SANS 3001-NG Standards

JOSEPH VAN HOUTEN

Technical Materials Specialist

October 2015

CONTENT

Part Description Applies to

NG1 Administration, handling

and maintenance of

a nuclear density gauge.

Everyone

NG2 Validation of standard

calibration blocks for

nuclear density gauges.

Owner of standard calibration

block set

NG3 Calibration of a nuclear

density gauge.

Agent or supplier

NG4 Verification of a nuclear

density gauge.

Agent or supplier

NG5 Determination of in situ

density using a

nuclear density gauge.

Everyone

A pictorial illustration of the working of the NG Series is

given in figure (i). It will be noted in the figure that

calibration is only required if the gauge fails the

verification test. If after calibration the gauge still fails

the verification test, calibration is to be repeated. If

this cycle continues the gauge needs to be carefully

inspected, maintained and if appropriate repairs carried

out. The gauge can only be used for field density tests

with a current verification certificate.

The series recognises that for the roads industry the

direct transmission depth modes 100 mm, 150 mm and

200 mm are critical. Provision is made for a verification

certificate where non critical modes do not comply.

The use of the gauge for moisture contents and asphalt

densities is not recommended when checking

conformance with specifications.

Due to the large volume of nuclear gauges in the

country and the lack of sufficient government

laboratories with testing capacity, the series is based on

self-regulation by the Agents and Suppliers of nuclear

density gauges; using the Standards NG2, NG3 and NG4.

In the event of a problem with a particular gauge

verification this can be tested by having the gauge

verified by an Agent or Supplier who was not involved in

the original verification.

Agents and Suppliers can either have their own sets of

standard blocks complying with NG2; or they may make

use of an independent set of standard blocks providing

that the set has been validated according to NG2.

The following pages give a brief overview of each of the

five standards, their use and for whom they are

intended.

FIGURE (i): PICTORIAL OVERVIEW OF NG SERIES

SANS 3001-NG1 (Requirements of ownership)

NG1 deals with the legal and health requirements for owning and operating a gauge.

Applies to: Owners of nuclear gauges

The ownership, responsibilities and use of a nuclear density gauge (group IV hazardous substances) are regulated in terms of the relevant national legislation (The Hazardous Substances Act No 15 of 1973) and the requirements of the Department of Health. Should any conflict arise due to the interpretation of the requirements of the legislation and this part of SANS 3001, the legislation takes precedence. Although the quantity of radioactive material contained by the gauge is small, the owner and operator of a gauge are to be familiar with and comply with the requirements of the legislation.

Applies to: Other parties

Clients, Engineers and Contractors should be aware of the obligations of ownership and where appropriate see that these are correctly applied.

ADMINISTRATION covers:-

1. The Holder of Authority (ie the buck stops here!).

2. Responsibility.

3. Radiation Protection Officer (RPO).

4. Internal rules outlining correct working procedures.

5. Records, registers and log books – covering Standard counts, Maintenance and Verification of the gauge.

6. Disposal of the gauge.

7. Offences.

HANDLING:- health and safety issues.

TRANSPORT:- correct procedures for transporting gauges (often ignored) and the correct documents to be kept with the gauge including emergency procedures.

STORAGE:- how and where the gauge may be stored.

MAINTENANCE:- annual maintenance and leak test certificate.

VERIFICATION AND CALIBRATION:- Verification is required when new, after each annual maintenance or after repairs. Calibration is only necessary when Verification fails.

BRIEF SUMMARY

SANS 3001-NG2 (Validation of standard calibration

blocks)

Applies to:

Owners of standard calibration block sets.

Agents who Verify and Calibrate gauges need to be aware of the need for validation of standard calibration block sets to the CSIR BE block set.

NG2 covers the validation of a set of standard blocks. Previously various bodies in South Africa held sets of standard calibration blocks which were not linked to one another. Because of the nature of the gauge and the empirical correlations applied to arrive at reasonable densities for voided materials in the field; no one has been able to provide a traceable density related to solid objects (ie the blocks). As a result, depending on the suppliers set of standard blocks, gauges supplied in South Africa varied because there was no national bench mark.

This standard gives a procedure to link the sets of standard blocks to the set held by the CSIR, Built Environment in Pretoria.

The following is an important explanatory extract from the Principles paragraph in the standard:-

Nuclear density gauges do not provide a direct reading of the density of a material. The gauge emits gamma radiation from a Cesium source indirectly in the backscatter mode, or from a probe in the direct transmission mode which passes through the material. The radiation, having passed through the material, is measured by detectors located in the base of the gauge and converted by a microprocessor into a density reading, using an empirical correlation. The denser the block the smaller the amount of radiation that is measured in a set time.

Validation of a set of standard blocks is carried out by comparing the nuclear density gauge count and wet density readings on the set of standard calibration blocks, with counts and wet density readings on a reference set of blocks held by the relevant national body (CSIR BE), ranging from 1 500 kg/m3 to 2 700 kg/m3. The differences between the two sets of wet density readings are subtracted from the assigned densities for the reference set. The densest block consists of granite or aluminium, while the other composite blocks are made of alternating laminations of aluminium and polymer. The blocks are given assigned values because experience has shown that correlations based on measurements of solid material differ from those taken on voided materials (typical of materials as measured in the field).

Type 1 gauges have an inbuilt moisture adjustment, and identify the

polymer components of the composite blocks as moisture and

accordingly reduce the displayed wet density. For the A block,

having a density of between 1 500 kg/m3 and 1 600 kg/m3, this

reduction is approximately 35 kg/m3. For the B block having a

density of between 2 000 kg/m3 and 2 100 kg/m3, this reduction is

approximately 15 kg/m3. The type 2 gauge has no inbuilt moisture

adjustment. As a result of the different performance in measuring

moisture between the two gauge types, the national reference set

of blocks held by the relevant national body (see foreword) has two

sets of assigned values as given in table 1. As an example the values

given in table 1 are those used for the Republic of South Africa.

Table 1 — Assigned wet density values for the CSIR BE set of blocks

1 2 3 4

Block Composition

Type 1 gauge Type 2 gauge

kg/m3 kg/m3

A Polymer/Aluminium 1 505 1 540

B Polymer/Aluminium 2 025 2 040

C Granite 2 620 2 620

SANS 3001-NG3 (Calibration of a nuclear gauge)

Applies to: Agents or Suppliers who Verify and Calibrate gauges.

Calibration is only required when a gauge fails the Verification test NG4. Some of the newer gauges have internal self-calibration procedures which are acceptable provided that the gauge passes the NG4 Verification test. Thus NG3 gives a manual procedure to calibrate a gauge and is not mandatory. The Standard requires that the set of standard calibration blocks used in the test has been validated against the National set at the CSIR BE (see NG2).

The manual calibration process consists of taking a series of readings on the three standard calibration blocks. The gauge counts are translated into wet density and moisture content using empirical algorithms. The density algorithm contains three constants that can be adjusted for each gauge reading mode (backscatter, 50 mm depth and so on).

The constants are calculated using the following:

a) an equation supplied by the gauge manufacturer;

b) the average counts on the three calibration blocks;

c) the standard reference block counts (see SANS 3001-NG2); and

d) the assigned wet density values for the blocks (obtained during the standard block validation described in SANS 3001-NG2).

Composite blocks have separately assigned wet density values for a type 1 gauge and a type 2 gauge, depending on whether the gauge makes a moisture adjustment (see SANS 3001-NG2).

In road construction the performance of the gauge in the depth modes 100 mm, 150 mm and 200 mm are considered to be critical. As a result more data is gathered in the procedure for these three modes than in the other modes.

In order to reduce the variations in measurement on the standard blocks due to the random nature of the radiation counts and the shifts caused by switching the gauge off and on (see NG5 5.2), the calibration procedure requires six sets of readings on each of the calibration blocks. This results in one calibration operation taking about one and a half days to complete. This is significantly longer than in the past but assists in providing a much closer calibration of the gauge.

SANS 3001-NG4 (Verification of a nuclear density

gauge)

Applies to: Agents or Suppliers who Verify and Calibrate gauges.

This standard test is required yearly after the annual maintenance and leak tests, or after repairs to the gauge.

This standard is the critical one in the NG series in ensuring that a gauge is performing satisfactorily. It requires that the set of standard calibration blocks used in the test has been validated against the National set at the CSIR BE (see NG2).

The verification process consists of taking a series of readings in all the reading mode positions and depths on the three standard calibration blocks. The average wet density reading for each mode position is compared with the assigned value for the block. The outcome of the verification is determined by evaluating the important properties (p) obtained from the three operational modes, namely, moisture content, backscatter and direct transmission.

Calculations are made using a statistical procedure which incorporates a large data set of historical results for gauges used in South Africa over a period of 30 years. The verification process for the critical modes used in road construction, the direct probe modes 100 mm, 150 mm and 200 mm, verifies the following essential performance issues for compliance:

a) the degree of variability, which reflects the general operational condition of the nuclear gauge;

b) bias, which shows the quality of calibration; and

c) conformance with historical records of overall gauge performance.

This portion of the verification process is much tighter than the tolerances previously applied in gauge calibration. As a result the possible difference in gauges verified on the same block set has been significantly reduced.

For gauges used in road construction the backscatter, moisture content and asphalt modes (AS), which are considered to be non-critical properties (see SANS 3001-NG5), are evaluated for bias only, using a tolerance of 1% for backscatter and direct modes; and 2% for moisture content. In the past these tolerances were applied to all the modes.

A gauge which indicates successful conformance in the

critical direct probe modes but has non-conformances in

the other modes may be issued with a verification

certificate provided that the non-critical non-

conformances are clearly noted.

The formulae for the statistical calculations are given in

the standard, together with a worked example in the

annex. However, because of the complex nature of the

calculations a software program (NuclearDensity) has

been produced to carry out these calculations, record

the outcome and generate a verification system. The

program is user friendly and can be customised to reflect

the details of the Agent doing the verification, keep

records of all tests (history) and produce the necessary

test data that is to accompany the verification

certificate.

The program is available on the SANRAL website.

SANS 3001-NG5 (In situ density testing)

Applies to: All people who have anything to do with the tests, interpretation of results, conformance with specifications, etc.

The standard covers the use of the gauge in the field to measure in situ density.

The following extract from the paragraph covering 5 Principles in the Standard is important in the understanding and use of the gauge:-

5.1 Nuclear density gauges do not provide a direct reading of the density of a material. The gauge emits random pulse gamma radiation (not a steady stream) from a Cesium source indirectly in the backscatter mode; or from a probe in the direct transmission mode which passes through the material. The radiation having passed through the material is measured by detectors located in the base of the gauge and converted by a microprocessor, using calibrated empirical algorithms, into a wet density reading. Moisture readings are obtained by counting slowed neutrons emitted by a neutron radiation source in the gauge and measured by a detector in the base of the gauge.

5.2 Due to the random pulse variation the testing on standard calibration blocks has shown that without moving the gauge, for a set of 10 × 1 min counts the resulting densities can vary over a range of up to 0,5 % of the block density. Also by switching the gauge off between sets of 10 × 1 min counts the average for each of the sets of 10 counts can vary over a range of up to 0,4 % of the block density. As a result it is clear that an individual gauge reading cannot be an exact statement of density. By using the 15 s count option on the gauge the ranges given above are doubled. The standard requires the use of 1 minute counts for field densities. A comparison between commonly available makes of gauges showed similar properties for each make.

The standard covers the following:-

Gauge requirements:

1. Reference to NG1 for administration, handling and maintenance. Preparatory requirements before drawing a gauge from the store.

2. The gauge standard counts.

3. Surface preparation of test area and creating the test hole.

Reliability of test results:

test positions, random sampling, timing of counts,

orientation of gauge, number of counts, moisture contents

and asphalt testing. Attention is drawn to certain ground

conditions which may affect the test outcome.

Special conditions

Detailed step by step test procedure: including the taking

of moisture content samples and backfilling of holes

Calculations and Test report

Baie Dankie

Thank You

Nkosi