Determine the type of device that you require. To measure pressure are actually used pressure gauges, pressure-vacuum gage, vacuum gauges and low-pressure devices. Most of them are intended for measuring of excess pressure in special conditions. Devices of the same type are usually made on a common standard, therefore interchangeable.
Find out what range of pressure will operate the device. This will require data on the operating pressure in the system which will serve the manometer, and about the range of possible values. To operating pressure add 25-30% and you will get the desired characteristics, which must comply with the gauge you purchase.
Consider the nature and characteristics of the environment in which you must work the device. It can be air, water, steam, oxygen and its variants, acetylene. In such cases, you will need technical gauges.
Buying a device for measuring the pressure in the oxygen, be aware that the oxygen pressure is more capricious and require additional care and maintenance (degreasing). In some cases, the pressure measurement can be carried out in aggressive environments (ammonia, chlorine and so on); for such conditions will need special devices that are resistant to environmental conditions.
Determine the required diameter of the housing of the measuring device. If the conditions allow to get close to the gauge at a safe distance, install the pressure gauge medium in size. Relative inaccessibility is more convenient to take readings from a device having substantially larger diameter. The diameters of the devices range from 40 to 250 mm.
Each device for measuring the pressure also has the class of precision: from 0.15 to 4. The smaller this index, the more accurate the device. Choose the required accuracy, in accordance with the features of the system to be controlled. Does not make sense to acquire high-precision pressure gauge, if the pressure can vary quite widely. High accuracy, typically required when conducting some research work.
If you plan to use the pressure gauge on the specialized manufacturing (e.g. food industry), then you probably need a device with separators of environments. For precise selection, please contact for advice to professionals, specifying the type and characteristics of the environment in a particular industry.