohm gives about 18 mW per gauge, which is fine for aluminum. Run all checks at 5 V.