Water Activity Measurements at NMISA€¦ · Calibration •Relative humidity meters –normally calibrated in a climatic chamber or against reference salt solutions depending on

Water Activity Measurements at NMISA

Regina MnguniNLA Test & Measurement

2018T206

Background

• The NMISA started a cross cutter programme called Africa Food and Feed Reference Material (AFFRM).

• Water activity is very important in food substances, therefore its relevance in the programme.

• To measure this accurately, water activity meters needs to be calibrated.

• To support this programme, the humidity laboratory had to be accredited for the water activity meters.

Introduction

• Humidity has different parameters which includes:

- Dewpoint Temperature

- Relative humidity

- Water Activity

- Moisture content

• Water activity (Aw) is free water/moisture available in a material (e.g food) as opposed the chemically boundmoisture.

• Directly related to the equilibrium relative humidity (%ERH)

Introduction

• %ERH – relative humidity of air at which there is no net exchange of moisture with a nearby substance.

• Water activity is expressed on a scale of 0 aw to 1 awwhile %ERH is 0 to 100 %ERH.

• Aw is one the most critical factors in determining quality and safety of goods we consume every day.

• It affects shelf life, safety, texture and smell of food.

• Controlling the water activity is very important in food industry as low water activity prevents microbial growth (increases the shelf life).

Water Activity Measurements

• Water activity is the ratio of the water vapour pressure of a material to the water vapour pressure of pure water under the same conditions.

• 𝐴𝑤 =𝑝

𝑝0=

%𝐸𝑅𝐻

100

• p = water vapour pressure of the material

• p0 = vapour pressure of pure water

• %ERH = Equilibrium relative humidity

Water Activity Measurements

• Water activity measurements are mainly used in the food, paper and pharmaceutical industries.

• Packaging and storage conditions of the products are very critical for these industries

Instruments

• Water activity instruments measure the amount of free water present in a sample.

• They are humidity instruments with a compartment/chamber for sample holding sealed from the surrounding.

• These are either chilled mirror hygrometers or relative humidity meters with capacitive or resistive sensor.

• Some instruments have detachable measuring chambers while others are embedded in the instrument.

Instruments

• Different types are available commercially

Instruments

Calibration

• The traceability chain is the same as for relative humidity

Chilled mirror hygrometers

Contact thermometers

Unsaturated salts/ampoules

Relative humidity meters, saturated salts & water

activity meters

Humidity dewpoint

generators

Calibration

• Relative humidity meters – normally calibrated in a climatic chamber or against reference salt solutions depending on the temperature and humidity range.

• With the water activity meters the procedure might be different because of the location of the sensor.

• At NMISA – unsaturated salt solutions (ampoules) as reference standard for relative humidity and water activity

Calibration

• A pad is placed in a sample holder and soaked with the ampoule content of certain humidity (e.g 5 %rhor 0,05 aw).

• It is then placed in a chamber and sealed off to create %ERH

• After stabilisation, water activity and temperature readings are recorded

• The results of the meter are then compared with the ampoule’s certificate

Ampoules

Quality assurance of results

• The NMISA humidity lab went through international assessment to include calibration of water activity meters on the scope of accreditation.

• Any new method needs validation before accreditation

• The laboratory participated in the bilateral interlaboratory comparison

• The comparison was between NMISA and RotronicUK (reference lab)

Quality assurance of results

• Rotronic UK supplied two water activity meters as the comparison artefacts

• The two were measured from 0,05 aw to 0,95 aw

using the ampoules as the reference standards.

• The corrections at each measured value were used as the comparison results

• The normalised error, En ratio method was used for the evaluation of measurement results.

Quality assurance of results

• En ratio equation

• 𝐸𝑛 =𝑥𝐿𝑎𝑏−𝑥𝑅𝑒𝑓

𝑈𝐿𝑎𝑏2 +𝑈𝑅𝑒𝑓

2

• Where En = normalised error

xLab = measurement results of the participating lab (NMISA)

xRef = measurement result of reference lab (Rotonic UK)

ULab = Expanded Uncertainty of participating lab (NMISA)

URef = Expanded Uncertainty of the reference lab (Rotronic UK)

Results

Table 1

Nominal Humidity

(aw)

Reference value (aw)

NMISA (aw) En ratio

0,050 0,001 -0,002 -0,38

0,100 0,001 -0,003 -0,55

0,200 -0,005 -0,002 0,33

0,350 -0,001 0,001 0,15

0,500 0,002 0,007 0,40

0,750 0,000 0,003 0,16

0,950 -0,002 0,006 0,41

Figure 1

-0,020

-0,015

-0,010

-0,005

0,000

0,005

0,010

0,015

0,020

0,000 0,200 0,400 0,600 0,800 1,000

Co

rre

ctio

n (

aw

)

Nominal Value (aw)

Aw-Therm SN 0061382591

NMISA Results Rotronic UK results

Results

-0,020

-0,015

-0,010

-0,005

0,000

0,005

0,010

0,015

0,020

0,000 0,100 0,200 0,300 0,400 0,500 0,600 0,700 0,800 0,900 1,000

Co

rre

ctio

n (

aw

)

Nominal Value (aw)

Aw-Therm SN 0061382591

NMISA Results Rotronic UK results

Comparison Artefacts

Results

Table 2

Nominal Humidity

(aw)

Reference value (aw)

NMISA (aw)

En ratio

0,050 0,001 -0,001 -0,28

0,100 0,001 0,001 0,04

0,200 -0,004 0,003 0,87

0,350 -0,001 0,005 0,62

0,500 0,001 0,009 0,56

0,750 0,002 0,005 0,18

0,950 -0,002 0,007 0,46

Figure 2

-0,020

-0,015

-0,010

-0,005

0,000

0,005

0,010

0,015

0,020

0,000 0,200 0,400 0,600 0,800 1,000

Co

rre

ctio

n (

aw

)

Nominal Value (aw)

Hygrolab 3 SN 60308720

NMISA Results Rotronic UK results

Results

-0,020

-0,015

-0,010

-0,005

0,000

0,005

0,010

0,015

0,020

0,000 0,100 0,200 0,300 0,400 0,500 0,600 0,700 0,800 0,900 1,000

Co

rre

ctio

n (

aw

)

Nominal Value (aw)

Hygrolab 3 SN 60308720

NMISA Results Rotronic UK results

Conclusion

• Satisfactory results were produced from the bilateral interlaboratory comparison.

• The NMISA humidity laboratory is now accredited for the calibration of water activity meters.

• The accredited range is from 0,05 aw to 0,95 aw

• The calibration and measurement capability (CMC) ranges from 0,005 aw to 0,014 aw

• See SANAS website for the accreditation scope