Transducers & Transmitters
The calibration of pressure transmitters and pressure gages is incredibly similar. I know the task can seem somewhat daunting at first, but I think we can relieve some of the stress that surrounds the job by explaining some underlying concepts first, and if you have the ability to calibrate pressure gages you most likely can calibrate transmitters.
What purpose to pressure transmitters serve
In a world where automation and integration is ever increasing, all sorts of sensors are used to monitor processes and enable computers to make decisions about how to react to changes in a system. A pressure transmitter can monitor a process and when the controller sees a change in pressure that requires a response the CPU might open a valve to reduce the pressure or add material to increase the pressure.
Analog control signals
For the computer to understand what a pressure transmitter it trying to communicate, it needs to understand what language it is speaking. Most data acquisition modules can interpret multiple “languages,” we call them control signals. These come in a number of different forms, but they can mainly be broken up into millivolt signals (mV), volt signals (V), and milliamp (mA) signals.
- Millivolt signals are the most varied and least expensive. They are also typically the least accurate. These will typically have ranges of 0 to 25, 50 or 100 mV however they also come in the millivolt per volt variety which adds a layer of complexity. I am putting together an upcoming blog based on calibration curves where we will look at the characterization of mV/V signals.
- Volt signals are common and require three or four wires to communicate. These will most often be in the ranges of 0-1, 5, or 10 volts, or 1 to 5 volts. These will typically be adjustable for the zero and span, or full range, of the transmitter.
- Most milliamp sensors only require two wires because they carry a voltage and adjust the “flow” of electricity to communicate the signal. The most common range for these is 4 to 20 mA, but 0 to 20 mA is seen from time to time.
When we look at the values of a control signal they will have a pressure input and an electrical output. So, a 0 to 100 PSI/0 to 10 volt pressure transmitter will output o volts at 0 PSI and 10 volts at 100 PSI. These are designed to be linear (unless otherwise noted) so an input of 50 PSI should give us an output of 5 volts. This example is very simple to understand because the math is not complex, but in order to calculate more complex ranges all you need to do is set up a ratio.
Use the above formula to solve for the variable you don’t know, by populating the three values you do know and then cross multiply. Remember if you are using a control signal with an offset from zero like 4 to 20 mA, the actual range of the control signal is 16 mA, so you’ll need to subtract 4 mA from the control signal if that is your input to the formula, or add it if the control signal is the output of your formula.
How to perform the calibration
The calibration of a transmitter is the same as a gage. We are just going to read the control signal with a meter instead of reading the gage or digital display of the device. We will power the device with a power supply set to the appropriate voltage and make connections to a meter to read the output. It is a good idea to refer to the manual for wiring and voltage requirements, because even sensors made by the same manufacturer can have different requirements. My procedure will have me zero the meter as needed, and then apply pressure at 20, 40, 60, 80, and 100% of the full pressure range and record the output of the transducer at each of these points. Then, I will reduce the pressure to 20% of full scale and record that reading as well. I will then convert these electrical signals to pressure values and report them on the calibration certificate.
How to adjust a transmitter
If the transmitter is adjustable, there will likely be two potentiometers that adjust “zero” and “span.” Perform a zero adjustment first and then apply the full scale pressure and adjust the “span” potentiometer. Remove the pressure and check the “zero” again, if it is in spec, you are done. If it is not, then you will need to go back and forth between “zero” and “span” a few times until you have it dialed in. When a transmitter’s “zero” and “span” impact each other, we refer to them as interactive. When you have this condition, it is a good idea to go a little over or under the adjustment (depending on which way you are needing to adjust) to reduce the number of times you need to go from zero to 100%.
If the transmitter is not adjustable, all is not lost. In this case, we have two options:
- Full loop calibration – connect the transmitter to the control system and use “software” adjustments to correlate zero and full scale readings. This method is actually the best way to calibrate any sensors on a control system because it takes into account all parts of the system when comparing measurements.
- Characterize the transmitter – this can be done without the control system. We will simply read the output signal of the transmitter and plot the values from the calibration in excel. Then, we can use those values to calculate a “calibration curve” for the transmitter (I will have an in depth look at this in an upcoming blog on calibration curves).