Choosing a Gauge: Gauge Construction

Gauge construction.
When you know what to look for, you can tell in advance which gauges can stand up to the elements you work in.

Sensors:

Welded Sensor: It’s not generally possible to tell whether a sensor is fully welded by looking at the gauge, so check with the manufacturer. Non-welded sensors use o-rings or even thread tape inside the gauge. O-rings can degrade, and both o-rings and thread tape have the potential for leaks. 

Moisture: If the specification warns you to use clean, dry air, the sensor does not have an isolating diaphragm. It may not be suitable because moisture or liquid water will eventually cause sensor failure.

Diaphragm: Look for sensors with isolating diaphragms that are gas/liquid compatible and protected from damage. Good designs use a filter or a very small pressure port opening to keep small screwdrivers or cotton swabs away from the sensor.

Sensor Technology: Currently, piezoresistive sensors with oil isolation provide the best combination of performance and value. They are highly repeatable and handle overpressure conditions well. Sensors employing bonded strain gauges or thin film strain gauges rely on the deformation of a metal diaphragm. This makes them similar to mechanical pressure gauges, in that over-pressure can cause a permanent shift in calibration.

Also, some designs in low-cost digital gauges should only be used with liquids. Rapid changes in pressure cause readings to be unstable until the strain gauge reaches thermal equilibrium, up to a minute later. To test for this, zero the gauge, apply full-scale pressure using air or nitrogen, then vent the gauge and see if it returns to zero.

Enclosure:

The gauge enclosure should be compatible with hydraulic and other fluids typically used. Solvents and hydraulic fluids can attack polycarbonates, so look for a metal enclosure. For service near saltwater, marine-grade materials are optimal. Most, if not all gauges, use liquid crystal displays. If the gauge does not have a hard plastic or glass window, dropping a tool onto the display will destroy the LCD.

Also, ask what happens when the gauge is dropped. This information is generally not in the brochure or the operator’s manual, but gauges are often dropped, so this information is important to know.

Batteries:

Find out if the gauge’s battery life meets the application need and whether it uses standard batteries. Lithium batteries are an excellent technology, but a 9V lithium is expensive and difficult to find. Many gauges require a degree of disassembly to change the batteries and are often surprisingly difficult to reassemble. In some designs, you need to be careful not to damage the sensor or its cable when replacing batteries.

Finally, it is critically important to take the time to actually test and use a gauge. Spec sheets and brochures only tell part of the story. Is it easy to use? How good is the zero stability? How repeatable is it? Does it drift? What happens when it’s dropped? Can you read the gauge in direct sunlight or from a distance? Does the gauge have a built-in backlight? Actually trying the gauge can answer all of these questions and should be part of the evaluation process before you buy it.