BIRK-A W 69-0883

A STRAIN GAUGE LOAD CELL FOR THE MEASUREMENT OF COMPRESSIVE LOADS UNDER CONDITIONS OF HIGH HYDROSTATIC PRESSURE

by

A. W. BIRKS and C. LUDLOW

The Queen's University, Belfast

The design and development of a strain gauge load cell suitable for use under high hydrostatic pressures is described and the method of bonding the resistance strain gauges is gil'en in detail.

pressure gauge connect ion

load

specimen 11/51:77~~~~

dial gau

Blackhawk hydraulic ram

inlet valve

low pressure oil inlet

Fig. 1. General arrangement of apparatus for com­pression testing under hydrostatic fluid pressures up to 100 000 l~f/in2.

Introduction The load cell was designed to measure the com­

pressive load imparted to a rock sample subjected to a very high hydrostatic pressure of up to 100000Ibf/in 2.

Since the compressive end load was applied to the specimen by means ofa ram (Fig. I), the rod of which passes through high pressure seals, a direct measure­ment of the load from the ram pressure would give ri se to errors owing to frictional losses at the sliding seals. Moreover, the frictional losses could be expected to vary as the hydro-static pressure varied and also as wear occurred. Hence it was considered desirable to measure the load by means of a load cell inside the high pressure enclosure.

Requirements The requirements for the load measuring system

were: (a) a maximum compressive load of 20 toof, (b) a sensitivity of not less than 10 m V for full range, (c) the effects of off-centre loading to be as small as

possible, (d) the overall diameter and the clearance volume to

be kept to a minimum, (e) a specimen anvil diameter of 0·750 in.

Design of Load Cell The load cell consists of an axially loaded hollow

circular cylinder and ttTe-load is communicated to the cylinder by means of a core bar and taken up by a flange which also carries the high pressure fluid seals. The details are illustrated in Fig. 2. Resistance strain gauges are bonded to the cylinder and compensating gauges are bonded to an un-strained cover cylinder. The gauges are connected to form one half of a Wheat­stone bridge circuit and the leads are brought out through high pressure cone seals.

The basic configuration is similar to that used by Pugh & Gunn 1, except that three axial strain gauges radially spaced at 120 degrees are connected in series. Tinsley foil type gauges were used, type number LSG 1/5/eN/E, 120 ohm, 1.0 in. active leng·th.

The gauges were bonded by means of Araldite strain gauge adhesive with hardener HY 956, the proportion being 100 parts resin (by weight) to 10 parts hardener.

~ I ~ ~ /active gouges compensatingr~ I" '/ l " I · d f\.. V ~ on cy In er gouges on cover L " ~ ~

~V/V~ f'..VV'"

a~VV~~ ~~r;~~; ~v~~

load

Fig. 2. Load cell.

The adhesive was cured at 60 °C for 12 hours (oven cooled). Printed circuit terminals were fitted and the assembly was further cured at 60 °C for 5 hours. The surface pretreatment will be described separately for each load cell, since it was changed during develop­ment testing.

Interconnecting leads from gauge to gauge were of 38 SWG Lewmex insulated wire and the connections between load cylinder gauges and compensating gauges were of 7/0048 P.T.F.E. insulated flexible wires (B.I.c.c. Ltd. Spec. EL. 1930).

"

10 /' V

V • ,/"

~ . v V

,../ . V

2 V V

load ton '

0 ./ 2 • • • 10 " " ,. 18 20

- 1'00 tqtm 2Cij'XIO X¥lOO ~J)OO 5q)OQ 6QOO!l 7O,COO ao.ooo 9(1.000 1OQ,000 hy dr~IQtic p!'Hsurt.· Ibl /in 2

Fig. 3. Calibration curves.

2

Calibration The load cell was calibrated in an Arnslt:r testing

machine of 35 tonf capacity, type 699. A d.c. constant­current supply (10 V nominal) was used and the bridge output was recorded on a digital voltmeter (Solartron Type LM 1420) with a resolution of 2·5 fLV/digit.

The load cells were load cycled ten times, each full load cycle taking about 10 minutes. In each case, no zero shift was observed after the third load cycle. Off­centre loading effects were assessed by carefully off­setting the loading bar which was in contact with the anvil (core bar) of the load cell. A typical calibration curve is shown in Fig. 3.

In order to determine the effects of hydrostatic pressure on the load cell characteristics, the high pressure containment of the rock sample testing machine was utilised, the pressure being generated by means of an intensifier.

No axial compressive load was applied and the bridge output was recorded at increments of pressure up to a maximum of 100 000 Ibf/in 2 approximately. The pressure was measured by means of an Astra­gauge, of the Bourdon-tube type, the calibration of which was verified against a dead-weight piston gauge.

The bridge output due to hydrostatic pressure only is given on the same graph as the load calibration curve (Fig. 3) so that the true axial load can be deter­mined for any combination of axial load and hydro­static pressure.

Load Cell Development The type of strain gauge, the adhesive and the curing

procedure remained as described, but the surface preparation of the various load cells was changed and different oils were used in the machine. It is therefore convenient to describe the performance and subsequent changes separately for each load cell.

Load Cell No. 1

The surface finish was cross-hatched with silicon carbide paper and the surface was degreased and cleaned with acetone ultrasonically immediately prior to the bonding of the gauges. The gauges protected by thin polythene sheet were tightly bound with nylon cord which was removed after curing of the adhesive. This technique gives a very thin glue line. Calibration showed linearity within O· 5 per cent of full-range output and the effect of an offset of the loading by 0·125 in. was barely measurable. The unit was considered satisfactory and was assembled into the rock testing machine. Its pressure sensitivity was determined using Plexol 201 oil.

The hydrostatic pressure was increased up to 100000 Ibf/in 2 and the strain gauge output was noted.

This was about 1· 5 mY, but after several pressure applications a marked zero shift was observed, a fault eventually traced to electrical leakage at the high pressure lead-throughs. The leakage was due to con­tamination by brake fluid from the initial proof test.

Further tests up to 50000 lbf/ in 2 were satisfactory until at the eighth application of pressure the strain gauges became faulty. Examination of the load cell revealed that the gauges had separated from the surface of the cylinder and that the leads and fibreglass terminals had lifted. The compensating gauges were not affected.

Load Cell No.2

The surface finish was changed to a 120/220 mesh grit vapour blast and the surface, as before, was cleaned with acetone immediately before bonding of the gauges.

The calibration was completed with results very similar to those obtained for the first cell. Testing was then started with Plexol 201 oil and with loads and pressures similar to those for No. 1 cell. Eleven tests were taken before the load cell output again indicated that a failure had occurred. The maximum pressure applied was 70000 Ibf/in 2.

Examination revealed that the epoxy resin backing of the strain gauges was attacked by the Plexol 201 oil; the bond appeared to be intact and the Araldite adhesive was unaffected.

The choice now was either to change the type of gauge or to use a different oil. It was decided to change the oil.

Load Cell No.3

The surface preparation was the same as that for cell No.2. Calibration gave very similar results and tests were taken with castor oil (Castrol castor oil, code "First Cut 0311") which was known from previous work to have a pressure freezing point of about 90000 lbf/ in 2 at room temperature.

Since castor oil is easily contaminated and can be electrically conductive, samples were checked on a conductivity bridge prior to filling of the rig.

The load cell completed twenty tests with loads up to 20 tonf and pressures up to 20 000 Ibf/in 2 before erratic output indicated failure. Before compressive testing with cast iron samples, the cell had also com­pleted nineteen pressure cycles up to 98 000 lbf/in 2.

Inspection revealed that the bonding was completely satisfactory but that one active gauge foil had separated from the backing and peeled back. A hair­line crack is thought to have formed in the foil and, as the oil drained off the surface, it allowed the foil to lift. However, the life of the load cell was considered

3

reasonable and there were no signs of attack by the castor oil.

Load Cell No.4

Re-gauged in exactly the same way as the cell No.3, this cell has up to now completed 56 tests at loads up to 7 tonf and pressures ranging from 14000 lbf/i n 2 to 100 000 Ibf/in 2. Frequent inspection has revealed no defects and a re-calibration on the Amsler machine has given results O· 7 per cent lower at full range than the original calibration.

Ancillary Equipment As an analogue display was preferred for the actual

tests, the read-out instrument used in conjunction with the load cells was a potentiometric indicator 3 with a circular scale and with a range O-lOmV. Span and zero controls are included in this instrument and a separate constant-current power supply provided by the manufacturers of the indicator energises the strain gauge bridge.

The compression (displacement) of the rock specimen is measured in addition by recording of the movement of the loading ram piston. This is done by means of a variable resistance potentiometer type transducer with a full range of 0·500 in and with a resolution of about 0·0005 in. The indicator is simply a micro-ampere meter used as a voltmeter, which measures the voltage developed across the potentio­meter slider, a I -4 V mercury cell supply being used for simplicity.

Conclusion The design of the load cell and the method of bond­

ing the foil strain gauges has proved satisfactory for compressive loads up to 20 tonf with a superimposed hydrostatic pressure up to 98 000 Ibf/in 2.

The authors acknowledge the help given by Messrs. Rolls Royce Ltd. , Dundonald, who kindly undertook the vapour blasting of the components of the load cell. The assistance of Mr. P. D. Peters (Department of Mechanical Engineering) and Mr. D. Wright (Depart­ment of Geology) is also acknowledged.

References 1. H. LL. D. PUGH and D. A. GUNN, " A strain gauge

load cell for use under high hydrostatic pres­sures." National Engineering Laboratory Report No. 143.

2. Plexol 201. Manufactured by ROHM and HAAS Co. and sold in the U.K. by Lennig Chemicals.

3. Type BPIO manufactured by CONTROL INSTRU­MENTS LTD .. Alfreton Road, Derby.