Gauge error and what is accuracy class: how to choose - Setafi

The accuracy of the pressure gauge is determined by the percentage of inaccuracy, which sets the range of possible values. This percentage is always indicated in the description of the device in accordance with the requirements of regulatory documents. About what accuracy classes exist, and how to determine the error, is described in the presented article.

Accuracy classes and redline

First of all, you need to figure out what the accuracy class of a pressure gauge is. This is the name of the marginal error (in absolute values), which is allowed for a particular model. For example, if it is indicated that the device belongs to class 0.4, this means that the error is small (only 0.4%), and the pressure gauge scale is quite accurate.

If, for example, 4.0 is indicated, this means that the maximum permissible deviation is 10 times greater, which is significantly large (4%). It is quite clear that the lower the value, the more accurate the readings given by the equipment. The definition of all possible accuracy classes is introduced by GOST 2405-88. According to this regulatory document, 6 classes are distinguished:

1. 0,4.
2. 0,6.
3. 1,0.
4. 1,5.
5. 2,5.
6. 4,0.

Some sources give additional values ​​\u200b\u200b- 0.15 and 0.25. However, they are not presented in GOST itself.

Also of practical importance is the red line on the pressure gauge. This is the name of the line that shows on the scale the maximum allowable pressure in the system. Those. this is a critical value, exceeding which can lead to emergency situations. The requirements for the red line on the pressure gauge include its presence in the factory version of the device or the installation of a separate metal plate that fits snugly against the glass.

Calculation of error by accuracy class

The rules for installing pressure gauges require independent determination of the error, depending on the accuracy class indicated on the device. First of all, you need to know that the class is determined depending on the diameter of the device. The larger the diameter and the larger the scales, the smaller the error. The already mentioned GOST 2405-88 provides a table with the definition of 6 classes.

Gauge diameter	Accuracy class
0,4	0,6	1,0	1,5	2,5	4,0
40	—	—	—	—	+	+
50	—	—	—	—	+	+
60	—	—	+	+	+	+
100	—	—	+	+	+	—
160	—	+	+	+	+	—
250	+	+	+	+	—	—

If you figure out how to choose a pressure gauge for working pressure, you need to determine its maximum error by a simple calculation. This requires knowledge of the class value and the maximum measuring range. Moreover, where the red arrow on the pressure gauge stands does not matter.

You can show the principle of calculation with an example. The device measures up to a maximum value of 10 MPa, and its accuracy class corresponds to 1.0. Accordingly, 10 * 1.0 / 100 \u003d 0.1 MPa - this is the error. For example, if the arrow on the scale shows 2.2 MPa, then due to inaccuracy, a deviation in the range of 2.1-2.3 MPa (but no more) may be observed. This is how the pressure gauge is selected according to the working pressure.

Based on this, we can say that 1.5 is an error of 1.5%. The limit value in the classification of GOST 2405-88 is 2.5, i.e. error 2.5%. To choose the most accurate device, you should consider models with 0.4, i.e. with an error of 0.4%. For example, if the pressure gauge reads 3.2, then its error does not exceed 0.4 * 3.2 / 100 = 0.013. Those. the true pressure value is in the range of 3.187-3.213 MPa.

The procedure for checking pressure gauges

The marking of pressure gauges comes from the factory, where the primary verification is carried out. Those. all new instruments give accurate values ​​with an error that does not exceed the specified range. The period of validity of the primary verification is 1 or 2 years, depending on the specific model (the value is indicated in the instrument's passport).

After that, the scale of the pressure gauge needs to be recalibrated. The procedure is of great importance not only for private houses, but for multi-apartment buildings, as well as organizations of medicine, education and others.

Re-verification is carried out only in a licensed company that has the appropriate permission. Moreover, at a cost, this service usually costs about the same as a new device, or even more. After the end, a brand is put on the pressure gauge - the next verification is carried out after the same period has expired (i.e. 1 or 2 years).

This leads to 2 conclusions:

1. It is best to purchase a model whose factory verification is valid for 2 years.
2. Before handing over the device for recalibration, it is worth considering the costs - it may be better to purchase a new model. For example, if there were hydraulic shocks, pulsations, emergency situations in the heating system, then half of the devices do not pass verification. The pressure gauge readings must be accurate enough, so you have to purchase another model.

It should also be noted that the equipment must be operated under suitable conditions as described in the operating instructions. Typically, the device can withstand temperatures from -40 to +100 degrees, although it is better to keep it in a room with relatively warm air.

Now it is clear how to calculate the error of the pressure gauge. To do this, they are guided by the accuracy class and, depending on the specified value, determine the range of inaccuracy (from a smaller value to a larger one). If in doubt, you can put a new model for comparison or give the old one for verification.