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    A

    Absolute Pressure
    The pressure measured using an ideal vacuum as a zero reference point.
    Accuracy
    The combined error due to nonlinearity, nonrepeatability, and hysteresis expressed as a percentage of full scale output.
    Adaptive Tuning
    A self-tuning function that continuously monitors the dynamics of a process and makes adjustments to the control parameters to maintain a pre-determined setpoint.
    Agency Approval
    Certification of conformity to the requirements of various independent testing agencies such as Underwriters Laboratories or the Canadian Standards Association.
    Alarm
    A point in a process if the value increases above (high alarm) or decreases below (low alarm), causes an action by an indicator or controller.
    Ambient Conditions
    The condition(s) around the transducer (pressure, temperature, etc.).
    Ambient Pressure
    The pressure of the medium surrounding the transducer.
    Ambient Temperature
    The average or mean temperature of the surrounding air which comes in contact with the equipment and instrument under test.
    Amplifier
    An electronic device which boots or increases a small signal to a higher level, usually for transmission, scale convenience, or noise immunity.
    Analog Output
    A voltage or current signal that is a continuous function of the measured parameter.
    Analog Output Trim
    A calibration operation that allows adjustment of the output electronics to establish a value of current which conforms to a standard.
    Anisotrophy
    The situation where properties vary according to the direction in which they are measured.
    ANSI
    American National Standards Institute.
    Apparent Shear Rate
    The shear rate determined in capillary viscometers without making a Rabinowitsch correction for shear thinning.
    Apparent Viscosity
    The viscosity determined in capillary viscometry without making a Rabinowitsch correction for shear thinning.
    ASCII
    ASCII (American Standard Code for Information Interchange) is a widely used code defined by ANSI (the American National Standards Institute). It represents the alphabet, the numeral digits, and many punctuation characters as 7-bit binary code. Packed ASCII, used in HART communications, is a 6-bit subset of ASCII which uses only upper case and omits many of the punctuation marks. It is employed so as to fit 4-bytes in a 3-byte field so that communications can be improved.
    ASTM
    Abbreviation of American Society for Testing and Materials, an association for establishing standard testing and reporting procedures.
    Auto/Manual Station
    The controller function that allows the operator to select the Automatic or Manual control mode. In the automatic control algorithm the controller determines the control output. In the Manual mode, the operator determines the control output.

    B

    Background Noise
    The total noise from all sources of interference in a measurement system independent of the presence of a data signal.
    Bell 202
    Bell 202 is a US telephone standard. It uses 1200 Hz and 2200 Hz as 1 and 0 respectively, a 1200 baud. Bell 202 is a full duplex communications standard using a different set of frequencies for reverse communications. HART is a half0duplex communications standard so that the reverse pair of frequencies are not used.
    Bit
    A bit, or binary digit, which represents a single item of high/low, yes/no, or on/off information.
    Breakdown Voltage Rating
    The AC or DC voltage, which can be applied across the insulation portion of a transducer without arcing or conduction above a specific current value.
    BTU: British Thermal Unit
    The quantity of thermal energy required to rise one pound of water 1°F at or near its maximum density (39.1°F) (1055J).
    Byte
    A byte is a set of bits, typically 8, which is treated as an entity. Most computers handle data bits as bytes because it is a power of two. A byte with parity is a 9 bit used for error detection.

    C

    Calibration
    (1)A test during which known values of pressure are applied to the transducer and corresponding output readings are recorded under specified conditions.
(2)The matching of a pressure controller or indicator to the characteristics of a specific transducer. Most frequently done utilizing span resistor internal to the pressure transducer. Procedure is termed RCal.
(3)Adjustment of an instrument to standards of known accuracy and stability.
    Calibration Cycle
    Pressure calibration in both a descending and ascending mode.
    Catalyst
    A substance that changes the rate of a chemical reaction without itself undergoing a permanent change or becoming part of the molecular composition of the product.
    Checksum
    A checksum is an additional byte or bytes of data appended to a message group containing the arithmetic sum of all previous bytes. In HART communications, the checksum is truncated to the single least significant byte.
    Coefficient of Friction
    A measure of the resistance to sliding of one surface in contact with another.
    Cogswell’s Method
    An approximate method for extensional viscosity measurement.
    Compensation
    The addition of specific material or device(s) to counteract a known error.
    Configuration
    The process of setting parameters, values and data which will determine how a transmitter will operate.
    Consistency Index
    In the power-law viscosity model which describes the reduction of viscosity as the shear rate increases (shear thinning), m is the consistency index which is a function of temperature. This corresponds to the value of the viscosity for shear rate.
    Control Output
    The output signal from a controller to the manipulated variable in response to input signals from the controlled variable. (See direct acting; reverse acting).
    Controlled Variable
    A process variable which is to be controlled at some desired value by means of error; i.e. cold junction compensation for thermocouples.
    Controller
    A device which manipulates one process variable (RPM, Heat, etc.) to result in a stable condition of a second (controlled) variable (pressure, temperature, etc).
    Current Loop
    A two-wire loop in which the current through the wires is maintained according to a controlling device, usually a two-wire transmitter. The advantages of a current loop are longer distance signal transmission, better noise immunity, and the ability to power the two-wire transmitter throughout the same two wires. The most common current loop is 4 to 20 mA.
    Cycle Time
    The time usually expressed in seconds for a controller to complete one on/off cycle.

    D

    Damping
    An analog function that modifies the response time of the transmitter to smooth the output signal in the presence of a rapidly varying input signal.
    DC
    Direct Current.
    Dead Bank
    The range through which input can be varied without initiating observable change in output. (There is a separate and distinct input-output relationship for increasing and decreasing signals.)
    Dead Volume
    The volume of the pressure port of a transducer at room temperature and ambient barometric pressure.
    Descriptor
    A sixteen character text field for additional identification information about the transmitter. The Descriptor is a user settable entry.
    Differential Pressure
    The static pressure difference generated by the primary device when there is no difference in elevation between the upstream and downstream pressure taps.
    Digital Input
    Auxiliary input to an indicator or controller which performs a function via switch closure or opening. Typically used to reset a latched alarm, or to duplicate a front panel function such as selection of automatic or manual control modes.
    Digital Output
    An output signal, which represents the size of a stimulus or input in the form of a series of discrete quantities.
    Digital Trim
    A combination trim operation which includes the separate operations of Sensor Trim and Analog Output Trim.
    Direct Acting
    Control output action which increases as the process variable increases. In the case of an alarm, a direct acting alarm has its relay activated in an alarm condition.
    Disturbance
    An undesired change that takes place in a process(es) that tends to affect adversely the value of a controlled variable.
    Drift
    An undesired change in output over a period of time, of which change is not a function of the measurand.
    Duplex
    Duplex communications means communications in both directions (as opposed to Simplex, which is communications in one direction only).

    E

    End Point
    The output at zero pressure and full-scale pressure.
    Environmental Conditions
    All conditions to which a transducer may be exposed during shipping, storage, handling, and operation.
    Error
    The difference between the value indicated by the transducer and the true value of the pressure being sensed.
    Error Band
    The allowable deviation of output from specific reference norm.
    Excitation
    The voltage supplied by an indicator or controller to a transducer to provide its proper operating conditions.

    F

    Fahrenheit
    A temperature scale defined by 32°F at the ice point and 212°F at the boiling point of water at sea level.
    Failure Mode Alarm
    A transmitter output function that drives the analog output of the transmitter to a selectable value in the event of a failure of the electronics or sensor element. The output may be driven high, low, or assume the last valid value read by the transmitter.
    Filter (Electrical)
    A device to sort desired result from undesired. Electrically, a selective circuit which passes through certain frequencies, while attenuating or rejecting others.
    FM Approved
    An instrument that meets a specific set of specifications established by the Factory Mutual Research Corporation which sets industrial safety standards.
    Freezing Point
    The temperature at which the substance goes from a liquid phase to a solid phase.
    FSPR
    Full Scale Pressure Range. The design maximum value of input pressure that the transmitter is designed to measure.
    Full Bridge
    A Wheatstone Bridge configuration utilizing active elements or stain gauges.
    Full Scale Output
    The electrical output of the pressure device with full scale pressure applied. Usually expressed in electrical units (mV/V, V, mA).
    Full Scale Pressure
    The maximum pressure under which applicable performance specifications apply.
    Full Trim
    A sensor trim procedure in which two accurate, end-point pressures are applied to the transmitter’s sensing element and all output is linearized between the values. The end-point values should be equal to or slightly outside the Lower Range Value and Upper Range Value.

    G

    Gain
    The ratio of the change in output to the change in input, which caused it.
    Gain Adjustment
    Means of adjusting the full scale output of an amplified transducer.
    Gauge Pressure
    The difference between the local absolute pressure of the fluid and the atmospheric pressure at the place of the measurement.
    Ground
    The reference point of an electrical system, or alternatively, the local earth potential (earth ground).

    H

    Half Bridge
    2 active elements or stain gauges.
    HART Protocol
    HART, Highway Addressable Remote Transducer.
    Heat
    Thermal energy. Heat is expressed in units of calories or Btu`s.
    Hysteresis
    Deviation in output within the transducer range when first approaching this point with increasing pressure and then with decreasing pressure.

    I

    Indicator
    A device which monitors and displays the condition of a process variable without exerting any control action. Indicators may be equipped with alarms or other auxiliary outputs.
    Input and Output Resistance
    The resistance measured across the input (excitation) and output (signal) terminals of an unamplified transducer.
    Input Fail Safe
    Direction in which signal is driven in the event of a sensor failure. Upscale will drive the signal fully upscale, and downscale will drive the signal fully downscale. The control output will respond as if the sensor has not failed.
    Insulation Resistance
    The resistance measured between specified insulated portions of a transducer when a specific DC voltage is applied at room conditions.
    Integral
    PID parameter (also called reset) which monitors and corrects the error signal between the setpoint and the process variable.
    Intrinsically Safe
    An instrument which will not produce any spark or thermal effect, under normal or abnormal conditions, that will ignite a specified gas mixture.
    ISO
    The International Organization for Standardization, a worldwide federation of national standards bodies from some 140 countries. Equivalent to ASTM.
    ISO 9000
    Family of standards concerned with “quality management”. This means what the organization does to enhance customer satisfaction by meeting customer and applicable regulatory requirements.
    ISOTROPY
    The situation where properties are independent of the direction in which they are measured.

    J

    Jumpers
    Wire links that allow for changes to be made in input and output hardware configurations.

    K

    K-Value of PVC
    A measure of the molecular weight of PVC based on measurements of viscosity of a PVC solution. It ranges usually between 35 and 80. Low K-values imply low molecular weight (which is easy to process but has inferior properties) and high K-values imply high molecular weight, (which is difficult to process, but has outstanding properties).
    Kelvin (Symbol K)
    The units of absolute or thermodynamic temperature scale based upon the Celsius scale with 100 units between the ice point and boiling point of water. 0°C = 273.16K (there is no degree [°] symbol used with the Kelvin scale).

    L

    Linearity (End Point or Terminal)
    Linearity as referring to a straight line between end points.
    Linearity (or Nonlinearity)
    The maximum deviation of the transducer output from a defined straight line during increasing pressure in a calibration cycle.
    Loop Gain
    The product of the gains of all the elements in a loop.
    Loop or Transmitter Power Supply
    24 Volt DC (nominal) supplied by an indicator or controller power 2 or 4 wire transmitters.
    Lower Range Limit
    The lowest value of the measured variable that the analog output of the transmitter is capable of measuring. Lower Range Limit, LRL, is factory set and not modifiable by the user.
    Lower Range Value
    The lowest value of the measurand that the analog output of the transmitter is currently configured to measure. Lower Range Value, LRV, is a user settable entity.

    M

    Master
    The communication device which controls the operation of slave devices in a communications environment. In a Master-Slave operation, the Slave device can only respond when requested by the Master device.
    Maximum Diaphragm Temperature
    The maximum temperature of the process media to which the transducer tip below the mounting threads can be exposed. Maximum strain gage temperature is the maximum environmental temperature at which the strain gage housing should be exposed.
    Maximum Pressure
    Pressure that may be applied to a transducer without changing the transducers` performance beyond specified tolerances.
    Measurand
    A physical quantity, property or condition which is measured. The term measurand is preferred to “input”, “parameter to be measured”, “physical phenomenon”, “stimulus”, and/or “variable.”
    Message
    A 28 character text field for additional identification information about the transmitter. The Message is a user settable entity.
    Modem
    A modem (modular/demodulator) is a device for converting binary digital signals to and from an FSK, Frequency-Shift Keying, form. This allows communications over pathways with poor propagation characteristics.
    Mounting Error
    The error resultant from installing the pressure transducer, both electrical and mechanical.
    Multidrop
    A mode of operation for HART transmitters. Establishes a communication system where more than two devices are connected together on a single transmission line. In such system, each device must have a unique address. The HART protocol can be operated in multidrop mode, with up to 15 slave devices on one pair of wires. The analog (4-20 mA) signals cannot be used in this mode since they would simply add together in this mode. With HART multidrop mode, each slave device assumes a fixed output current value of 4 mA.

    N

    NEMA-4
    A standard from the National Electrical Manufacturers Association, which defines enclosures for indoor or outdoor use primarily to provide a degree of protection against windblown dust, rain, and/or splashing water.
    Newtonian Fluids
    Fluids which exhibit constant viscosities independent of the shear rate. Water, glycerin, oil and other small molecule fluids are Newtonian.
    Noise
    An unwanted signal which can contribute to errors in measurement. Examples are hum (power lines), radio frequency interference (RFI), electromagnetic interference (EMI), and broadband or white noise.
    Non-Newtonian Fluids
    Fluids having viscosities that depend on the shear rate. Polymer solutions and melts are non-Newtonian fluids. They also exhibit other non-Newtonian properties such as stress relaxation and normal stresses.
    Normally Closed
    The state of a switching device (relay or SSR) whose non-powered state is connected.
    Normally Open
    The state of a switching device (relay or SSR) whose non-powered state provides no connection.

    O

    Off-Line Configuration
    Those procedures and data exchanges which do not have a direct impact upon the Analog Signal which the transmitter outputs or reports and which do not effect the measurement data transmitted digitally by the transmitter. They typically refer to operations which can be performed on a transmitter on a bench with the transmitter removed from a process control system.
    On-Line Configuration
    Any operation which is performed on a transmitter which characterizes the response of the transmitter or which modifies its configuration parameters, values, or settings.
    Output
    The electrical signal, which is produced by a pressure applied to the transducer sensor.

    P

    Parallel
    Parallel communications provides a multipath avenue for the transmission of several, often as many as 8 bits, bits simultaneously. Serial communications is the transmission of digital information from one device to another on a single transmission path one bit at a time. The HART protocol uses serial asynchronous communications.
    Parity
    Parity refers to the appending of an additional bit to a byte of information for the purposes of error detection. Parity can be ODD or EVEN by agreement between the communicating parties. For ODD parity the extra bit is a 1 or a 0, so as to make the total of the ones in the byte add up to an odd number. For EVEN parity the total would add up to an even number. HART protocol appends an ODD parity bit to each byte transmitted.
    PID
    The control algorithm providing proportional control with automatic Integral and Derivative terms. Mathematically determines the control action to be performed.
    Polarity
    In electricity, the quality of having two oppositely charged poles, one positive and one negative.
    Polling Address
    A unique number in the range 0 to 15 used to identify a transmitter. In multidrop operation, the allowable range of values is 1 to 15. In analog operation, the only allowable value is 0.
    Positive Feedback
    A closed loop in which any change is reinforced until a limit is eventually reached.
    Potentiometer
    A variable resistor often used to control a circuit.
    Power Supply
    A separate unit or part of a circuit that supplies power to the rest of the circuit or to a system.
    Pressure Range
    The pressure values over which a transducer is intended to measure, specified by their upper and lower limits.
    Primary Loop
    The outer loop in a cascade system.
    Proof Pressure
    The maximum amount of pressure that can be applied to a pressure transducer without changing any specification. See maximum pressure.
    Proportional Band
    The PID parameter which determines the area in which the proportional control algorithm is operative.
    Proportional Control
    (1)Current or Voltage: Control algorithm which determines a continuous linear relationship between the input and the output.
(2) Time proportioning: Control algorithm that determines the time that a control output remains in the “ON” condition in a finite cycle. In this case, when the output is “ON”, it is fully on.
    Protocol
    A set of rules used in generating or receiving a message to insure reliable digital or other communications. It may involve transaction rules, message structure, coding, and physical signal characteristics.
    PSIA
    Pounds per square inch absolute. Pressure referenced to a vacuum.
    PSIG
    Pounds per square inch gage. Pressure referenced to ambient air pressure.
    PV-Process Variable
    This is the controlled variable in a control situation or the monitored variable in an indicated situation.

    R

    Range
    The upper and lower pressure limits that a transducer is required to measure.
    Rate Time
    The time interval over which the system variable is sampled for the derivative function.
    Reference Junction
    The cold junction in a thermocouple circuit, which is held at a stable known temperature. The standard reference temperature is 0°C (32°F); however, other temperatures can be used.
    Regrind
    Waste material that has been reclaimed by shredding or granulating.
    Remote Setpoint
    An analog input to a controller which allows the setpoint to be changed by a remote device. This can be another instrument (cascading), a PLC, or computer or it can be done manually.
    Repeatability
    The ability of a transducer to reproduce output readings when the same pressure value is applied to it consecutively, under the same conditions, and in the same direction.
    Reranging
    A procedure allowing the modifications of configuration values and parameters that change the transmitter 4-20 mA output settings in response to input pressure readings.
    Retransmission Output
    An analog output from an indicator or controller directly proportional to the value of the PV. Also called the recorder output.
    Reverse Acting
    Control output that decreases as the process variable increases. In the case of an alarm, the reverse acting alarm has its relay activated in the non-alarm state. This is also called fail-safe alarm.
    RFI
    Radio Frequency Interference.
    Room Condition
    Ambient conditions used for test purposes.
    RS-232-C
    RS-232-C is perhaps the most widely used standard for serial asynchronous communications. It was originally designed for interconnection of computer equipment, terminals, and modems over distances less than 50 feet. It is specified by the Electronic Industries Association, EIA. It defines the connectors, signal types, signal voltage characteristics used and provides a standard for the handshaking and control lines. Most implementations do not make use of all of the control lines or handshaking features. The RS-232-C port on most computers can be used for connection to a Bell-202 modem to provide HART communications.

    S

    Safe Overexposure
    The maximum pressure that can be applied to a transducer without changing its performance beyond specified tolerances.
    Self Heating
    Internal heating of a transducer as a result of power dissipation.
    Self Regulation
    The property of a process or machine which permits attainment of equilibrium, after a disturbance, without the intervention of a controller.
    Self Tuning
    Generic term for algorithms from a number of manufacturers which more or less succeed in tuning the PID parameters of controllers and control systems automatically. The term is non-specific, and individual manufacturers should be consulted regarding their algorithms
    Sensing Element
    The part of a transducer, which reacts directly in response to the pressure.
    Sensitivity
    The ratio of the change in transducer output to a change in the value of the pressure.
    Sensitivity Shift
    A change in the calibration slope.
    Sensor Trim
    A digital trim procedure which permits the adjustment of a digital process variable to a precisely known input pressure. Zero Trim and Full Trim are the two Sensor Trim functions.
    Serial
    Serial communications is the transmission of digital information from one device to another on a single transmission path one bit at a time. Parallel communications provides a multipath avenue for the transmission of several, often as many as 8 bits, bits simultaneously. The HART protocol uses a serial asynchronous communications.
    Setpoint
    An input variable which sets the desired value of a controlled variable.
    Shear Stress
    A tangential force divided by the area (FORCE/AREA) on which it is applied. The shear stress is equal to the viscosity multiplied by the shear rate (measured in units of pressure, i.e., MPa or psi). At the die lips under usual production conditions, the shear stress may reach values of 0.2 MPa (29.0 psi) or more. The usually accepted value for the onset of sharkskin in capillaries is 0.14 MPa (20.3 psi), although higher values are reported in industrial production. With additives the critical shear stress value might be pushed up to 0.5 MPa (72.5 psi).
    Shear Viscosity
    The ordinary viscosity that is the ratio of shear stress to the shear rate (see also VISCOSITY).
    Shield
    A protective enclosure surrounding a circuit or cable which is to protect it from an electrical disturbance such as noise.
    Shunt Calibration/Rcal
    A method of generating an electrical output to match the electrical output that would be given in response to an applied pressure. This is accomplished using a resistor to unbalance the bridge electrically rather than with strain introduced by applied pressure. With standardized shunt or Rcal, the same point (generally 80%) is chosen on the calibration curve so that all similar transducers calibrate at the same point to facilitate interchangeability.
    Signal Conditioner
    An electronic network that permits adjustments to match a particular transducer to a readout device. Generally included are provisions for adjusting for zero balance and span or sensitivity.
    Signal Conditioning
    To process the form or mode of a signal so as to make it intelligible to, or compatible with, a given device, including such manipulation as pulse shaping, pulse clipping, digitizing, and linearizing.
    Slave
    The communication device which is controlled by a Master device in a communications environment. In a Master-Salve operation, the Slave device can only respond when requested by the Master device.
    Smart
    A term used to describe any instrument which is microprocessor controlled and features advanced communications capabilities.
    Span
    The algebraic difference between the upper and lower range values.
    Span Turndown
    The ability to re-range a transmitter to lower ranges. The re-ranging allows the 20 mA signal to be adjusted to the lower range which provides improved resolution.
    Stability
    The ability of a transducer to retain its performance characteristics for a period of time and under a variety of conditions.
    Static Calibration
    A calibration recording pressure versus output at room temperature.
    Static Error Band
    The error band applicable at room temperature.
    Static Pressure
    The pressure of a fluid or gas at rest.
    Strain
    A technical term synonymous with deformation.
    Strain Gauge
    A measuring element for converting force, pressure, tension, etc., into an electrical signal.
    Surging
    An instability of melt pressure and flow rate in an extruder, which can be detected by a pressure gage at the tip of the screw (or at the die adapter), or by dimensional product variations.

    T

    Tag
    An 8 character text field used to identify a transmitter. The Tag is stored in the transmitter and is capable of being user modified.
    Temperature Effect on Span
    The percentage change in rated output per degree change in ambient temperature.
    Temperature Effect on Zero
    The percentage change in zero balance due to a change in ambient temperature.
    Temperature Range, Compensated
    The range of ambient temperature for which Thermal Zero Shift is applicable (temperature error). Operation outside this range may require re-calibration.
    Temperature Range, Operable
    The range of ambient temperature, given by their extremes, within which the transducer is intended to operate.
    Temperature Range, Storage
    The range of ambient temperatures, given by their extremes, at which a transducer may be stored or transported.
    Thermal Effect on Sensitivity
    The change in transducer full scale output due to the effects of temperature only.
    Thermal Effect on Zero
    The change in transducer zero pressure output due to the effects of temperature only.
    Thermal Mechanical
    Analysis In this technique, a sample is deformed under a static load as its temperature is being changed. Glass transition and softening points can be measured. The amount of orientation can also be measured by TMA.
    Thermal Sensitivity Shift
    The sensitivity shift due to changes of the ambient temperature from room temperature to the specified limits of the compensated temperature range.
    Thermal Zero Shift
    An error due to changes in ambient temperature in which the zero pressure output shifts. Thus, the entire calibration curve moves in parallel displacement.
    Thermosets
    Materials that undergo chemical reaction and can be hardened by application of heat and pressure. They cannot be softened again to make them flowable. Typical plastics in this family are melamine, urea, epoxies and phenolics.
    Transducer
    In the broadest sense it is a device (or medium) that converts one energy form to another. Therefore, items such as a windmill, electric light, or an automobile engine could be called a “transducer” – but, in common practice, the term is generally applied to devices that take a physical phenomenon (pressure, temperature, humidity, flow, etc.) and converts it to an electrical output.
    Transmitter
    A device which translates the low-level output of a sensor or transducer to a higher level signal which is suitable for transmission to a site where it can be processed further.

    U

    Upper Range Limit
    The highest value of the measured variable that the analog output of the transmitter is capable of measuring. Upper Range Limit, URL, is factory set and not modifiable by the user.
    Upper Range Value
    The highest value of the measurand that the analog output of the transmitter is currently configured to measure. Upper Range Value, URV, is a user settable entity.

    W

    Write Protect Mode
    A security feature employing password access to the transmitter’s configuration values that can prevent accidental or deliberate changes to the transmitter’s configuration data.

    Z

    Zero Adjustment
    Means of adjusting the zero pressure output of an amplified transducer.
    Zero Balance (Offset)
    The measured transducer output under room conditions with no pressure applied to the pressure port. For absolute pressure transducers, this value is measured at 0 psia. Gage and sealed pressure transducers have this value measured at atmospheric pressure.
    Zero Shift
    Any parallel shift of the input/output curve.
    Zero Trim
    A zero-based, one-point adjustment, typically used to compensate for mounting position effects of zero shifts caused by sensor drift with age.