Name: how to build a vacuum spring-transport package for spinning rotor gauges
Uploaded: 2016-04-07T14:00:00
Description: Watch this Scientific Journal Video about How to Build a Vacuum Spring-transport Package for Spinning Rotor Gauges at JoVE.com

The authors are thankful for the help of the NIST neutron imaging facility instrument scientist Dr. Daniel Hussey for assisting us with neutron radiographs.

1. Procure Non-custom Parts for the Spring Transport Mechanism
<ol>
	<li>Procure the springs, threaded rod, standoffs, nuts, and washers. These items are listed in the List of Specific Materials/Equipment. Purchase the springs and standoffs before manufacturing the ball holder. Use 18-8 stainless steel (except 303), or preferably 316 stainless steel, for all materials.</li>
	<li>Procure rotor. The rotor is a ball-bearing 4.5 mm in diameter made of 440C stainless steel or E52100 alloy steel.</li>
	<li>Procure the right-...

     

<ol>
	<li>Fremerey, J. K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+spinning+rotor+gauge.">The spinning rotor gauge.</a> J. Vac. Sci. Technol. A. 3 (3), 1715-1720 (1985).</li><li>Jousten, K., Jousten, K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Chapter+13,+Total+Pressure+Vacuum+Gauges.">Chapter 13, Total Pressure Vacuum Gauges.</a> Handbook of Vacuum Technology. , 573-583 (2008).</li><li>Berg, R. F., Fedchak, J. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=NIST+Calibration+Services+for+Spinning+Rotor+Gauge+Calibrations.">NIST Calibration Services for Spinning Rotor Gauge Calibrations.</a> NIST Special Publication. , 250-293 (2015).</li><li>Messer, G., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Intercomparison+of+Nine+National+High-vacuum+Standards+under+the+Auspices+of+the+Bureau+International+des+Poids+et+Mesures.">Intercomparison of Nine National High-vacuum Standards under the Auspices of the Bureau International des Poids et Mesures.</a> Metrologia. 26, 183-195 (1989).</li><li>Jousten, K., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Results+of+the+regional+key+comparison+Euromet.M.P-K1.b+in+the+pressure+range+from+3+x+10-4+Pa+to+0.9+Pa.">Results of the regional key comparison Euromet.M.P-K1.b in the pressure range from 3 x 10-4 Pa to 0.9 Pa.</a> Metrologia. 42 (1A), 07001 (2005).</li><li>Jousten, K., Santander Romero, L. A., Torres Guzman, J. C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Results+of+the+key+comparison+SIM-Euromet.M.P-BK3+(bilateral+comparison)+in+the+pressure+range+from+3+x+10-4+Pa+to+0.9+Pa.">Results of the key comparison SIM-Euromet.M.P-BK3 (bilateral comparison) in the pressure range from 3 x 10-4 Pa to 0.9 Pa.</a> Metrologia. 42 (1A), 07002 (2005).</li><li>Yoshida, H., Arai, K., Akimichi, H., Hong, S. S., Song, H. W. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Final+report+on+key+comparison+APMP.M.P-K3:+Absolute+pressure+measurements+in+gas+from+3+x+10-6+Pa+to+9+x+10-4+Pa.">Final report on key comparison APMP.M.P-K3: Absolute pressure measurements in gas from 3 x 10-6 Pa to 9 x 10-4 Pa.</a> Metrologia. 48 (1A), 07013 (2011).</li><li>Fedchak, J. A., Bock, T. h., Jousten, K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Bilateral+key+comparison+CCM.P-K3.1+for+absolute+pressure+measurements+from+3+x+10-6+Pa+to+9+x+10-4+Pa.">Bilateral key comparison CCM.P-K3.1 for absolute pressure measurements from 3 x 10-6 Pa to 9 x 10-4 Pa.</a> Metrologia. 51 (1A), 07005 (2014).</li><li>Fedchak, J. A., Arai, K., Jousten, K., Setina, J., Yoshida, H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Recommended+practices+for+the+use+of+spinning+rotor+gauges+in+inter-laboratory+comparisons.">Recommended practices for the use of spinning rotor gauges in inter-laboratory comparisons.</a> Measurement. 66, 176-183 (2015).</li><li>Rohl, P., Jitschin, W. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Performance+of+the+spinning+rotor+gauge+with+a+novel+transport+device+as+a+transfer+standard+for+high+vacuum.">Performance of the spinning rotor gauge with a novel transport device as a transfer standard for high vacuum.</a> Vacuum. 38 (7), 507-509 (1988).</li><li>Fedchak, J. A., Scherschligt, J., Sefa, M., Phandinh, N. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Building+a+spring-transport+package+for+spinning+rotor+gauges.">Building a spring-transport package for spinning rotor gauges.</a> J. Vac. Sci. Technol. A. 33 (3), (2015).</li><li>Hussey, D. S., Jacobson, D. L., Arif, M., Coakley, K. J., Vecchia, D. F. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=In+Situ+Fuel+Cell+Water+Metrology+at+the+NIST+Neutron+Imaging+Facility.">In Situ Fuel Cell Water Metrology at the NIST Neutron Imaging Facility.</a> J. Fuel Cell Sci. Technol. 7 (2), 021024 (2010).</li><li>Chang, R. F., Abbott, P. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Factors+affecting+the+reproducibility+of+the+accommodation+coefficient+of+the+spinning+rotor+gauge.">Factors affecting the reproducibility of the accommodation coefficient of the spinning rotor gauge.</a> J. Vac. Sci. Technol. A. 25 (6), 1567-1576 (2007).</li></ol>

The objective was to design a spring-transport mechanism with a sufficient holding force such that the rotor would remain immobilized during transport. Designing a robust spring-transport mechanism is not enough to insure the rotor will remain immobilized because, for example, dropping the mechanism from tall height onto a hard surface can produce an enormous shock. The force on the rotor can be greatly reduced by packaging the spring-transport mechanism such that it gently decelerates over a distance within the package,...

The spinning rotor gauge (SRG) is a high-vacuum gauge used to determine vacuum pressures in the range of 1.0 x 10-4 Pa to 1.0 Pa. It is fundamentally a rotating steel ball that is suspended between two permanent magnets. Electro-magnets are used to rotate, or &#34;spin-up&#34;, the ball to some frequency (typically 410 Hz); the ball is then allowed to freely rotate, but the rotation rate will decrease over time because of collisions of gas molecules in the vacuum system with the ball surface. Vacuum pressure is thus related to the deceleration rate of the steel ball or rotor. Figure 1 shows the essential elements of the SRG: the rotor, thimble, head with connecting cable, and electronic controller. The rotor, or ball, is contained within the thimble during operation and is normally not handled by nor is visible to the SRG user. The thimble is connected to the vacuum system. To operate the SRG, the head is slipped over the thimble. The head contains two permanent magnets and several sets of wire coils used for vertical and horizontal stabilization, driving the rotor, and sensing the rotation. The electronic controller interprets the signal from the sensing coil so that a pressure measurement can be made. For a rotor with ideal surface conditions, the deceleration rate is related to the vacuum pressure by fundamental physics. To make absolute pressure measurements using an SRG, a calibration factor, known as the effective accommodation coefficient, must be determined. The effective accommodation coefficient depends on the real surface conditions of the rotor, such as the roughness, adsorbed gases, and scratches. These factors tends to be stable over the course of its use. Additional details of SRGs can be found in other references.1-3
The SRG is used in applications where absolute vacuum measurements are required. For example, calibration laboratories often use SRGs as an absolute vacuum standard. In this case, high-vacuum gauges are calibrated by comparing their reading to that of the SRG. In turn, the SRG standard must be periodically calibrated by shipping the SRG to a primary calibration laboratory to have its accommodation coefficient re-determined. Primary calibration labs are usually National Metrology Institutes such as the National Institute of Standards and Technology (NIST). The primary lab determines the SRG accommodation coefficient by comparing its reading to a primary vacuum standard, and then returns the SRG to the &#34;secondary&#34; calibration lab. The SRG is also used as a transfer standard for the comparison of standards between calibration laboratories or National Metrology Institutes. In this application, the SRG is transported domestically or internationally between the various laboratories.4-8 During shipment, events can occur that change the accommodation coefficient. Prior to shipment, the rotor must be de-suspended and the head is removed; the rotor then rests on the interior wall of the thimble. During transport, the rotor surface is subject to change from the mechanical action between the rotor and thimble due to vibrations and shocks, or the surface may change due to the exposure of the rotor to atmospheric gas and humidity. These changes affect the long-term stability of the accommodation coefficient. Ideally, the rotor should remain in vacuum and immobilized during transport.
Historically, SRGs have been used as transfer standards in key comparisons of vacuum standards among national metrology institutes, where SRGs are internationally transported many times among the various institutes.9 During an early key comparison, it was found that the long-term stability of the SRG accommodation coefficient could be improved by utilizing a spring-transport mechanism which both immobilized the rotor and kept it under vacuum during transportation.1,10 Since then, the spring-transport mechanism has been used many times in international key comparisons. A recent study of the historical data showed that 90% of these comparisons had stabilities better than 0.75%, and 70% had stabilities of 0.5%.9 Therefore, using a spring-transport mechanism will, in most cases, yield a stability that is more than sufficient for most applications.
Until now, there has been little guidance in the literature on how to build a spring-transport mechanism. Early versions of these devices have been known to fail to fully immobilize the rotor, due to a combination of being insufficiently designed for robustness, and being mishandled during shipment. These early lessons show that it is important both to build a robust spring-transport mechanism, and to properly package it in a way that minimizes shock during transport. This later point is critical but often ignored. Here we will describe the construction of a robust spring-transport mechanism in addition to a properly constructed transport package. Our design is based on a few simple, tested, engineering principles that enable the construction of a durable spring-transport package that minimizes the possibility of failure during transport. We also describe our tests of the robustness of our design. Additional details of the test methods can be found in Fedchak et al. (2015).11

<table>
	<tbody>
		<tr>
			<td>Spring, 3 N/m</td>
			<td>Lee Spring (www.leespring.com)</td>
			<td>LC 042C 18 S316</td>
			<td>Outside diameter 0.240 in, Wire Diameter 0.042 in, Rate 17.1 lb/in, Free Length 2.25 in, Number of Coils 29.3&#160;&#160;</td>
		</tr>
		<tr>
			<td>8-32 threaded rod, 316 stainless steel</td>
			<td>McMaster-Carr (www.mcmaster.com)</td>
			<td>90575A260</td>
			<td>Type 316 Stainless Steel Fully Threaded Stud 8-32 Thread, 3&#34; Length.&#160; Cut to length specified in protocol</td>
		</tr>
		<tr>
			<td>standoffs, 8-32 Screw Size</td>
			<td>McMaster-Carr (www.mcmaster.com)</td>
			<td>91125A140</td>
			<td>18-8 Stainless Steel Female Threaded Round Standoff, 1/4&#34; OD, 1/4&#34; Length, 8-32 Screw Size</td>
		</tr>
		<tr>
			<td>nuts, 8-32</td>
			<td>McMaster-Carr (www.mcmaster.com)</td>
			<td>90205A309</td>
			<td>316 SS Undersized Machine Screw Hex Nut 8-32 Thread Size, 1/4&#34; Width, 3/32&#34; Height</td>
		</tr>
		<tr>
			<td>Split Lock-Washers, 316 Stainless Steel</td>
			<td>McMaster-Carr (www.mcmaster.com)</td>
			<td>92147A425</td>
			<td>Type 316 Stainless Steel Split Lock Washer NO. 8 Screw Size, .3&#34; OD, .04&#34; min Thick</td>
		</tr>
		<tr>
			<td>Steel Rotor</td>
			<td>McMaster-Carr (www.mcmaster.com)</td>
			<td>9292K38</td>
			<td>&#160;Bearing-Quality E52100 Alloy Steel, Hardened Ball, 4.5 mm Diameter</td>
		</tr>
		<tr>
			<td>Right-Angle Valve</td>
			<td>VAT Valve (www.vatvalve.com)</td>
			<td>54132-GE02-0001</td>
			<td>Easy-close all-metal angle valve, DN 40 (1.5&#34;)</td>
		</tr>
		<tr>
			<td>Shipping Container</td>
			<td>Allcases, Reekstin &#38; Associates (www.allcases.com)</td>
			<td>REAL1616-1205</td>
			<td>Zinc Hardware w/Zinc Handles, Rotationally Molded, light-weight, high-impact, Polyethylene Case with protected recessed hardware.&#160; 15.75&#34; X 15.88&#34; X 16.45&#34;</td>
		</tr>
		<tr>
			<td>Ester Foam</td>
			<td>Carry Cases Plus (www.carrycasesplus.com)</td>
			<td>ES-PAD 3&#34; Thick</td>
			<td>3&#34; Thick, 2lb Charcoal Ester Foam Pad, 24&#34; x 27&#34;.&#160;</td>
		</tr>
		<tr>
			<td>Ester Foam</td>
			<td>Carry Cases Plus (www.carrycasesplus.com)</td>
			<td>ES-PAD 1&#34; Thick</td>
			<td>1&#34; Thick, 2lb Charcoal Ester Foam Pad, 24&#34; x 27&#34;.&#160;</td>
		</tr>
		<tr>
			<td>Egg-carton ester foam</td>
			<td>Carry Cases Plus (www.carrycasesplus.com)</td>
			<td>ES-CONV</td>
			<td>ES-CONV, 2lb, 24&#34; x 27&#34; x 1 1/2&#34;.&#160; &#34;egg-crate&#34; ester foam.&#160;</td>
		</tr>
		<tr>
			<td>Foam Cutout, PE foam</td>
			<td>Willard Packaging Co. (www.willardpackaging.com)</td>
			<td></td>
			<td>Custom Foam Cutout.</td>
		</tr>
		<tr>
			<td>Spinning Rotor Gauge&#160;</td>
			<td>MKS Instruments (www.mks.com)</td>
			<td>SRG-3</td>
			<td>Controller, head, and thimble.&#160; Custom thimble must be used for the spring-transport mechanism</td>
		</tr>
		<tr>
			<td>Custom thimble</td>
			<td>MDC vacuum Inc. (www.mdcvacuum.com)</td>
			<td></td>
			<td>drawing must be submitted for custom part</td>
		</tr>
		<tr>
			<td>Detergent</td>
			<td>Fisher Scientific Co (www.fischersci.com)</td>
			<td>04-320-4</td>
			<td>Sparkleen 1 Detergent</td>
		</tr>
		<tr>
			<td>Acetone</td>
			<td>Fisher Scientific Co (www.fischersci.com)</td>
			<td>A18-S4</td>
			<td>Acetone (Certified ACS)</td>
		</tr>
		<tr>
			<td>Ethanol</td>
			<td>Warner-Graham Company (www.warnergraham.com)</td>
			<td>190 proof USP</td>
			<td>190 Proof USP ethyl alcohol</td>
		</tr>
		<tr>
			<td>Bolt set for valve</td>
			<td>Kurt J. Lesker (www.lesker.com)</td>
			<td>TBS25028125P</td>
			<td>&#160;B,N&#38;W SET,12 POINT,(25)1/4-28X 1.25&#34;FOR 2.75&#34;THRU,SILVER PLAT&#160;</td>
		</tr>
		<tr>
			<td>Silver-plated copper gaskets</td>
			<td>Kurt J. Lesker (www.lesker.com)</td>
			<td>GA-0275LBNSP</td>
			<td></td>
		</tr>
		<tr>
			<td>Spring Assembly (welding)</td>
			<td>Omley Industries, Inc. (www.omley.com)</td>
			<td>N/A</td>
			<td>The machine work and welding were done in NIST&#39;s shop. However, Omley industries was used as an alternative for welding the spring assembly.</td>
		</tr>
	</tbody>
</table>

how to build a vacuum spring-transport package for spinning rotor gauges

The spinning rotor gauge (SRG) is a high-vacuum gauge often used as a secondary or transfer standard for vacuum pressures in the range of 1.0 x 10-4 Pa to 1.0 Pa. In this application, the SRGs are frequently transported to laboratories for calibration. Events can occur during transportation that change the rotor surface conditions, thus changing the calibration factor. To assure calibration stability, a spring-transport mechanism is often used to immobilize the rotor and keep it under vacuum during transport. It is also important to transport the spring-transport mechanism using packaging designed to minimize the risk of damage during shipping. In this manuscript, a detailed description is given on how to build a robust spring-transport mechanism and shipping container. Together these form a spring-transport package. The spring-transport package design was tested using drop-tests and the performance was found to be excellent. The present spring-transport mechanism design keeps the rotor immobilized when experiencing shocks of several hundred g (g = 9.8 m/sec2 and is the acceleration due to gravity), while the shipping container assures that the mechanism will not experience shocks greater than about 100 g during common shipping mishaps (as defined by industry standards).

All of the components of commercial SRG are shown in Figure 1. This includes the rotor, thimble, head containing the permanent magnets and wire coils used for suspension and pickup, and the electronic controller. The small spring shown (Figure 1c) is used to retain the ball in the thimble; this retainer spring is not used in the spring-transport mechanism. The commercial controller and head are used in the spring-transport mechanism. The tines from the co...

Watch this Scientific Journal Video about How to Build a Vacuum Spring-transport Package for Spinning Rotor Gauges at JoVE.com

How to Build a Vacuum Spring-transport Package for Spinning Rotor Gauges

Here we describe how to build a robust spring-transport mechanism for a spinning rotor gauge. This device securely immobilizes the rotor and keeps it under vacuum during transportation. We also describe packaging that minimizes the risk of damage during transport. Tests show our design works for typical shocks during transport.

The spinning rotor gauge (SRG) is a high-vacuum gauge often used as a secondary or transfer standard for vacuum pressures in the range of 1.0 x 10-4 Pa to ...

The overall goal of this procedure is to build a spring-transport mechanism for the transport of spinning rotor gauges. The design presented here has been tested for robustness and provides the user with better long-term stability of the calibration factor. Spinning rotor gauges or SRGs are often used as transfer standards but damage to the rotor's surface received during transport can change their calibration factor also known as the accommodation coefficient.The advantage of using the spring-transport mechanism is that the rotor is immobilized and under vacuum during transport thus minimizing the possibility of changing the rotor's surface. Spring transport mechanisms have been used by a few national metallurgy institutes for more than 20 years but this example has been known to fail and the design today has remained unpublished. We realized that it's important both to build a robust spring-transport mechanism and to properly package it in a way that minimizes shock during transport.The text protocol reviews several precursor steps to preparing the spring-transport mechanism which include fabricating the parts for the mechanism and cleaning out the vacuum components. For the assembly, wear nitrile, vinyl, or latex gloves and never accidentally handle the vacuum parts bare handed. Several tools are needed.Be sure they are cleaned with ethanol or follow the procedure as described for vacuum parts. Additional parts include the valve, thimble, ball bearing, 18-millimeter 8-32 studs, thread adapters, nuts and lock washers, three-millimeter Allen wrench, and masking tape. With everything in order, proceed with the assembly.Begin the assembly by closing the right angle valve. Looking through the valve ports, identify the M6 stud centered on the valve seat. Then, screw the adapter snugly onto the M6 stud.Use a UHB clean wrench or pliers to get the adapter as snug as possible. Next, insert the 8-32 stud into the other end of the adapter and tighten it as snugly as possible. The 8-32 stud should extend through the adapter and push against the M6 stud.By making certain that the 8-32 stud pushes firmly against the M6 stud on the valve seat, we ensure that the adapter will be held firmly in place. Place a lock washer on the 8-32 stud and tighten a nut onto it using a clean socket driver or pliers. Next, screw a second nut onto the 8-32 stud until it nearly touches the first.Then, place a lock washer on the 8-32 stud by the standoff followed by the spring ball holder and tighten the holder until it compresses the lock washer. Now, check the length adjustment of the spring assembly. First, place the rotor in the custom thimble.Then attach a magnet to the custom thimble exterior and fix it in place with masking tape. Next, slide the thimble over the spring ball holder until the ball holder touches the ball and measure the gap between the valve flange and the thimble flange. If the gap is in the range of two to six millimeters it is acceptable.Otherwise rotate the standoff to adjust the gap. Three millimeters is ideal. Making certain the gap is within two to six-millimeter range ensures the mechanism will function as designed and will hold the bowl with sufficient force.Next, remove the thimble flange assembly and set it aside. Then, tighten the nut against the standoff and open the valve. Continue the assembly by attaching the two tines to the flange.Secure them with a lock washer and bolt provided by the commercial spinning rotor gauge. Remove the rotor by removing the masking tape and magnet and carefully allowing the rotor to roll out of the thimble. Now, rotate the tines so that they are square with one another and check their straightness by slipping the spinning rotor gauge head over the thimble.The head should slide on freely. Then, give the two bolts a final tightening and recheck for alignment. Place the rotor back into the thimble and secure it with the magnet and masking tape.Open the valve by turning it counterclockwise. The next step is to place a copper or silver-plated copper gasket on the valve port. Place the rotor into the thimble if it's not already there.Now, slide the flange thimble assembly over the spring ball holder. Orient the flange so that the set screw in the tines will point downwards when the valve is attached to the chamber. Finally, using quarter 28 bolts and nuts secure the flange to the valve.Then, close the valve and remove the magnet and tape holding the ball. This completes the spring-transport mechanism assembly. To use the spring-transport mechanism it must be properly mounted and the rotor must be properly suspended.This should only be performed by those who know how to use the spinning rotor gauge, the controller electronics, and are familiar with high vacuum technology. The first step is to unpack the spring-transport mechanism from its shipping container. Then, remove the endcap and foil from the open port.Next, using a new copper or silver-plated copper gasket and a quarter 28 bolt set, attach the open port to a DN40 port on the vacuum chamber. The orientation of the valve will depend on the orientation of the tines. When oriented properly, the tine with the set screw points downward.Then, slip the head over the thimble and orient the thimble vertically to within two degrees of accuracy. Now, evacuate the vacuum chamber to less than one one-thousandth pascal. Then, slowly open the spring-transport mechanism valve.Finally, attach the head to the controller, turn on the controller, and suspend the ball. After assembling the spring-transport mechanism, it was packed in foam inserts. The shipping container allows for 7.6 centimeters of foam surrounding the mechanism.Ester foam was packed above and below and PE foam was packed on the sides. In this configuration, the shock experienced by the spring transport mechanism was measured at less than 100 g even when dropped from 152 centimeters. If the foam was compressed during packing, even by a small amount, the shock increased by about 40%In this case, the foam was compressed roughly 1-1/4 centimeter over a span of 22-3/4 centimeters.After performing over 180 drop tests of one prototype model, a neutron radiograph image was made of the mechanism. The mechanism still worked, the ball remained secure to the thimble with the valve closed. Note that the valve on this prototype is smaller than the one in the protocol and is no longer manufactured.Once all the parts have been procured, the spring-transport mechanism can be assembled in just a few minutes. It's important to procure the custom thimble given in our design. The commercial thimbles do not have the proper length or interior geometry to completely immobilize the ball.Machine drawings of our custom parts are available as supplemental material. As described in our manuscript, it is critical to transport the spring-transport mechanism in a shipping container with at least 7.6 centimeters of foam packing surrounding it. This ensures that the rotor will remain immobilized during common shipping mishaps.

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