Performance effects on diaphragm seal applications

The diaphragm seal application will be exposed to various factors and conditions influencing the performance, such as varying process and ambient temperatures, location of the transmitter, fluctuating static and process pressures, choice of instrument, etc. All these factors have an effect on the performance of the diaphragm seal application. These are also the factors which have a performance specification written around them on product datasheets (e.g. accuracy). This specification describes the worst case tolerance for the independent factor. BaseCal takes into account the worst case tolerance or error for each of the following factors:

• • Process temperature effect: showing the error of a combined effect of the fill fluid characteristics (e.g. density) and its related expansion or contraction error of the seal volume. This is based on the volumetric displacement of the diaphragm, which is dependent on thickness, diameter and material of construction. All based on the maximum process temperature.
• • Ambient temperature effect: showing the error of a combined effect of the fill fluid characteristics (density) and its related expansion or contraction error of the system volume (capillary + transmitter) showing the effect based on the maximum ambient temperature deviation.
• • Mounting effect: this shows the error based on the (vertical) mounting of the transmitter and the diaphragm seals, taking also into account the change in density of the fill fluid based on the maximum ambient temperature deviation, and thus the pressure of the liquid column of the fill fluid.
• • Accuracy compensated with temperature effect: this shows the combined error based on the accuracy of the selected transmitter and the error of the selected diaphragm, taking into account the span effects related to the temperature effects.
• • Static pressure effect: this shows the error based on the static pressure effect of the selected transmitter and selected diaphragm seal application (only applicable for dP measurements).

Total probable error (TPE)
It is difficult to make a general statement on the performance of any diaphragm seal application based on only one single effect or factor. It is the combination of the set of factors that determines the total performance. Statistical mathematics shows that the most realistic way to combine the effects of independent factors is by using the root-sum-square (RSS) method instead of just summing or averaging them. The result of the RSS method is what we refer to as the Total Probable Error of the diaphragm seal application. BaseCal’s TPE is based on the following effects:

• • Process temperature effect
• • Ambient temperature effect
• • Mounting Effect
• • Accuracy at process temperature
• • Static pressure effect

The total probable error is the most likely error that will appear on the measurement outcome of the diaphragm seal application. It is based on the statistical mathematic combination of the worst case tolerances of the independent factors that influence the diaphragm seal application.