Pressure Transmitters

SOR Pressure Transmitters are durable and compact devices. They provide continuous pressure measurement and some models also provide a switch output. SOR Pressure Transmitters utilize a diaphragm-sensor design where a Wheatstone bridge circuit, containing two variable resistors, is placed on the diaphragm of the sensor. As pressure is applied to the sensor, the resistance of the variable resistors changes and a corresponding voltage difference in the circuit is monitored. The Pressure Transmitter can use this change in voltage to measure and record a change in pressure.

SOR recognizes that there is no industry convention with respect to terminology and definitions pertinent to Pressure Transmitters. This glossary applies to SOR Pressure Transmitters.

1-5V Output – A configuration of the Pressure Transmitter where the pressure range sensed is linearly translated to an electrical signal output that ranges from 1 to 5 volts. (This output provides low current consumption and therefore less power consumption.)

4-20mA Output – A configuration of the Pressure Transmitter where the pressure range sensed is linearly translated to an electrical signal output that ranges from 4 to 20 milliamps. (Unlike the 1-5V output, this output is not vulnerable to signal loss over long distances.)

Absolute Pressure (psia) – The sum of gauge pressure (psig) and atmospheric pressure (14.7 psi, 1.01 bar).

Adjustable Range – The span of pressure between upper and lower limits within which the Pressure Transmitter may provide continuous monitoring and solid state switch actuation/deactuation.

Ambient Temperature – The temperature of a Pressure Transmitter’s internal electronics.

Best Fit Straight Line (BFSL) – A straight line drawn exactly through the center of a group of data points.

Burst Pressure – The pressure at which mechanical failure and loss of pressure containment within the transmitter will occur.

Compensated Temperature Range – The range of temperatures that a Pressure Transmitter’s accuracy has been tested over. (If used outside of this range the accuracy of the transmitter is no longer valid.)

Continuous Output – The constant sensing, translating, and transmitting of pressures by the pressure transmitter.

Continuous Output Accuracy – Defines the maximum possible deviation of a transmitter’s output from the perfectly measured value. (Sometimes defined as a percentage of the upper range limit, i.e. “0.10% URL”, so if the upper range limit is 100 psi then the value of the output could vary from the actual input by up to 0.1 psi.)

Dead Band – The pressure range, centered on the set point, through which an input can be varied without initiating an observable response, usually defined as a percentage of the span. (Also known as hysteresis, differential, reset value, and reset.)

Diaphragm Seal – A flexible barrier used to protect the Pressure Transmitter’s sensor and keep it isolated from the process.

Differential Pressure (psid) – The difference in pressure between two separate points in a system.

Differential Pressure Transmitter – A Pressure Transmitter with the capability of measuring differential pressure.

Digital Compensation – A function of the transmitter where it collects inputs over a specific time interval and outputs the average of those inputs. This function can be used to eliminate the transmitter’s reaction to rapid changes in pressure. (Also known as damping.)

Dual Seal – A combination of a primary seal and a secondary seal that require the failure of two independent seals in order for the process fluid to leak out of the isolated process connection port.

EMC Protection – The prevention of both emission and reception of electromagnetic waves through the transmitter’s housing.

Explosion Proof (Flame Proof) – A device that is able to contain an internal explosion without letting the explosion escape to the atmosphere.

Fail-safe State – the state that the Pressure Transmitter’s switch output reverts to if power is removed from the transmitter; either open or closed. (For the 800 series the fail-safe state is always open.)

Full Scale Pressure Range (FSPR) – The maximum pressure range that a Pressure Transmitter is designed to measure.

Gauge Pressure (psig) – Pressure measured against atmospheric pressure.

HART – Highway Addressable Remote Transducer (HART) is a digital communications protocol used to read and configure a Pressure Transmitter’s output.

Hermetically Sealed – The method of sealing an electrical switching element so that it is unaffected by ambient surroundings. The enclosure is completely sealed from the environment using fusion techniques such as welding, soldering, brazing or glass-to-metal fusion.

Hydrostatic Pressure – The pressure produced by the weight of a resting liquid above the measurement location.

Ingress Protection (IP) Rating – A classification of the level of protection that a device’s body provides against the intrusion of objects, dust, and water.

Intrinsically Safe – A term used for devices that don’t generate or consume enough power to cause combustion.

Long Term Stability – The amount of change in a device’s measurement under the exact same conditions, over a period of time.

Loop Powered – A device that is powered by the 4-20 mA current from a power supply that is connected in series with the device.

Loop Resistance – The total resistance that the device causes within the power supply loop.

Modbus RTU Serial Communications – A communication protocol by Modicon systems that provides the capability to read and configure a Pressure Transmitter’s output.

Normally Closed – A switch preset where the switch is closed until the set point is reached and then it opens.

Normally Open – A switch preset where the switch is open until the set point is reached and then it closes.

Over Pressure – The maximum pressure that a device can withstand before the performance of the device is affected.

Pressure Transducer – A device that senses pressure and translates it into an output voltage signal.

Pressure Transmitter – A device that senses pressure and translates it into a 4-20 mA output signal.

Process Temperature – The temperature of the process media that the transmitter is monitoring.

Repeatability – The amount of change that a device’s output experiences at the same measurement point, over a specific number of cycles through the device’s measurement range.

Response Time – The amount of time that a device’s output takes to reflect a change in the pressure being measured.

Set Point – The specific pressure at which the Pressure Transmitter’s switch output is configured to actuate/deactuate on the rising or falling pressure.

Solid State Switch Output – An electronic switching function that switches on or off when a small voltage is applied to its terminals. (This voltage is applied to the solid state switch when the transmitter senses that the pressure has reached the set point.)

Span – The difference between the lowest point and the highest point in a configured measurement range. (For example, if a 250 psi transmitter is adjusted to measure a range of 50 psi to 200 psi, then the span is 150 psi.)

Static Line Pressure – The total pressure at a specific point within a pressurized pipe.

Switch Output Accuracy – The maximum possible deviation of the Pressure Transmitter’s switching pressure to the perfectly measured pressure. (Sometimes defined as a percentage of the upper range limit, i.e. “0.25% URL”, so if the upper range limit is 100 psi then the transmitter’s switching pressure could vary from the actual pressure by up to 0.25 psi.)

Turndown Ratio – The ratio between the highest and lowest possible span settings for a transmitter. (For example, if a 1,000 psi transmitter has a turn down ratio of 5:1, then the highest possible span is 0 to 1,000 psi and the lowest possible span is 0 to 200 psi.)

Upper Range Limit (URL) – The highest value that the transmitter can possibly measure.

Vacuum – Pressures that are less than atmospheric pressure.

Wetted Materials – The materials that are exposed to the process media.

Zero – The lowest value that a Pressure Transmitter is currently configured to measure.