How to Correctly Choose Laboratory Scales

Everything in the world has weight, and for this reason, proper weighing and weighing methods are very important. Therefore, ensuring that the scales you use for weighing measure correctly is the most critical aspect of this process. Scales used in every field have different characteristics and sensitivity levels. For this reason, you should use the correct laboratory scale in the correct application. The same situation applies to laboratory scales used in the manufacturing sector. So how can you select the correct laboratory scale?

When selecting a laboratory scale, you should consider what purpose you will use the scale for (pharmaceutical weighing, chemical substance weighing, analytical work, etc.). Each laboratory scale has a different readability range. You will see this on the laboratory scale as the number of decimal places after zero.

For example, a laboratory scale of 0.000001g has a readability of 1μg. The smallest value that the laboratory scale can read (the scale's readability), along with many other technical and physical properties such as the scale's weighing capacity, should enable you to meet your requirements. When beginning laboratory scale selection, you should remember that there are two types of laboratory scales with the technical specifications you want in weighing. These are: 1. Laboratory analytical scales display the weighing result in the range of 4 to 7 decimal places (0.0001g) after the decimal point. This number of decimal places is referred to as readability. The most common analytical scales display results with four to five decimal places after zero and are generally used to weigh chemicals and pharmaceuticals in powder, liquid, or solid form. Maximum capacity can reach up to 500g. Micro scales, which are part of analytical scales, display results up to six decimal places for applications requiring extremely precise measurements. Ultra-micro scales are the most precise measuring scales, and you can perform measurements up to 7 decimal places (0.1 μg) on these scales. 2. Laboratory precision scales are devices that can perform measurements between 1 decimal place and 3 decimal places (0.1 g-0.001g) after the decimal point. When you require less sensitivity and readability compared to analytical scales, you can use these scales. Additionally, if you wish to perform high-capacity weighing, since these scales have higher weighing capacities than analytical scales and can perform weighing at kilogram levels, you can also use these scales for your high-capacity weighing applications. Laboratory scale manufacturers have specified technical characteristics to guide users in selecting their products. These are: a) Weighing range/maximum weighing capacity, b) Readability, c) Scale accuracy, d) Repeatability, e) Linearity, f) Minimum weighing amount (USP 41), g) Operating temperature, h) Accuracy class, i) Weighing pan, j) Computer connectivity, k) Scale internal chamber, l) Scale enclosure.

a) Weighing Range

The scale's weighing range is determined by the scale manufacturer, and you should carefully follow this and select the scale for the range in which you will perform the highest weighing. You should always keep in mind that if a weight greater than the maximum weighing weight is placed on the scale, damage to the scale may occur. For this reason, when selecting a scale, you can select a scale with a capacity of twice the weight of the sample you will weigh during the application. For example, if you regularly perform 100g weighing, you can select a scale with 200g capacity.

b) Readability

This is a parameter related to the scale's maximum weighing capacity. You should pay attention to the mass of the measured sample and the accuracy with which this sample will be measured. If the analysis is performed incorrectly, it can lead to a situation where your expectations or desired results do not occur. For this reason, if you are selecting a scale for weighing that requires 0.0001g reading, you should select a scale with 0.00001g reading capacity. In this way, you will have achieved the best precision for your weighing.

c) Scale Accuracy

Regarding the accuracy of the analytical scale, if you are performing 100g weighing and wish to work with 1mg accuracy using a scale with 0.01g / 10mg readability, you cannot say whether the weighing result is 1mg. If you have a scale with 1mg readability suitable for you and take accuracy into account, a 0.1 mg resolution (readability) analytical scale will allow you to obtain a more accurate result.

d) Repeatability

This is the ability to obtain the same result in repeated weighing of the same sample under the same measurement conditions. Many of the questions from you relate to parameters defined as "repeatability" in commercial samples concerning analytical scales. The better the scale's repeatability, the more accurate your results will be.

e) Linearity

This is expressed as the scale error over the scale's weighing range. If you are using the scale over a very limited weighing range, you should perform calibration at the point where you will perform the weighing. This is very important. Because since the error is tested during measurements, the error is determined for the entire weighing range and adjustment is made.

f) Minimum Weighing Amount (USP 41)

Especially if pharmaceutical weighing is involved, you must determine the minimum amount at which you can start weighing on your scale, which is determined by the USP and depends on your scale's technical characteristics. How you will perform this operation is specified by USP 41. Additionally, scale supplier companies specify this in their own technical documentation, and these supplier companies certify this for you. Your responsibility here is to select your scale according to the amount you will start weighing with and certify it.

g) Operating Temperature

These are various temperatures at which the scale manufacturer guarantees proper measurement. For this reason, since the operating temperature environment differs everywhere, the scale should be calibrated in the environments where you will use it. Otherwise, you will have caused incorrect measurements.

h) Accuracy Class

This is expressed in terms of maximum permitted errors in relation to the verification unit. If you are using your scale in transactions with equivalent value in weighing, you should have selected your scale with this feature and should have your scale certified by the national legal metrology authority.

i) Weighing Pan

You should select the pan to be at least equal to the size of the smallest sample to be measured. You can also select whether the pan is round or square shaped based on your application. As standard, we recommend selecting the weighing pan as a pan made with stainless steel technology (non-magnetic). The enclosure blocking air flow should be made of plastic without electrostatic charge, as selecting this will provide greater precision in your work.

j) Computer Connectivity

Nowadays, almost all laboratory scales contain software. For this reason, depending on your requirements, you can select scales with USB or RS-232 connections. Additionally, if a wide display is desired for the application on the scale, a touch screen and color display would be useful features for you.

k) Scale Internal Chamber

It is most commonly coated with aluminum (especially in legal metrological scales). Additionally, you can select a scale with a plastic-coated interior that provides lower cost and resistance to chemical substances. You can make the appropriate selection depending on your use and your work.

l) Scale Enclosure

Analytical scales include an enclosure inside the scale that blocks air currents from the external environment to the sample. Depending on your purpose for using the scale (if you require high precision), you can select different enclosure sizes. However, selecting a scale with a large enclosure can provide you with an advantage in situations where the size of the sample you will weigh will be large. In most scales, the weighing mechanism is directly below the weighing pan and these scales are more cost-effective. In more precise scales (d=0.1 mg), the weighing mechanism is located at the back of the scale. Additionally, if the electrostatic charge of the environment or the ionic charge of the liquid being weighed is a problem for you, you can select a scale with systems that eliminate these. Additionally, depending on your preference, you can make your selection to include a system that allows the enclosure doors to open automatically or manually. This will provide you with opening your scale enclosure without vibration, thereby increasing your precision in your work. In some cases, with samples containing substances in ionic form, you may obtain incorrect weighing results in this process. In this situation, it would be beneficial for you to prefer scales where the weighing mechanism is quite far from the weighing pan (the weighing mechanism is at the back of the scale). When selecting a scale, if you will use a metrological scale, you should check whether the device has a type approval certificate. During the verification process, the approving bodies will check for you whether the scale has appropriate metrological characteristics and temperature error within limits. If you wish to keep the weighing results in the scale's memory in the scale memory, you should also consider this option when selecting the scale. Or you should select scales with the feature that results can be transferred to the computer environment through intermediate connections. Apart from these features, in cases where you want scales to have certain features or wish to make additional connections, you can select scales with appropriate features. Among these, if you want to print your results as output with a printer, you should consider versions with GLP features or versions where you can only get simple printout. If you will use a scale for pipette calibration, you should select scale features that meet this requirement. If you also want to perform density measurement, you should also separately request this density kit. All these features mentioned are reflected in the product differently in different scales. Here, as a user, you should first determine the technical specifications (weighing capacity, scale repeatability, readability, scale accuracy, etc.) and physical features (enclosure and pan size, computer connections, etc.) desired for your primary weighing applications, and then select a scale suitable for you, which would be more appropriate. Mustafa Yetim / Sales & Application Specialist Laboratory Products - Sartonet Separation Technologies Inc.