McFarland Densitometer: Parts, Principle, and Operation

Densitometer is laboratory equipment that measures the density or turbidity of photographic objects by measuring the transparency photometrically. The most commonly used densitometer in the microbiology laboratory measures the McFarland standard for antibiotic sensitivity testing. 

The densitometer used in a microbiology laboratory is hence named the McFarland densitometer. It is helpful laboratory equipment as an alternative method for preparing McFarland standards of bacteria and fungi cell suspension. The McFarland densitometer is calibrated to measure turbidity in the range of 0.3-6.0 McFarland units. 

Principle of Densitometer

Like any spectrophotometer and colorimeter, a densitometer also uses the principle of photometry. The light from the stabilized source passes through the optical system inside the densitometers. The light absorption depends on the density of the substances. The absorbance is indirectly proportional to the density, i.e., the denser the substance, the lesser the absorbance. The intensity of light rays transmitted (It) and the intensity of incident ray (Io) is measured to generate optical density. Here, the optical density (OD) or absorbance (A) is the ratio of the intensity of the incident light ray to the intensity of the transmitted light ray {A or OD= -log (It/Io)}. In the case of a McFarland densitometer, the optical density is digitally displayed as McFarland units instead of absorbance.

Parts of McFarland Densitometer

The parts of the McFarland densitometer include; a light source, digital display, tube holder, power switch, and cable for external power supply.

1. Light source: Tungsten light acts as the light source for the densitometer. It is enclosed inside the compact design.  
2. Digital display: The readings are displayed digitally in the bright LED (light-emitting diode) display. Here the readings are displayed as McFarland units. 
3. Tube holder: It is detachable and can fit 16 mm or 18 mm glass tubes. 
4. Cable: It helps to supply electric current for the smooth running of the equipment.
5. Power switch: It is a small switch in the front part near the LED display, and turning it on is the first step for operating the equipment.

Extra Accessories for McFarland densitometer 

These accessories may or may not be provided with the McFarland densitometer.

1. Glass tubes: The McFarland densitometer supports glass tubes of 16 mm and 18 mm in diameter. 
2. Battery: Three AA batteries are required to operate DEN-1B by Grant-Bio as a replacement for the electrical supply. 
3. CKG16 kit: For calibrating the densitometer that uses 16 mm glass tubes. It consists of 0.5, 1.0, 2.0,3.0, and 4.0 McFarland turbidity standards.
4. CKG18 kit: It is used for the densitometer that uses 18 mm glass tubes. The kit has 0.5, 1.0, 2.0,3.0, and 4.0 McFarland turbidity standards. 

Operating the McFarland Densitometer

Steps for Operating McFarland Densitometer

1. Connect the cable or place 3 AA inside the battery holder. 
2. Then turn the power switch on.
3. Firstly place a glass tube without suspension. The reading should be zero. 
4. Pour at least 2ml of the prepared suspension into a glass tube of the preferred diameter.
5. Place the glass tube in the tube holder.
6. Once operated, it takes one second to display the McFarland standard. Dilute or mix the suspension as per requirement.   

Calibrating McFarland Densitometer

1. The McFarland standard is provided along with the equipment. 
2. Both 16 mm and 18 mm diameter glass tubes should be used for calibrating.
3. Firstly place the tube labeled McFarland standard 0.5 into the tube holder. See the reading. If the reading does not show 0.5 McFarland units, shake the glass tube and repeat the step.
4. Then, simultaneously place the tubes labeled McFarland standard 1.0, 2.0, and 4.0. If the reading, even after proper shaking, shows less or high, the equipment requires maintenance, and contacting authorized companies is suggested. 

Things To Consider

1. Always calibrate the equipment before use.
2. Perform repair and maintenance from time to time. 

References

• Grant SCI UKC – Grosseron. Densitometers. (n.d.). Retrieved November 1, 2022, from https://www.grosseron.com/Assets/Client/images/GROSSERON/Schema/6234576.pdf
•  Den-1, densitometer (suspension turbidity detector) – biosan. DEN-1, Densitometer (suspension turbidity detector). (n.d.). Retrieved November 1, 2022, from https://biosan.lv/marketplace/pdf/201/