Like most makers of oceanographic instrumentation, Applied Microsystems offers two types of pressure sensors: the strain gauge pressure sensor and the quartz pressure sensor.   Given the dramatic price difference between the two types of sensors, the vast majority of Applied Microsystems instruments are shipped with strain gauge pressure sensors.  The table below compares the two types of sensors.

Pressure Sensor Choices

	Pros	Cons
Strain Gauge The strain gauge pressure sensors are silicon wafer strain gauges. These sensors consist of a Wheatstone bridge on a silicon wafer. Pressure on the silicon causes a resistance change on the resistors in the bridge.	Less expensive Greater number of range choices Smaller footprint	Lower Accuracy (0.05%FS) Requires temperature compensation
Quartz Quartz sensors operate by subjecting the quartz crystal to a pressure induced stress. This changes the resonant frequency of the crystal. The output from these sensors is two frequencies, pressure and temperature. The two frequencies are measured and used to compute pressure.	Greatest Accuracy (0.01%FS) Higher resolution ( Minimal hysteris and drift	Expensive Larger footprint makes availability impossible in smaller housings Requires temperature compensation Fewer range choices Fragility and susceptibility to shock

Given Applied Microsystems' detailed calibration methodology, instruments with strain gauge sensors are frequently shipped with an RMS error that is better than our specified field accuracy of 0.05%FS. It is not unusual for the RMS error on our pressure calibrations to be 0.028%FS or 0.029%FS. 

Pressure Specifications

	Accuracy	Precision	Resolution	Response
Strain Gauge	+/- 0.05%FS	+/- 0.03%FS	+/- 0.005%FS	10 milliseconds
Quartz	+/- 0.01%FS		+/- 0.000001%FS

Absolute error is different from relative error and is a function of the pressure range that is chosen and the relative accuracy of the sensor.  For example:

• a pressure sensor with a range of 2000 dbar and a relative accuracy of 0.05%FS will have an absolute error of 1 dbar (ie. 0.05 * 0.01 * 2000 dbar)
• a pressure sensor with a range of 4000 dbar and a relative accuracy of 0.05%FS will have an absolute error of 2 dbar (ie. 0.05 * 0.01 * 4000 dbar)

Many vendors of oceanographic instrumentation refer to accuracy and precision interchangeably.  They are not interchangeable.  In effect, accuracy refers to how well a sensor performs against a known third party standard.  For example, a temperature sensor may be +/- 0.001 C, as compared to a Black Stack themistor module.  Precision refers to the repeatability of the readings of a given sensor.   A sensor is precise when it repeatedly provides the same reading, regardless of how accurate that reading is.

A good analogy is a dart board.  The thrower of darts is accurate when he or she is able to reach the target, the bulls-eye.  He or she is precise if, having thrown three darts, all three land in the same location, irrespective of whether or not that location is the bulls-eye.

Applied Microsystems offers calibrated pressure sensors in the following ranges, measured in dbar.  NB: Dbar and metres are roughly similar:

• 0 to 50 dbar
• 0 to 100 dbar
• 0 to 200 dbar
• 0 to 500 dbar
• 0 to 1000 dbar
• 0 to 2000 dbar
• 0 to 4000 dbar
• 0 to 5000 dbar
• 0 to 6000 dbar

For an explanation of the relationship between calibrated pressure range and absolute accuracy, please see below.

For a detailed explanation of how we calibrate Applied Microsystems' pressure sensors - including a listing of Frequently Asked Questions - please click here.

Applied Microsystems designs and manufactures sensors - conductivity, temperature and pressure - that have unusually fast response times.  Rapid response times are important in a wide variety of scenarios.  However, if you intend to do vertical profiling - where accurate mapping of the thermocline or picnocline is important - or if you plan to calculate salinity or density - where mismatches between sensor readings can generate spiking, then rapid sensor response times should be a critical decision factor in your choice of supplier.