A small pressure difference can coexist with substantial line pressure
A differential-pressure instrument measures the difference between its two pressure inputs. The requested measuring range therefore does not describe every pressure that the assembly must withstand. WIKA's differential-pressure training material treats differential range, working pressure and overpressure as separate selection items.
This article is general selection guidance, not a pressure-rating approval or a procedure for operating valves on a live installation.
Keep three descriptions separate
- Measuring task: the required lower and upper DP values, units, sign convention and usable calibrated span.
- Working conditions: the expected pressure at the high and low ports, their pressure reference and the process temperature.
- Exceptional conditions: the possible imbalance or pressure exposure during startup, shutdown, blockage or an approved service activity.
Ask the manufacturer which limits apply to the exact sensor, process connection, manifold and seal assembly. A working-pressure limit, a one-sided overload statement and a burst value are different specifications. Do not use a burst value as an operating limit.
A labelled arithmetic example
Consider hypothetical simultaneous inputs of 2.50 bar(g) at the high port and 2.30 bar(g) at the low port. Both use the same gauge reference. Their difference is 0.20 bar, or 20 kPa.
These are fictional teaching values, not WESIVO specifications or a customer measurement. The subtraction defines the pressure difference only. It does not establish an acceptable configured range, working-pressure rating, overload allowance or uncertainty. Two equal port pressures would give zero DP without proving that the assembly is depressurised.
Read the ordered configuration's documents
The Yokogawa EJA110E general specifications illustrate why the document review has several parts: they list measuring span/range, static-pressure effects and operating limits separately. Their numerical limits depend on the stated model and configuration. They are not ratings for a WESIVO 780C or another transmitter.
Request the applicable document revision and identify any temperature, material or option conditions attached to a pressure limit. Where the duty can expose only one side to pressure, describe that condition explicitly and ask for confirmation rather than assuming a common working-pressure figure covers it.
Verify measurement performance separately from pressure suitability
Meeting a pressure limit does not by itself demonstrate the required DP measurement performance. Ask which calibrated span, reference conditions and applicable temperature or static-pressure effects support the quoted performance. Retain the basis of the review rather than comparing isolated accuracy numbers.
Also specify whether the receiving channel requires DP or a derived variable. The sensor configuration, output scaling and receiving input must describe the same intended quantity; the correct output type cannot be inferred simply from the word transmitter.
A compact RFQ pressure envelope
Provide the normal and maximum pressure at each port, the required DP range, possible reverse DP, pressure reference, temperature, medium, connection drawing and signal requirement. Identify unresolved startup or maintenance conditions. Ask for written confirmation of the ordered assembly and its applicable pressure and performance documents.
The WESIVO 780C page and original pressure catalogue are configuration-review entry points. A catalogue range-selection notation is not automatically a maximum working-pressure or one-sided overload rating. Submit the complete conditions through the product or contact page; do not transfer the external examples' ratings, approvals or procedures to WESIVO.
差压较小，不代表管线压力也小
差压仪表测量两个压力输入的差，因此所需测量量程不能描述整个组件必须承受的所有压力。WIKA 差压培训资料把差压范围、工作压力和过压作为不同的选型项目。
本文属于通用选型说明，不是压力额定批准，也不是运行中阀门操作程序。
分开写清三类条件
- 测量任务：所需差压下限、上限、单位、正负方向和可用标定量程跨度。
- 工作条件：高、低压接口预计承受的压力、压力参考与过程温度。
- 异常条件：启动、停机、堵塞或批准的检修活动中可能出现的不平衡或压力暴露。
请厂家说明具体传感器、过程连接、阀组和密封组件各自适用的限制。工作压力限制、单侧过载说明和爆破值是不同规格，不能把爆破值当作运行上限。
明确标注的算术示例
假设同一时刻高压侧为 2.50 bar(g)，低压侧为 2.30 bar(g)，两者使用同一表压参考。其差为 0.20 bar，即 20 kPa。
这些是假设教学值，不是 WESIVO 参数或客户测量结果。相减仅说明压力差，不能据此判定配置量程、工作压力额定、过载余量或测量不确定度。两侧压力相等时可以显示零差压，但这并不能证明组件已经泄压。
查看具体订货配置的文件
Yokogawa EJA110E 通用规格体现了为什么需要分别核对文件：其中分列测量量程、静压影响与运行限制，数值适用性取决于其注明的型号及配置，不能作为 WESIVO 780C 或其他变送器的额定值。
索取适用文件版本，识别压力限制附带的温度、材质或选项条件。工况可能仅对一侧施压时，应单独说明并要求确认，不能默认一个共同工作压力数值已经涵盖该情况。
压力适用性与测量表现分别复核
满足压力限制，本身不等于满足所需差压测量表现。应索取报价性能对应的标定跨度、参考条件及适用的温度或静压影响，保留复核依据，而不是只比较孤立的精度数字。
同时说明接收通道需要差压还是某个派生变量。传感器配置、输出换算和接收输入应对应同一目标量，不能仅由“变送器”这个名称推定正确输出方式。
简明询价压力条件清单
提供各接口正常及最大压力、所需差压范围、可能的反向差压、压力参考、温度、介质、连接图和信号要求；标出未明确的启停或维护条件，要求按订购组件书面确认适用压力及性能文件。
WESIVO 780C 页面及原版压力样册是配置复核入口。目录中的量程选择写法不自动等于最大工作压力或单侧过载额定。请经产品或联系页面提交完整条件，不把外部实例的额定值、批准或操作程序移植到 WESIVO 产品。
Discuss your application
Send your medium, operating conditions, connection, output, quantity and destination. WESIVO will review the configuration before quotation.