As a purveyor of the broadest range of accelerometer calibration systems, we support customers with everything from simple hand-held 1G exciter check devices, compact all-in-one portable systems, desktop automated accelerometer calibration workstations, to the lowest uncertainty laser based accelerometer primary calibration systems. Routinely, customers on a quest to save a little money ask us if they can supply their own Digital Volt Meter (DVM) as part of the system… and being a customer focused company, we always consider the viability of the request. Like most things in life, the answer often is a conditional MAYBE…
For example, users can pair a calibrated DVM with an all-in-one exciter/source/reference accelerometer portable calibrator capable of excitation up to 10 kHz. This pairing allows the user to generate traceable (through the DVM) calibrations by reading the RMS value of sensor output with the DVM. So the customer asks us:
Question: “Can I supply my own DVM?”
Answer: “Well…. Maybe.”
“Maybe” for a number of reasons – all of them in the best interest of the customer and their accelerometer calibration well being. Let’s take a look at them…
- It depends upon the bandwidth. The DVM market (especially the lower priced) is often joyfully indifferent to the world of frequency and expects that 1 kHz is more than enough bandwidth for anyone! Rarely is this enough bandwidth for accelerometer calibration in anything other than the most rudimentary industrial calibrations. For example, DVM Models like the Fluke 114 and 170 are, by design, only specified to 1 kHz bandwidth. The problem is that above 1kHz, for example, lower bandwidths will exhibit a frequency dependent roll-off in reading. While a 1kHz bandwidth DVM may specify +/- 1% at 1kHz, by 10kHz the DVM can be expected to read low by as much as 30-50% of true signal level. Alternatively, we would recommend/supply a higher bandwidth, higher precision model.
- Aside: When using the K9100C for calibration to ensure the best uncertainties, we provide a Fluke 289 because it offers a sufficient bandwidth of 100kHz and high precision (0.25%).
- It depends upon the system and the degree of automation. In the illustrative example of this article, there is no integration or automation of the DVM. As long as the user has verified the proper bandwidth and uses the applicable uncertainty during calculation, a user supplied meter can be suitable, accurate and cost effective. In a more automated case, such as a higher throughput laboratory accelerometer calibration workstation, the use of the DVM is often integrated under software control for automated reporting of voltage values to the calibration system software. In this workstation case, the software generally requires a specific manufacturer and model number to operate properly.
- In cases where the customer does have the proper manufacturer and model number of meter to supply in a computer controlled, automated system, it is still prudent for the customer and vendor to agree on a small integration fee to ensure that the unit is shipped to the vendor, properly verified, integrated into the system and checked out/warranted for system performance.
- Aside: Most modern accelerometer calibration workstations have eliminated the DVM RMS approach entirely (due to susceptibility to errors from exciter mechanical distortion and/or extraneous environmental motion input) and instead use the accuracy of digital signal analyzer (DSA) and/or a frequency based method such as the Discrete Fourier Transform (DFT) or Fast Fourier Transform (FFT) for speed and reduced uncertainties.
- It depends on the customer tolerance for risk and down time. In critical laboratories and/or those with high throughput needs, it may be important to have spare components on hand to ensure continued operation in case of component failure. Ask your calibration system vendor what service stock they carry to help ensure your maximum customer uptime.
- Aside: it may also be useful to access calibrated “loaner” or service stock components to keep a calibration system up and running even while the original components are being re-certified during their annual recalibration.
- It depends in which country each party operates. Certain countries allow flexible temporary importation (and ultimately a tax free return) of products for repair or integration, while other countries assume that every border crossing is a permanent one. In these countries, it is all but impossible to return the integrated equipment to the customer without paying additional duty on the product.
As you can see, there is a lot to think about when taking on in-house accelerometer calibration. But don’t worry, we will help you build the control and confidence you need. More questions? Just ask us… we’re here to help you.