Regarding cable entry
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There is a sample guide for selecting flowmeters, and the dimensions for the cable inlet are as follows: M20x1.5 G1/2" NPT1/2" .......... Why are there several options? What determines the choice?
It is determined based on the specifics of the selected cable, as well as the requirements for cable connections.
These are the threads of the interfaces; when selecting the interface threads for the instrument, it is also necessary to design the threads of flexible tubes or cable sealing connectors for connection purposes to match them.
M20X1. 5 Plain thread, nominal diameter 20, pitch 1. 5 G1/2 1/2″55-degree cylindrical pipe thread, NPT1/2" 1/2″60-degree tapered pipe thread – in imperial units. The choice depends on the type of cable sealing connector you are used to using; generally, for thermocouples and similar devices, domestic-made connectors are mostly of the M20X1 type. 5, while most transmitters use 1/2 NPT
It’s not chosen based on anything specific; rather, it’s you who decides what specifications you need.
Here, it mainly refers to the thread type used for connecting instruments to cables. M20X1.5 is a standard metric thread with a nominal diameter of 20 mm and a pitch of 1.5 mm. G1/2 1/2″55-degree cylindrical pipe thread, NPT1/2" 1/2″60-degree tapered pipe thread – in imperial units. Most domestic instruments use M20X1.5 threads; those from abroad mostly use 1/2” NPT threads. Personally, I think 1/2” NPT is better; it has a certain taper, which results in better sealing.
I think it should also be determined based on the selection of other devices in the same batch or existing spare parts, in order to maximize versatility, reduce inventory, and save financial and human resources – that is, to take economic considerations into account.
Based on individual preferences*, and also because there are many remote sensors in the field and these sensors do not come equipped with sealing connectors, the same type of electrical interface is chosen to facilitate ordering and ensure consistency in spare parts. My usual practice is to use 1/2NPT for transmitters, and temperature sensors as well as control valves without feedback signals of type M20X1.5; generally, 1/2NPT provides better sealing.
M20X1.5 ordinary thread, nominal diameter 20, pitch 1.5; G1/2, 1/2″55-degree cylindrical pipe thread; NPT1/2", 1/2″60-degree tapered pipe thread. Pay attention to the threads when using an external explosion-proof bypass tube.
Tapered pipe threads certainly offer better sealing performance than straight pipe threads