Lets suppose there is a DP transmitter with maximum sensing limits of -5000 to +5000 mmWC and a calibrated range of 0 to +2500 mmWC. Now, if I use this transmitter for sensing pressure outside the calibrated range but within its sensing limits (lets say LRV= -1000 mmWC and URV= -250 mmWC), what will happen to its accuracy and linearity? Do I need to calibrate it again for the range I am going to use?

 

Assuming your transmitter is factory calibrated for a negative DP range, then using it over some other range than it was originally configured for is just a matter of changing the LRV/URV to the new range settings.

But you should be aware that some smart transmitters are factory calibrated in the negative range, some are not.

For example, older Honeywell ST3000 would measure and report negative DPs, but unless the unit was purchased with the CC option that covered a negative DP range, there was some degree of uncharacterized response resulting in error, except for the draft range transmitter which was 'compound characterized'. All of the newer models ST700/800 are factory "compound characterized" for negative DP.

As with all brands of DP transmitters, the touted accuracy/uncertainty spec is at reference conditions, at some limited turndown, which unless the DP is used in a cal lab is unlikely to be its operating conditions. But all the vendors have calculations for determining the uncertainty at operating conditions over the turndown in-use.

The spec/data sheets for these devices spell out this stuff, with some degree of cryptic terminology.

 

Are you saying that I don't need to calibrate it again for the range I am going to use? Just changing the LRV and URV is sufficient?

 

There are too many vendors out there to make blanket statements, given that in my experience, some pressure transmitters were not factory calibrated (characterized) for negative ranges), unless that option was purchased when the transmitter was manufactured.

I do believe that all DP transmitters are factory calibrated for positive ranges. But negative ranges, no.

If you're using it in a negative range, you should find out from the manufacturer whether it has been factory calibrated (characterized) for negative ranges.

If it has not been factory calibrated for negative ranges then your ability to calibrate it in the field is quite limited, given the limited functionality of the trim functions on the input. It is not likely to provide specification grade accuracy.

If the vendor confirms that that model is factory calibrated for negative ranges, then yes, just re-range it with different LRV and URV.

The transmitter will sense and report over a negative range, the question is whether the negative range is characterized. I suspect that it is, but my experience shows that not everything is obvious about how DP are characterized. You should investigate and find out.