621/622 Series Explosion-Proof Pressure Transmitters

A: In many industrial applications, the terms pressure transmitter, pressure transducer and pressure sensor are used interchangeably. Traditionally, a transmitter referred to an instrument with a current output signal such as 4 mA to 20 mA, and a transducer referred to an instrument with a voltage output signal such as 0 Vdc to 10 Vdc. Today, all three terms are commonly used to describe electronic pressure measurement devices, so a NOSHOK pressure transmitter or transducer is often considered a type of pressure sensor.

A: When these terms originated there was a distinctive difference between the two. A transmitter was referred to as an instrument with a current signal, for example 4 mA to 20 mA, and a transducer was referred to as an instrument with a voltage signal, for example 0 Vdc to 10 Vdc. As time has progressed these terms are now commonly interchanged for reference to either output signal.

A: Proof pressure, which is higher than the full scale pressure point, is the limit that you can go to without affecting the performance and calibration of the transducer. The burst pressure, on the other hand, is the limit that you can go to before there is pressure chamber rupture and damage. An overload limit specification used sometimes means that proof and burst ratings are identical.

A: Radio Frequency Interference (RFI) and Electromagnetic Interference (EMI) refer to the effects electrical noise can have on instruments. RFI frequently comes from hand held walkie-talkies and EMI comes from AC motors in the vicinity of the instrument. ESD, or Electrostatic Discharge, comes from many sources including the application itself. CE compliant transmitters and transducers incorporate protection techniques and components to minimize most of the interference.

A: Most diaphragm seals can be used with pressure transducers and transmitters. The key is to assemble and fill the seal properly, being careful not to entrap air in the fill fluid.

A: The steam syphon is necessary in steam pressure applications. It is important to isolate the transmitter sensing diaphragm from the high temperature encountered with steam pressure applications.

A: As with other pressure measurement instruments including gauges, pressure pulsations and spikes are issues with pressure transmitters. Whenever the pressure of an incompressible fluid is measured, there is the potential for pulsations and spikes, which can damage pressure transmitters. An orifice installed in the pressure connection by NOSHOK can protect the transmitter from damage. Where there is the possibility of clogging the small orifice, an attachable piston snubber is recommended.

A: All pressure measurements are inherently differential in theory. Gauge pressure is referenced to ambient atmospheric pressure, and absolute pressure is referenced to a vacuum contained in an evacuated chamber within the transmitter. The level measurement is also a differential measurement, with its reference to ambient atmospheric pressure. In order for the submersible level measurement to be referenced to atmospheric, the cable contains a vent tube which runs the complete length of the cable and vents to atmospheric pressure at the junction box connection, which is out of the liquid.

A: The transmitter measures the hydrostatic pressure produced by the liquid level higher than the point where the instrument is located. The higher the liquid, the higher the pressure.

A: Voltage output transducers are available with a fourth connection which is electrically the same as the power supply common to connect to wiring configurations that require it.

A: A smaller diaphragm is less easily damaged, and positioning the sensors directly behind the diaphragm minimizes fill fluid and enables active temperature compensation directly at the point of measurement.

A: A turndown ratio, also commonly known as rangeability, refers to the ratio between the full-scale range and the minimum point of measure, indicating the range in which an instrument can accurately measure the media. For example, if a pressure transmitter has a maximum calibration range of 0 to 300 psi and a turndown ratio of 10:1, the span can be adjusted anywhere between 0 to 30 psi and 0 to 300 psi. The higher the turndown ratio, the higher the rangeability, which can also help minimize required inventory.