BGB Projects LPR Handbuch

Registered Office: 18 Craig Avenue, Flixton, Manchester, M41 5PT – Tel: +44 (0) 161 748 3645 – Fax: +44 (0) 161 748 3645

Company Registration No: 333 7082 – VAT No: 693 2194 16

Dr J P Broomfield

Mr K G Davies

57 Brook Rd

Flixton, Manchester

M41 5RY, UK

Tel: +44 (0) 161 748 3645

Fax: + 44 (0) 161 748 3645

Email: corrosioneer@btconnect.com

BGB Projects

LPR Corrosion Rate Meter

Technical Information

INSTRUCTION MANUAL – LPR METER

1. INTRODUCTION

The LPR Corrosion Rate Meter measures rates of corrosion of metals in corrosive

environments. There are many similar devices available, these are usually intended to

be used with corrosion probes of a relatively small surface area. The LPR Corrosion

Rate Meter is specifically designed for measurement of corrosion of steel reinforcement

in concrete where the surface area of the test electrode (rebar) can be large.

The meter uses the linear polarisation resistance (LPR) measurement technique and

controls the test electrode potentiostatically. The meter reads both the corrosion

potential (Ecorr) and the corrosion rate (in µm/year) of the test electrode.

Additionally, the meter can read Dallas Semiconductor DS2401 (or similar) electronic ID

number chips (i-buttons). These may be used to uniquely identify measurement

locations.

The meter stores up to 512 measurement results in non-volatile memory, time and date

stamping each one.

The LPR Corrosion Rate Meter is housed in a rugged polycarbonate enclosure and is

designed for a rough site use.

INSTRUCTION MANUAL – LPR METER

2. QUICK-START GUIDE

•Connect electrodes to the LPR Meter

oConnect reference electrode (half-cell) to the RE terminal (BLACK)

oConnect test electrode (rebar) to the WE terminal (GREEN)

oConnect auxiliary electrode to the AE terminal (YELLOW)

oEnsure that the SE (BLUE) and WE (GREEN) terminals are

shorted

•Power on the LPR Meter by turning the key switch

oAs the meter powers up check that the battery voltage is at least

10.5V - recharge the meter otherwise.

•Note the reading number, shown as for example 321.

•Press the GREEN button

oCheck that the measured Ecorr is plausible (+100 to -600 mV for

steel in concrete) and steady.

•Press the GREEN Button again

oCheck that a corrosion rate in the expected range is displayed (0.5

to 5 μm/year for sound concrete, 5 to 50 μm/year for mediocre

concrete and over 50 μm/year for actively corroding steel. Zero or

very low corrosion rate usually indicates a problem with the

auxiliary electrode connection.

oWait 60 seconds until the measurement completes

•Press the GREEN button again.

oReview the stored data if required or proceed to the next

measurement

•Press the GREEN button again for next measurement.

oetc.

•Power off the meter and disconnect the leads.

INSTRUCTION MANUAL – LPR METER

3. CONTROLS AND CONNECTORS

3.1. Controls

The LPR Corrosion Rate Meter is operated by two push buttons, green and red. A menu

system is used - the user selects an appropriate action by pressing one of the push

buttons. For most simple measurements only several presses of the green button are

required.

The meter is switched on and off using the key switch. The key may be removed only in

the ‘off’ position.

3.2. Connectors and leads

There are four 4mm screw terminals for connections to the test cell and a single round

connector for battery charger, communications and probe ID reading.

The test cell connectors are as follows :

RE – Black Connection to reference electrode.

SE – Blue Voltage sensing connection to test electrode.

WE – Green Current carrying connection to test electrode.

AE – Yellow Connection to auxiliary electrode.

In normal use the SE (Blue) and WE (Green) terminals should be shorted using the

link provided.

If long (more than ~ 5 meters) connections to the test electrode are necessary it is

recommended that the voltage sensing connection to the test electrode is used. This

compensates for any ohmic drop in the test electrode connection and improves

measurement accuracy in this situation. Simply connect a fourth wire in parallel with the

WE (GREEN) connection to the SE (BLUE) terminal. In all other cases short the SE

terminal to the WE terminal at the LPR Meter.

The five pin circular connector connects to the battery charger, probe ID chip or to a RS-

232 serial link.

INSTRUCTION MANUAL – LPR METER

4. MENU SYSTEM AND OPERATION

On power up the meter runs internal diagnostics and

displays the CCSL logo.

Corrosion

Control Services

The meter then displays its identification number and

displays the main battery voltage.

Meter # 101

Battery: 12.5V

Next, the meter shows its current clock settings

(day/month/year hour:minute). Meter # 101

15/10/2004 23:59

Finally, the present reading number is shown. The

reading number auto-increments whenever a set of

results had been stored. The meter can store up to 512

sets of results.

At this point the user can choose either to read the

measurement point ID or LPR measurement. Pressing

the left (RED) button selects ID read, pressing the right

(GREEN) button selects LPR measurement.

Reading # 001

Pressing the left (RED) button reads the measurement

point electronic identification. Use the supplied test lead

(5 pin to two 4mm plugs) to connect the meter to the

measurement point identifier connector.

The read ID is stored with the measurement results. If

no valid ID chip is found to be connected the meter

displays and stores 0000-0000 as the identifier.

ID: 53FE-1234

LPR>

Pressing the left (RED) button at this point returns the

meter to the previous display and the ID read may be

repeated if necessary.

Reading # 001

Selecting LPR by pressing the right (GREEN) button

initialises a measurement of the corrosion potential.

The measured value of the potential between the test

electrode and the reference electrode is displayed.

The flashing right pointing arrow prompts the user to

start the actual corrosion rate measurement by pressing

the GREEN button. This stage may be aborted by

pressing the RED button.

Ecorr: -234.5 mV

Go >>

Pressing the GREEN button starts the corrosion rate

measurement. The meter first initialises the

measurement and autoranges. A *WAIT* prompt is

shown for a few seconds during this time.

Ecorr: -234.5 mV

Rate: *WAIT* 03

INSTRUCTION MANUAL – LPR METER

The meter then shows the measured corrosion rate in

μm/year and a count timer showing the number of

seconds into the measurement. The corrosion rate

reading is updated approximately once per second.

Ecorr: -234.5 mV

Rate: 70.1 19

The corrosion rate measurement may be terminated by

pressing the RED button at any time during the

measurement process. Otherwise the corrosion rate

measurement proceeds for 60 seconds. On completion

the meter displays the measured corrosion potential

and the corrosion rate.

Pressing either button exits this display and leads to the

next menu.

Ecorr: -234.5 mV

Rate: 70.1 um/y

This display confirms that the measurement results had

been stored and shows the stored location number.

At this point the user can either review the results of the

last measurement (RED button) or proceed to a next

measurement (GREEN button).

Stored 101-001

The review display simply cycles through all the

recorded parameters. Date and time are shown first.

Stored 101-001

15/10/2004 23:59

The measurement location ID number is shown next (if

absent the display shows 0000-0000-0000).

Stored 101-001

0123-4567-89AB

This is followed by a display of the recorded corrosion

potential (Ecorr).

Stored 101-001

Ecorr: -123.5 mV

Next, the polarised potential is shown. Normally this will

be about 20mV more positive than Ecorr.

Stored 101-001

Epol: -103.4 mV

The recorded corrosion rate is shown next.

Stored 101-001

Rate: 70.1 um/y

The measurement range, measurement time and error

flags are displayed next.

Stored 101-001

R:3 t:60 e:00

The meter battery voltage at the end of the

measurement is shown last. The display then cycles

back to the date/time screen.

Stored 101-001

Battery: 12.0 V

Pressing the RED button exits the preview and returns

to the previous screen.

Stored 101-001

The meter may be safely turned off whenever this last screen is shown – the

measurement results are stored in the non-volatile memory just prior to the display of

this screen at the completion of the measurement.

INSTRUCTION MANUAL – LPR METER

5. OPERATING PRINCIPLE

5.1. Summary

The LPR Corrosion Rate Meter uses the LPR measurement method to determine the

corrosion rate of the connected test electrode.

The measurement is carried out potentiostatically, using 3 electrodes and a 20 mV

anodic polarisation.

5.2. Measurement Sequence

When the user selects a measurement start the internal analog to digital converter

measures the potential between the reference electrode (RE) and the test, or ‘working’,

electrode (WE). This is shown to the user as the corrosion potential, Ecorr.

When satisfied that a valid Ecorr is being measured, the user starts the polarisation

sequence. The meter connects the auxiliary electrode (AE) to the potentiostat circuitry

and polarises the test electrode to the measured Ecorr voltage. This is done by setting a

voltage equal to Ecorr on an internal digital to analog converter.

Next, the meter applies a 20mV anodic overpotential and measures the resulting

polarisation current. The polarised potential of the test electrode is also measured (Epol).

The ‘sense’ terminal is used to carry out this measurement, as to avoid any ohmic drops

in the test electrode lead. During the 60 seconds of measurement the meter monitors

the polarisation current and autoranges the current sensing circuitry as required.

The corrosion rate is computed from the value of the polarisation current (ΔIapplied) and

from the polarisation overpotential (ΔE = Ecorr – Epol)

The linear polarisation resistance measurement technique (LPR) relies on a

measurement of the slope of the current vs. voltage response of the corroding interface

at or near its natural free corroding potential. It can be shown that this slope,

(ΔIapplied/ΔE), is related to the corrosion current by the Stern-Geary equation:

corr applied ca

=×

⎛

⎝

⎜⎞

⎠

⎟

23.

ββ

where βcand βaare the cathodic and anodic Tafel slopes. For practical purposes the

equation can be simplified to:

EBB

corr applied

=×=

where B is an empirically determined constant and Rpis the ‘polarisation resistance’ of

the corroding interface.

In order to calculate the actual penetration rate Icorr is substituted to the Faraday

equation, giving:

INSTRUCTION MANUAL – LPR METER

penetration I

area time atomic weight

F valency density

corr

=××

××1

Thus, for example, to convert to cm/year for iron:

penetration rate B

=× ×××× ××

×365 24 60 60 5585

9 645 10 2 1

787

The LPR Meter calculation assumes a Stern-Geary constant B of 60 mV, a test

electrode surface area of 100 cm2and shows the corrosion rate in μm/year. To convert

to other values it will be necessary to adjust the corrosion rate measurement result

accordingly.

On completion of the measurement the circuitry de-polarises the test electrode back to

Ecorr and disconnects the auxiliary electrode from the potentiostat.

5.3. Limitations

There are two major uncertainties in the measurement of corrosion rate of steel

reinforcement in concrete.

The first is that of the precision of the Stern-Geary parameter which depends on the

exact electrochemical conditions at the corroding interface. The value of 60 mV is a

reasonable compromise for passive steel embedded in concrete.

A second uncertainty is that of the test electrode area being polarised. The 100 cm2test

electrode surface area used in the calculation is only a nominal value, in practice the

measurement results should be adjusted for the actual test electrode area present.

Further errors can be caused by the resistivity of the concrete surrounding the test

electrode (rebar) and by the placement of the reference electrode relative to the auxiliary

electrode and to the rebar.

In practice, however, the reinforcement surface should be passive and ‘corroding’ at a

very low rate of 1μm/year or less. To put the sensitivity of the measurement into

perspective, consider that the wavelength of visible red light is about 0.68 μm. Active

corrosion processes on the rebar surface will result in an increase of the corrosion rate

by several orders of magnitude to 50-100 μm/year, which is easily detectable and

measurable.

INSTRUCTION MANUAL – LPR METER

6. RS-232 COMMUNICATIONS

6.1. Summary

The meter communicates via a RS-232 interface. An adaptor for connection of the LPR

Meter to a standard 9 pin RS-232 serial lead (male to female) is supplied.

All communications are at 9600 baud 8N1.

Communications may be carried out using a terminal emulator program (e.g. Windows

HyperTerminal), other suitable software or the supplied application (LPR232.EXE)

6.2. Protocol

All serial communications are at 9600 baud 8N1 (eight bits, no parity, one stop bit). No

handshaking is used as normally the host machine will be fast enough to handle the

relatively slow data throughput.

Three commands are available :

*D Dump stored data

*C Clear memory

*T Set LPR Meter clock

Note that <CR> or <LF> input terminator characters are not required.

When powered up, the LPR Meter automatically checks

whether an electrical connection to a RS-232 device is

present. If a RS-232 connection is found, the meter

display will confirm this and the meter will wait for

incoming RS-232 characters. Power off the meter and

disconnect the RS-232 lead to terminate the

communications and to return to normal operation.

RS-232 Ready

The hex value of any received character is displayed to

assist in serial link troubleshooting.

RS-232 Ready

6.3. *C – Clear Memory

Send *C to clear all stored data. The stored data will be irreversibly erased.

All stored data is deleted and the next available data

slot pointer is set to 1

Clearing...

INSTRUCTION MANUAL – LPR METER

6.4. *T – Set Time and Date

Send *TYYMMDDHHMM.

For example, to set the date and time to 23:59 on 15th October 2004, send

*T0410152359.

Note that no checks are carried out for plausibility of the time and date value. Please

check that the value displayed immediately after this operation is correct. The ‘seconds’

value is set to zero on receipt of the final *T command character.

The meter will set its internal real time clock to the

required time, read back the clock time and show it on

the display.

Time set to :

15/10/2004 23:59

6.5. *D – Dump

Send *D to retrieve all stored data.

The output may be captured into a text file and further analysed in a spreadsheet or

similar.

After a 10 second delay the meter sends back all stored

data as 512 comma delimited lines. The reading

number currently being sent is shown on the display.

Sending...

123

The data output format is :

01, Meter number as set by switches on the LPR circuit board

123, Reading number from 1 to 512

DD/MM/YYYY, Reading date DD-day, MM-month, YYYY-year,

(e.g. 15/10/2004 for 15th October 2004)

HH:MM:SS, Reading time HH – hour, MM – minute SS – second

(e.g. 23:59:06)

XXXX-XXXX-XXXX, ID number (0000-0000-0000 if there was none connected)

+DDD.D, Ecorr in mV

(Corrosion potential of the test electrode prior to polarisation)

+DDD.D, Epol in mV

(Potential to which the test electrode was polarised – normally

+20 ±2 mV more positive than Ecorr)

DD.DDD, Measured corrosion rate in um/year

DD.D, Battery voltage in volt, recorded at the end of the measurement

DD, Polarisation time in seconds

D, Polarisation current measurement range

(5=1MΩ, 4=100kΩ, 3=10kΩ, 2=1kΩ, 1=100Ω)

D, DAC error (if Ecorr over +/-1024 mV)

D Rate error (corrosion rate negative etc.)

<CR><LF> Carriage return and line feed characters

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