Sensors and transmitters: instrumentation basics

A sensor responds to a physical quantity such as pressure or temperature, and a transmitter turns that response into a standard signal a controller can read. Getting a good measurement depends as much on choosing the right technology and installing it properly as on the instrument's published accuracy.

What a sensor does, and what a transmitter adds

The sensing element is the part in contact with the process. A resistance temperature detector (RTD) changes resistance as it warms. A thermocouple produces a small voltage. A pressure sensor has a diaphragm that flexes and a strain gauge or capacitive cell that reads the movement. On their own, these outputs are small, non-linear and easily disturbed by long cable runs and electrical noise.

The transmitter conditions that raw signal. It powers the sensor where needed, linearises the reading, compensates for things like ambient temperature, scales it to a configured range and sends it out as a standard signal, most often 4–20 mA, sometimes with digital data on top, or over a fieldbus or Ethernet protocol. Many transmitters have the sensor built in; others, such as temperature transmitters, often sit in a head or a panel and take a separate sensor.

Modern "smart" transmitters add diagnostics, remote configuration and sometimes a second measured value. That is useful, but only if the controller or a configuration tool is set up to read it. A transmitter reporting a sensor fault that nobody sees is no better than one that cannot report it.

The common measurements

Pressure

Pressure transmitters read gauge pressure (relative to atmosphere), absolute pressure (relative to a vacuum) or differential pressure (the difference between two points). Differential pressure is the workhorse: it is used across filters, to infer flow across an orifice plate and to infer level in a tank from the weight of the liquid column.

Temperature

RTDs, most often platinum elements, are stable and accurate over the ranges found in buildings and most process work. Thermocouples cover higher temperatures and respond quickly but are less accurate and need the right extension wire. Thermistors are common in HVAC sensors where cost matters and the range is narrow. Most process temperature sensors sit in a thermowell so they can be removed without draining the line.

Level

Level can be measured by differential pressure, radar, ultrasonic, float, capacitance or guided wave methods. Each has blind spots. Ultrasonic sensors can be fooled by foam and vapour, differential pressure readings depend on the liquid's density, and floats can stick in dirty service.

Flow

Flow meters use many principles: differential pressure across a restriction, magnetic (for conductive liquids), vortex, ultrasonic, thermal and Coriolis. The choice depends on the fluid, the range of flows, the pressure loss you can accept and the accuracy you need.

Specifying an instrument

A good specification answers the questions the supplier would otherwise guess at:

• Range and span. The normal operating value should sit well inside the calibrated range, not at the edge. A transmitter ranged far wider than the process needs loses resolution.
• Accuracy. Check whether it is stated as a percentage of span, of upper range limit or of reading. These give very different errors at low values.
• Process conditions. Maximum and minimum pressure and temperature, the fluid, and whether it is dirty, corrosive, viscous or likely to freeze.
• Wetted materials. The parts that touch the process must suit the fluid and any cleaning chemicals.
• Environment. Enclosure rating, ambient temperature and, where it applies, the hazardous location classification. Equipment in classified areas must be certified for that class and installed to the Canadian Electrical Code.
• Signal and power. 4–20 mA, digital protocol or both; loop powered or separately powered; local display or not.
• Maintenance. How it will be isolated, calibrated and replaced, and whether a technician can reach it safely.

Installation decides the measurement

Most poor readings come from installation rather than the instrument. Flow meters need a length of straight, undisturbed pipe upstream and a shorter length downstream, because valves and elbows swirl the flow; the manufacturer states how much. Pressure transmitters need an isolation valve or manifold so they can be removed and checked without shutting down the process. Temperature sensors need enough immersion in the flow to read the fluid rather than the pipe wall.

Other details that matter:

• Impulse lines for liquid service slope so that gas can vent back to the process; for gas service they slope so that liquid drains back. Lines exposed to cold need heat tracing or a different approach.
• Transmitters are mounted where vibration, heat and direct sun will not affect them, and where a technician can reach them without a ladder in a hazardous spot.
• Signal cables are shielded where needed, with the shield grounded at one end, and kept apart from power and drive cables.
• Every instrument carries a tag that matches the instrument index and the loop drawing.
• A test connection or calibration port is provided where in-place checks are planned.

A typical case

A heating plant reports that the supply water temperature on the operator screen looks steady, yet the boilers short-cycle and some zones run cold. A technician compares the transmitter reading with a reference thermometer in a nearby test well and finds the two disagree. The sensor turns out to be in a thermowell that is too short for the larger pipe it was moved to during a renovation, so it reads partly the pipe wall. A properly sized well, a check of the transmitter range against the controller scaling, and an as-left calibration record fix the reading, and the boiler sequence then behaves as designed.

Common mistakes

• Choosing the instrument from a catalogue accuracy figure without checking process conditions, so it reads well on the bench and poorly in service.
• Ranging a transmitter to the instrument's limits instead of the process range.
• Installing a flow meter wherever the pipe happens to be accessible, regardless of straight run.
• Leaving out isolation valves to save cost, so the instrument can never be checked without a shutdown.
• Configuring the controller's scaling differently from the transmitter's range. The loop then reads a believable but wrong number. Our guide to 4–20 mA current loops explains how scaling works.
• No calibration plan after commissioning. See instrument calibration for how to set one up.

In classified areas, such as some fuel, gas and process spaces, the extra rules for equipment selection and wiring are covered in our guide to electrical equipment in hazardous locations.

How ehZee can help

We select and specify instruments for the process conditions, prepare the instrument index and installation details, and review installations and existing measurements that are giving trouble. Design drawings are sealed by a licensed professional engineer where required, and installation is carried out by licensed electrical contractors and qualified instrumentation trades. Learn more about our instrumentation and controls service.

Common questions