Sensor manufacturers subject their designs to a series of tests simulating actual operating conditions to determine the on-line operating characteristics. Standard definitions are given in the Scientific Apparatus Makers Association (SAMA) publication PMC-20, Measurement and Control Terminology.

A sample performance report on a motion-balance

sensor is given below

Description

Pressure-sensing mechanism Bourdon tube (316 stainless steel) Electric transmission Output-signal ranges

±10 volts d-c, ±50 mV d-c, 0 to 100 mV d-c Operating Conditions

Ambient temp, nominal, 75°F, reference, calibration ±5°Г, normal, 40 to 140° F, operative limits, -10 to 200° F Supply voltage nominal, 118 volts a-c, normal, 107 to 127 volts, operative limits, 100 to 1 35 volts Frequency nominal, 50 or 60 H7, normal, 48 to 62 Hz, operative limits, 45 to 75 Н/

Ambient temp, effect 7ero-shift error/100°F temp, change, — 1% range span, Range-shift error/100°F temp change + 1% range span

Reference Performance Characteristics (°0 range span)

Accuracy 0.5%

Dead band 0.2%

Hysteresis 0.5%

Linearity 0.25%

Repeatability 0.25%

Design Data

Source impedance a-c signal coil, 200 ohms, d-c signal demodulator, 180 ohms

Minimum external load a-c transmitted signal, 2000 ohms, d-c transmitted signal, 30,000 ohms Maximum ripple 0.15% a-c ripple

Case classification NEMA (National Electrical Manufacturers Association) type 2 or N1 MA type 7D Over-range protection ll/4 times max. scale measured pressure

Performance data on a force balance sensor is given as

Description

Pressure-sensing capsule 316 stainless steel Electric transmission 2 wire d-c Output signal range 10 to 50 mA d-c Operating Conditions

Power supply 63 to 85 volts d-c Supply voltage effect 0 25% per 10-volt variation Performance characteristics (% range span)

Accuracy 0 5%

Dead band 0.005%

Repeatability 0.15%

Design Data

Output load limits 600 ohms (+10%, -20%)

Case classification NEMA type 4, hazardous area Class I Group D, Div. 1

4- 3.7 Transmitting Devices

(a) Pressure Switches. These are widely used to actuate alarms or initiate sequential operations. A Bourdon tube or similar sensor is linked to a snap-acting mechanical switch.

(In some cases an enclosed mercury switch is used.) The switch may be indicating or nomndicating, have range­setting capability, and provide necessary logic at pre­determined pressures.

(b) Electric Modulating Transmitters. These produce an electrical output proportional to input pressure (or force) applied to the sensor. Either the motion-balance or the force-balance principle may be involved. The output may be a voltage or a current of suitable value and range for input to readout devices, such as recorders, indicators, computers, and control loops to action equipment. A sample circuit for the motion-balance example of Sec. 4-3.6 is shown in Fig. 4.23. Forms of the linear voltage differen­tial transformer (LVDT) mechanism and a sample output curve are shown in Fig. 4.24.

(c) Pneumatic Modulating Transmitters. Differential — pressure sensors installed with one side open to the atmosphere and the other side connected to a pressure source can be used The device shown in Fig. 4.35 can be used and the pneumatic force-balance principle applied to obtain a pneumatic output proportional to sensor gage pressures at connection H (or L, as desired).