The Ashford Instrumentation range of direct drive Pneumo gauges can be bench calibrated in exactly the same way as a conventional pressure gauge.

Technical Advantages

The advantage of direct drive pneumo gauges over conventional C- tube gauges is simple. There is only one moving part, no gears, linkages or springs to wear or break. The unique helical-wound elastic element is attached directly to the pointer shaft. This eliminates the need for any movement-amplifying gears or linkages (the parts of a C-tube that fail most frequently). Direct drive gauges maintain their structural integrity and accurate calibration in the most demanding environments, even during prolonged and rugged use.

Safety Features

Direct drive pneumo gauges have a very low volume elastic element and solid front case that provide the safest gauge possible. C-tube gauges have a relatively large volume elastic element that in cases of catastrophic failure could spill large amounts of process media. Not all direct drive gauges offer the industry standard solid front for case safety. Our configuration of solid front and rear blowout features provides the customer with the safest possible case style.

Certified Accuracy

Our direct drive pneumo gauges have a high repeatable accuracy, instantaneous action, and are manufactured as standard to class 0.25 (+/- 0.25% full scale). The full scale accuracy rating is applicable in both rising and falling pressure and each gauge comes with a calibration certificate traceable to the National Institute of Standards and Technology (NIST)

Before calibrating there are a number of things to consider:

1.Accuracy Of Master Equipment. To calibrate any pressure gauge the master test equipment you use must be at least four times more accurate than the gauge you are testing at the point of reading. The master instrument must be traceable to a National or International Standard. In accordance with the requirements of the European Standard EN837-1, the direct drive pneumo gauge should be tested from at least 10 equally spaced points rising pressure and 10 points falling pressure. The gauge should be lightly tapped before the reading is taken.

Example:

0-70 MSW Gauge @ 0.25% accuracy is accurate to +/- 0.175 msw at any point. The master must be accurate to +/-0.04375 msw at point of reading (i.e four times greater). This would be expressed as 0.0625% FSD over a span of 0-70 msw. Alternatively a master instrument with an accuracy expressed as 0.0625% reading (or better) could be used.

In this example an instrument with an accuracy class of say 0.005% over a range of 350 msw (0-35 bar) would not be accurate enough for this calibration.

2.Scales Conversion / Specific Gravity. The S.G of Sea Water will vary around the world. When calibrating an instrument with a sea water scale it is vital the S.G is known. If the master calibration equipment is also scaled in sea water, the S.G should match that of the instrument on test. If the master equipment is calibrated in Bar it is possible to use a mathematical formula to convert bar to metres sea water. Alternatively the manufacturer should be able to supply you with a conversion back to PSI or Bar. The Ashford Instrumentation range of direct drive gauges uses the following conversion values:

1 Meter Sea Water = 1.46042 PSI

1 Metre Sea Water = 0.10069 Bar

3.Temperature. Direct drive pneumo gauges should be calibrated at 20C +/-2C since this is the temperature at which the initial setting and calibration were made. It is worth noting that the accuracy of all direct drive gauges are severely affected by changes in both ambient and process temperature. The degree of inaccuracy depends upon the materials of construction and can vary by manufacturer but it is generally accepted as being between 0.3% and 0.5% FSD (Full Scale Deflection) for every +/-10 Kelvin. Given the accuracy of the direct drive pneumo gauge is 0.25% this means that a shift of around 7C either side of 20C could give an error in reading equal to the maximum error allowed. This assumes that the instrumentation was 100% accurate at 20C.

Once the instrument to be tested has been moved into the calibration area it is important that it is allowed to settle. If the environment from which it has been removed is significantly different from the temperature within the calibration area (that should be regulated at 20C as previously stated), we recommend it is left for a minimum of 24 hours to allow for full heat sink. If the master calibration equipment is also temperature sensitive the same rules should apply.

The temperature at which the calibration takes place should also be measured on an appropriate calibrated device and the results recorded on the instrument calibration certificate.

4.Mounting. It is vital that the instrument on test (and the test equipment if applicable) are mounted in the correct orientation for the test. Unless clearly stated on the dial, all pressure gauges including the premium caisson gauge should be calibrated in the vertical position. Any deviation away from this will result in significant errors in reading.

5.Calibration Media. Direct drive pneumo gauges are degreased during manufacture therefore they must be calibrated using an oil free media. Calibration must be made using gas or clean air as the extra weight of a fluid will have a marked difference on the defection of the bourdon tube. We recommend Nitrogen be used as the calibration media.