CALIBRATING d.p.Ts

Poor calibration of d.p.Ts and a failure to take line pressure into account can result in costly measurement errors. But help is at hand. BY David Jackson

Differential pressure transmitter testing by deadweight instrument, courtesy Tradinco

Differential Pressure Transmitters (d.p.Ts) are frequently used in fairly critical and cost-sensitive applications. But inappropriate calibration techniques can lead to significant measurement errors which, in turn, can result in lost profits, and sometimes excessive tax liability.

Problems arise when d.p.Ts are calibrated with the low pressure side open to atmosphere. In nearly every case, the application of d.p.Ts involves elevated line pressure acting on both the low and high pressure side of the transmitter. If the normal operating line pressure is not applied during calibration, the validity of the exercise comes in for serious questioning.

Independent evaluation of the effect of line pressure on d.p.Ts in the UK, France and the Netherlands underlines its importance. Of 16 d.p.Ts from 14 different manufacturers the following resulted. Six failed to meet the manufacturer’s specification for line pressure effects; two had span changes at maximum specified line pressure at least four times the accuracy specification; while 13 were significantly affected by line pressure.

The maximum zero shift observed was 6.9%, while the maximum span change was 4.2%.

Generally, d.p.Ts were calibrated at five different line pressures between atmospheric pressure and the maximum pressure recommended by the manufacturers. A calibration of at least six points was carried out at each line pressure for rising and falling readings. The main explanation for the d.p.Ts’ output change with line pressure is mechanical distortion of the wetted parts. Although distortion of the jointed cast body and silicon-filled measuring capsule is extremely small, the high degree of sensitivity means the change is significant.

Realistic environments

For this reason, a range of cost-effective Differential Deadweight Testers (DDTs) has been developed to enable technicians to undertake d.p.T calibration under conditions which reproduce realistic pressure environments.

Instruments come with a variety of line pressure and accuracy ranges to international standards. The operating medium can be air, oil or water.

The DDTs consist of a measuring piston and balance piston, each with its own pressure generating and control system, connected by a common cross-connecting valve. When the valve is open, weights are placed on the measuring piston and balance piston, equal to the required line pressure. The system is then pressurised so that both pistons are floating. Small adjusting weights are added to the balance piston until a state of equilibrium is reached, with both pistons at their mid-operating position, dropping at their natural fall rate. Once `cross float’ equilibrium is achieved, the common cross-connecting valve is closed.

As long as the balance piston is rotating and floating at the same reference height, the initial line pressure is maintained. The differential pressure measurement is made by each additional weight placed on the measuring piston. As the differential pressure is generated by the addition of extra weights, the resolution can be extremely small.

The main uncertainties associated with DDT are: that of establishing the effective area of the measuring piston and its rate of change as a function of pressure; the uncertainty of the weights; and the deviation from the measuring and balance piston `cross float’ equilibrium. This affects the zero differential pressure point and remains constant throughout the differential calibration.

The change in relative height between the measuring and balance piston after achieving the `cross float’ position is yet another factor. Similarly, the change in temperature in the measuring and balance piston after `cross float’ will have a bearing. This can be reduced by ensuring both piston and cylinder are manufactured from materials with a low linear thermal coefficient of expansion, such as tungsten carbide.

* David Jackson is managing director of Tradinco Instruments.